Epidemiology and the Extent of Mental Retardation

Psychiatric Perspectives on Mental Retardation

Epidemiology and the Extent of Mental Retardation

]. Dale Munro, M.S .W., C.S.W.*

Mental retardation is the most common lifelong handicap in "developed" nations and likely consumes more professional and financial resources than any other disabling condition. 5 · 48 Professionals unfamiliar with the field frequently underestimate the enormous problems that must be confronted when attempting to serve people with mental retardation. They often fail to recognize that only among people with mental retardation is there the potential to find all three types of disability-intellectual, physical, and emotional-in one person. Within the mental retardation field, probably no other single topic has been studied as thoroughly as epidemiology and the prevalence of and conditions associated with mental retardation. Epidemiologic research has had a rich history dating back at least to 1811 when Napoleon ordered a census of "cretins" to be made in one of the Swiss cantons. 62 Little came of this, but throughout the nineteenth century and especially since the turn of this century, researchers repeatedly have seemed preoccupied with securing accurate epidemiologic data. The purpose of this article is to review many of the epidemiologic studies of mental retardation completed in this century. Emphasis will be on difficulties in conducting epidemiologic research and on prevalence rates and ways demographic and intellectual factors influence these estimates. The relationship between mental retardation and other impairments, such as physical and psychiatric problems, will be discussed. However, no exhaustive attempt will be made to analyze etiologic factors.

EPIDEMIOLOGIC STUDIES From clinical, budgeting and planning standpoints, it is important to have accurate estimates of how many individuals with mental retardation *Clinical Consultant and Social Work Supervisor, Oxford Regional Centre, Woodstock, Ontario, Canada

Psychiatric Clinics of North America- Vol. 9, No. 4, December 1986

591

592

J.

DALE MUNRO

there are in a given population. Table 1 shows prevalence rates and other information from studies conducted in many countries dating back to 1908. The results reveal wide variability in prevalence estimates. To explain these differences, two problems in conducting such research must be examined: first, how mental retardation is defined; and second, how data are collected. Difficulties in Defining Mental Retardation Obtaining universal agreement regarding what is meant by the diagnostic category "mental retardation" has always presented problems. Contributing to the confusion is the fact that the terms mental deficiency, mental handicap, mental subnormality, oligophrenia, and, more recently, developmental handicap and developmental delay frequently have been used interchangeably with mental retardation in various countries. Yet, at other times, these terms have had different meanings. For the purposes of this article, the terms will be considered synonymous. It is also important to note the use of the term developmental disability in the United States since the 1970s. Developmental disabilities are defined as severe, chronic disabilities attributable to mental or physical impairment, including mental retardation, cerebral palsy, autism, and other neurologic disorders closely related to mental retardation. Further complicating a review of epidemiologic research is the fact that different criteria have been used when attempting to define who is and who is not mentally retarded. First, the most commonly used criterion is based on a "statistical model" using psychometric test scores. An IQ score two standard deviations below the mean usually is the accepted cutoff point for diagnosing m ental retardation. Second, diagnosis sometimes is based on a "pathologic model" derived from a medical orientation. This model is concerned primarily with detecting physiologic dysfunction and developmental delays, as well as the symptoms and etiology of a particular disability or disease. Finally, mental retardation has been defined according to a "social systems perspective" that views mental retardation as a socially defined role or status. This viewpoint suggests that individuals are mentally retarded only if they have been so labeled by a social system of which they are a member, commonly the school system. Thus, many children are not regarded as mentally retarded until they start school and many are no longer regarded as such once they leave. 93 Across North America, the most widely accepted definition of mental retardation is that of the American Association on Mental Deficiency (AAMD). 43 This definition states the following : "Mental retardation refers to significantly subaverage general functioning existing concurrently with deficits in adaptive behavior, and manifested during the developmental period." This pragmatic definition follows a statistical model, but also combines some characteristics of a pathologic model, along with consideration of an individual's current adaptive behavior. Methodologic Concerns Discussing epidemiology in the mental retardation field would be easier if IQ scores were distributed according to the normal curve. Since this is not the case, the prevalence of mental retardation must be estimated

EPIDEMIOLOGY AND THE EXTENT OF MENTAL RETARDATION

593

utilizing more elaborate methods: epidemiologic surveys. 21 The methods used in studying prevalence fall into five categories, although many investigators combine these methods: (1) "agency surveys" involving accessing records of schools, clinics, institutions, and other agencies; (2) "household surveys" involving house-to-house canvasses; (3) interviewing key informants in a community such as clergy, teachers, police, or social workers; (4) clinical assessments of subjects involving medical examination or psychometric testing; and (5) extrapolating information from census data or conducting sample surveys. As exemplified by Jones' 1976 research, 61 attempts to extrapolate figures from census data tend to lead to lower reported prevalence rates as many families either do not recognize or do not wish to identify their relatives as being mentally retarded. As well, agency surveys often report lower prevalence estimates than other methods. One reason is that many persons with mental retardation are unknown to agencies unless they present severe academic, behavior, health, or other problems. Furthermore, some agencies may decline involvement in prevalence surveys if they have concerns about possible breaches of client confidentiality. 21 Other factors can also contribute to differences in prevalence rates among studies. For instance, the methods of inquiry were practically identical in the Royal Commission 112 study of 1908 and Lewis'82 investigation of 1929 conducted in England and Wales. Yet, Lewis' estimate of the prevalence of persons with mental retardation was almost double that made 20 years earlier. His figures were considered more accurate for three reasons. First, all areas surveyed were investigated by Lewis himself, rather than using several different researchers as in the 1908 study. Second, in the 20 years between surveys, much greater attention had been paid to the subject of mental retardation and widespread development of agency services had occurred. Third, it appears that the Royal Commission inquiry included many persons with mental retardation under other categories such as epileptic or "uncertified insane." Another important consideration when analyzing results from different prevalence surveys relates to their heavy reliance on standardized IQ tests. Unfortunately, research employing psychometric data has not always indicated the specific tests used, what IQ cutoff scores constituted a diagnosis of mental retardation, or whether the test used had been standardized on populations comparable to those under investigation. In general, when reviewing prevalence studies, it is difficult to find two studies comparable with respect to their methodology.78 Methods of sample selection, casefinding, and the diagnostic criteria used frequently differ. Therefore, it should be no great surprise that various surveys have reported substantially divergent prevalence estimates.

The 3 Per Cent Versus 1 Per Cent Controversy After analyzing some of the more recent studies listed in Table 1, there appears to be considerable consistency in the cross-national comparisons; all are below 1 per cent prevalence (that is, fewer than 10 persons with mental retardation per 1000 population). In the United States, with a population of approximately 250 million, a 1 per cent prevalence rate would

CJl

'° """ Table 1. Prevalence of Persons with Mental Retardation from Several Epidemiologic Surveys INVE STIGATORS (YEAR PUBLISHED)

POPULATION LOCATION OF STUDY

Royal Commission' 12 (1908) England and Wales (urban & rural)

SIZE

-

PREVALENCE RATE AGES SURVEYED

METHOD

all

Agency survey Key informants Medical examinations

UPPER IQ LIMIT

PER 1000

no IQ assessment

4.6

Lewis (1929)

England and Wales (urban & rural)

623,000

all

Agency survey Key informants Medical examinations Group and individual IQ testing

70

8.6

Lemkau et al. (1942)

Baltimore, Maryland (urban)

54,600

all

Agency survey

69

12.2

Bremer (1951)

Norwegian village above the Arctic Circle (rural)

1325

all

Key informants

no IQ assessment

55.6

Denmark (rural)

4130

90

13.3

Fremming (1951)38

Medical examinations all

Key informants Medical examinations

Lin (1953)

Taiwan, Republic of China (semi-rural)

20,000

all

Agency survey Key informants Medical examinations

no IQ assessment

3.4

Book (1953)l3

Northern Sweden (rural)

-

all

Parish registers Agency survey Key informants

no IQ assessment

10.3

._

Medical examinations·

New York State Department of Mental Hygiene (1955)

Onondaga Coun ty (urban & rural)

342,000

Essen-Moller (1956)

Southern Sweden (rural)

2550

0-17

Agency survey

all

Medical examination of every member

t:I

above and below 90

35.2

70

9.8

;:..

t"' t"1

S:::: c:

of population from three parishes

z

"0

Wishik (1956)

Georgia

Akesson (1961)

Southern Sweden {rural)

Levinson (1962)

Maine

Goodman & Tizard (1962)40 Middlesex County England (urban)

54,291

0-20

36.6

tTI

68

17.5

8t'1 ::: 0 r

Questionnaire survey of all schools and child-serving reside ntial facilities

75

25.0

0-14

Agency survey

50

2.5

all

Agency survey

70

3.4

Key informants Ho usehold survey Medical examinations

79

all

Agency survey Key informants Medical examination

148,000

5-20

451,800

7533

"O

0

()

-<

>

z ti ...,

:i: t'1

Kushlick (1963)

Wessex, England (urban & rural)

Kennedy et al. 67 {1963)

Five southeastern U.S.

1,837,000 1800

6-14

states

Jastak et al.59 (1963)

Scally & Mackay

124

(1964)

Delaware

Northe rn Ireland (urban & rural)

tTI

;x ...,

2117

1,435,400

Richardson & H iggins (1965)

Alamance {urban) & Halifax {rural) Counties, North Carolina

150,000

Taylor e t al. 139 (1965)

Oregon

215,000

Lemkau & lmre79 (1966)

Maryland {rural)

14,500

10-64

15-19 0-20

Surveyed an all-black population with low socioeconomic backgrounds

69

Unique design using 3 alternative definitions of me ntal retardation . Persons were conside red retarded if they fell in the lowest 25% {9% or 2%) of intellectual and social measures .

2% Level 9% Level 25% Level

Agency survey

184.0 4.0 20.0 83.0

r'l

...,z 0

"rj

~

t'1

z ...,

>

50

4.7

Agency survey Household survey Psychometric testing

70

0-20

Agency survey

75

19.0

1-59

Household survey

69

82.0

79. 0 (Alamance) 77. 0 (Halifax)

r ::0 t'1 ...,

> 00 ti

> ...,

0 z

Extensive psychome tric testing

Kushlick (1966)

Salford, England (urban)

Drillien et al. 28 (1966)

E dinburgh, Scotland (urban)

155,000

all

Agency survey

70

4.4

39,500

7.5- 14.5

Agency survey

69

11.3

CJl

'°

CJl

"'"'

Table 1. Prevalence of Persons with Mental Retardation from SeveraLEpidemiologic Surveys (Continued) INVESTIGATORS

(YEAR PUBLISHED)

PREVA LENCE RATE

POPULATION

LOCATIO N OF STUDY

SIZE

O'l

AGE S SURVEYE D

METHOD

UPPER IQ LIMIT

PER 1000

8274

8-10

Agency survey

50

3.7

Camberwell, England (urban)

38, 460

10-14

Agency survey

50

3.9

Mercer (1973)94

Riverside, California (urban)

85,000

all

A.B. & 70 IQ A.B. & 85 IQ < 70 IQ > 85 IQ

9.7 34.7 21.4 72.9

Reynolds (1976)ll 3

Queensland, Australia (urban & rural)

396,200

0-16

Agency survey

55

3.4

Kirk (1978) 69

Dutchess County, New York {urban)

220,000

all

Agency survey

83

5.5

Jones (1979)61

United States (urban & rural)

150,000

3+

no IQ assessment

4.3

Birch et al. 12 (1970)

Aberdeen, Scotland (urban)

Wing (1971)147

A "social systems" and agency survey approach; and a "traditional clinical" (IQ and adaptive behavior) approach.

Utilization of census data from re presentative sample of country

no IQ assessment

6.7

85

5.4

no psychiatric assessme nt

5.0

70

9.5

Agency survey

70

9.4

Government health surveillance registry using over 80 information sources

90

4.0

Hagberg et al. 47 (1981)

Gothenberg, Sweden

450,000

8-12

Fishbach & Hull34 (1982)

Manitoba (urban & rural)

967,042

all

Agency survey Province-wide survey of age ncies

Lundy (1983)87

Ontario (urban & rural)

8,625,105

all

Special formula used for estimating prevalence based on Jones (1979). Data validated from agency records and

Jacobson & Janicki 58 (1983)

New York State {urban & rural)

Richardson et al. 117 {1984)

Britain {urban)

Baird & Sadovnick5 (1985)

British Columbia {urban & rural)

other research.

-

all

13,842

5-22

2,700,000

all

Statewide nee ds assessment survey of those receiving special services

(A dapted and updated from Lapouse, R. , and Weitzner, M.: E pidemiology. In Wortis, J. (ed. ): Mental Retardation: An Annual Review I. New York, Grune & Stratton, 1970, page 198; with permiss ion. Studies not included in the original table have reference numbers noted bes ide investigators' names.)

'--<

0

;.. r' t
~

c::

z

~

0

EPIDE MIOLOGY AND THE EXTE NT OF ME NTAL RETARDATIO N

597

indicate there are 2.5 million persons with mental retardation. In Canada, with a population in the range of 26 million , the I per cent rate suggests there are 260,000 individuals with mental retardation. Yet, until about two decades ago, it usually was estimated that 3 per cent of the general population was mentally retarded, 127· 134 particularly after President Kennedy's Panel on Mental Retardation 107 incorporated the 3 per cent estimate. Still, as early as 1933, Doll 27 suggested that the prevalence of mental retardation might be closer to 1 per cent, and by 1960, Dingman and Tarjan 26 began to present cogent arguments opposing the 3 per cent figure. They suggested that the prevalence rate for "visible" mental retardation is approximately 1 per cent. A few years later, two well-documented reports 94 · 138 argued that the 3 per cent prevalence model is "a myth" predicated on four assumptions: (1) diagnosis of mental retardation is based essentially on an IQ below 70; (2) mortality of individuals with mental retardation is similar to that in the general population; (3) mental retardation is always identified in infancy; and (4) once made, diagnosis does not change. To clarify these inaccurate assumptions, let us briefly consider each in turn. In explaining the first assumption, it should be noted that a 3 per cent prevalence rate is an artifact of IQ testing based on a theoretical normal distribution curve. Traditionally, mental retardation was defined as an IQ score more than two standard deviations below the population mean . This placed 2.27 per cent (usually rounded off to 3 per cent) of the population in the mental retardation range. Yet, this 3 per cent figure is not supported by results from recent epidemiologic studies that have found the rate of labeled mental retardation to be less than 1 per cent. A major reason is that the most widely accepted definitions of mental retardation are not unidimensional (that is, based only on IQ scores) . For instance, the A. A. M. D . definition cited earlier establishes a diagnosis of mental retardation not only on subaverage intellectual functioning, but also requires that an individual manifest impaired adaptive behavior. This double criteria system reduces prevalence rates, as so well illustrated by Mercer. 94 The second assumption 'ignores the fact that the mortality rates of individuals with mental retardation are inversely correlated with IQ, with only persons who are mildly retarded having life expectancies approaching those of the general population. Therefore, incidence (new cases) of mental retardation at birth may be as high as 3 per cent, but prevalence (all cases at any given time) is closer to 1 per cent. In explaining the inaccuracy of the third and fourth assumptions, it is clear that all persons with mental retardation are not diagnosed at the same age. As well, a diagnosis of mental retardation can change over time and is often age dependent. This will be discussed further in the next section. Rather surprisingly, literature as recent as 197686 has suggested that many professionals and jurisdictions continued to adhere to the 3 per cent prevalence figure and sometimes have used IQ cutoffs higher than 70 when diagnosing mental retardation. Evidence forwarded within the past three years 58 indicates this is ceasing and that a figure in the 1 per cent range is growing in acceptance. This has come about for the following reasons. First, professionals today use more restrictive diagnostic criteria consistent with

598

J.

DALE MUNRO

recent trends toward normalization, mainstreaming, diversion of persons who are disabled to generic community services, and unlabeling people previously identified as mentally retarded. 87 Second, to some degree, efforts to reduce poverty and nutritional, social, and educational deprivation likely have contributed to the perceived decline in prevalence. Third, medical research has had some effect on the prevalence of mental retardation. Greater accessibility to genetic counseling and abortion services in cases of high-risk pregnancy has helped reduce the number of damaged infants. There are now medical treatments and prevention for syndromes such as cretinism, galactosemia, and phenylketonuria, and improved obstetric techniques have lowered the incidence of brain damage at birth.57 For political and fund-raising reasons, some special interest groups continue to oppose the l per cent prevalence figure in favor of a higher rate, as was reported by Kirk. 69 Nevertheless, there is good reason to be skeptical of the 3 per cent figure.

DEMOGRAPHIC INFLUENCES ON PREVALENCE Age From information presented in Table 2, it is apparent that there is considerable uniformity among cross-national studies with respect to the relationship between prevalence rates and age. Most surveys show substantial increases in prevalence from the preschool ages (0 to 4 years) to the latency stage (5 to 12 years). Before school begins, parents may not be aware of their child's developmental impairments. It is primarily children with more severe levels of retardation who are identified during preschool days. Children are more likely to be identified as mentally retarded when they become involved in activities outside the home, especially after starting school. Performance expectations rise and developmental delays become more visible. Depending on which study is cited, prevalence tends to peak either in the middle tee nage years or the early twenties. If the rise is in the adolescent years, it usually is explained by the fact that developmental problems become more obvious with increasing demands on the individual in the school system. At the same time, people at this stage also must face more subtle social pressures and are expected to show more judgment and controlled behavior (for example, sexual conduct). Initial institutional admissions commonly have occurred during this period. 119 The finding of prevalence peaking in the early twenties is often attributed to the fact that at this stage, vocational expectations tend to bear heavily on individuals with intellectual deficits. This is particularly true in investigations oriented toward identifying persons with more severe levels of retardation. 5 • 61 • 74 For young adults in the age range from 22 to 34, prevalence decreases. Once the critical period of school attendance is over, many persons formerly labeled mildly retarded (the infamous "6-hour" variety of mental retardation 106) again are assimilated fully into their community, join the ranks of the dull-normal, and cope more or less successfully with day-to-

Table 2. Prevalence of Persons with Mental Retardation By Age per 1000 Population

!Tl

AGE INVESTIGATORS (YEAR PUBLISHED)

0

5

10

15

20

Lewis (1929)

1.2

15.5

26.5

10.8

Lemkau et al. (1942)

0.7

11.8

43.6

30.2

·Bremer (1951)

82.l

4.6

76.9

Essen-Moller (1956)

9.0

16.0

21.0

19.0

Wishik (1956)

8.7

57. l

60.8

19.4

35

40

5.7

7.2

45

50

5.4

45.5

55

65+

3.9

6.4

43.2

2.6

57.6

3.5

60

4.9

8.3

8.1

4.1

New York State Department of Mental Hygiene (Onondaga County, 1955)

8.6 1.9

33.6

2.3

7.1

3.9

s:

0

r

12.2

0 CJ -<:

55.6

>-

3.4

44.5

6""t'1

35.2

z

0

>-I

:i: t'1

!Tl

18.4

12.5

Akesson (1961) Jastak et al.

8.4

6.7 39.3

30

48 .8

0.3

Lin (1953)

25

ALL ACES

37.2

10.0

16.0

3.0

8.0

9.8 36.6

14.2

19.7

22.7

17.7

17.6

8.3

17.5

x>-I t'1

z

>-I

0

'Tl

~

t'1

(1963) 59

4.9 19.5 93.1

2%

3.8 13.3 62.8

9% 25% Richardson & Higgins (1965) Alamance County Halifax County

48.0 43.0

81.0 92.0

97.0 87.0

89.0 870

Le mkau & Imre (1966) 79

72.4

74.9

84.7

103.9

3.3 22.8 95.l

4.0 20.0 83.0 79.0 77.0

z>-I

>-

r ::i:i t'1

,,>-

>-I

0 Mercer (1973) 94

0.7

Jones (1979) 61 Swedish statistics (1979)32

2.2 0.6

Fishback & Hull (1982)" Lundy (1984)87 Baird & Sadovnick (1985) 5

5.4

11.5

16. l

4.3

5.8

5.7

2.0

2.3

5.2

2.6

4.2

7.1

4.2 5.3

3.5

5.6

4.0 6.8

7.7

82.0

1.3 5.5

12.6 4.7 6.6

2.4 1.5

81.2

9.0

7.4 7. 7

8.3

1.2

80 .0

4.9 7.2

3.2

9.7 3.5

1.8

2.4

2.1

3.9

3.5 1.4

1. 7

>-

::l

0

z

4.3

1.2

3.8

0.4

5.4

1.9

5.0 4.0

(A dapted and Updated from Lapouse, R., and Weitzner, M. : Epidemiology. In Wortis, J. [eel.]: Mental Retardation: An Annual Review. New York, Grune & Stratton , 1970; with perm ission. Studies not included in the original table have reference numbers noted beside investigator's nam es.)

Coll

ca ca

600

J.

DALE MUNRO

day demands, often in marginal socioeconomic circumstances. 119 Many, at this point, no longer are perceived as mentally retarded. For middle-aged adults in the 35 to 54 year range, there is a more substantial decline in the prevalence rate . For persons in the "seniors" age groups, prevalence continues to decline steadily between 55 and 64 and sharply after age 65. In explaining this pronounced fall after age 65, Lundy87 argues that there may be at least three possible explanations. First, after age 65, persons with mental retardation are more difficult to track because they switch from special disability pension benefits to pensions made available to all senior citizens. Second, many generic community services for the general senior population become available to individuals with mental retardation, and perhaps these support services take up the slack. Third, retirement brings relief from the performance expectations of the working years, and many finally may discard the label of mental retardation and live within normal bounds. Thus, when comparing prevalence data from various studies, it is particularly meaningful to compare age-specific rates. The important facts from the viewpoint of prevalence are (1) approximately 75 per cent of individuals identified as mentally retarded are adolescents or younger, with nearly 70 per cent being of school age; (2) the initial diagnosis of mental retardation is established in practically 100 per cent of cases before adulthood; and (3) the majority of individuals diagnosed as mentally retarded lose this label during late adolescence or early adulthood. 138 Either these previously labeled persons continue to be extremely disabled in later life but are unknown because the services they need are unavailable-in which case society is failing to fulfill its obligations to them-or these individuals are no longer mentally retarded in any true sense and do not need special protection and services- in which case we had better change our concept of what real mental retardation is. 44 Sex Most prevalence studies have reported higher rates of males with mental retardation than females, as is apparent in Table 3. Even a recent investigation completed in the rather remote Cook Islands reported a similar trend. 108 Two extensive literature reviews 33• 78 completed in the late 1960s found only one survey, by Lemkau et al., 80 that showed a reversal of this pattern. Clearly, sex makes a difference in one's chances of being labeled mentally retarded. Some research61 suggests males are almost twice as likely to be judged mentally retarded as females . There are at least three explanations for this higher prevalence among males. First, males seem more susceptible to extrinsic factors or agents that can damage the CNS and impair intellectual development. Congenital anomalies are more prevalent in boys, as are prematurity, neonatal death, and stillbirth.44 Second, there are hereditary factors contributing to an excess of males such as Xlinke d mental retardation.52• 75• 125 Third, in Western cultures, boys tend to express their frustrations in more overt, aggressive ways than girls. Therefore , problematic boys of preschool and elementary school ages face a greater possibility of coming to the attention of authorities and being

601

EPIDEMIOLOGY AND THE EXTENT OF MENTAL RETARDATION

Table 3. Prevalence of People with Mental Retardation by Sex per 1000 Population I NVESTIGATORS (YEAR PUBLISHED)

AGE RANGES

Lewis (1929)

MALES

FEMALES

All rural urban

42.0 22.6

37.2 19.2

10-14 20+

50.5 5.7

36.8 7.6

Bremer (1951)

All

62.0

49.0

New York State Department of Mental Hygiene (Onondaga County, 1955)

0-17

44.7

25.3

Essen-Moller (1956)

All

11.4

8.1

All

2.9

2.3

3.4 23.5 89.6

3.0 14.8 72.0

100.0 85.0

59.0 67.0

84.5

79.4

4.0 3.6

3.1 2.8

Lemkau et al. (1942)

Kushlick (1963)

10-64

Jastak et al. (1963) 59 2% level 9% level 25% level

0-20

Richardson & Higgins (1965) Alamance County Halifax County Lemkau & Imre (1966) 79

1-59

Reynolds (1976) 113

0-16 rural urban

Jones (1979) 61

3-4 5-13 14-17 18-21 22-34 35-54 55-59 60-64 65+ All ages

3.7 5.2 6.2 7.4 7.2 5.9 4.3 2.4 2.2 5.5

0.6 3.5 5.3 4.0 3.9 2.7 2.8 1.2 1. 3 3.1

Fishbach & Hull (1982)34

0-5 5-11 12-17 18-34 35-64 65+ All ages

1.5 10.0 12.8 5.4 4.3 0.6 5.8

0.9 6.3 12.3 5.6 3.7 0.4 5.0

All

8.8

6.6

Baird & Sadovnick (1985) 5

(Adapted and updated from Lapouse, R., and Weitzner, M.: Epidemiology . In Wortis, J. (ed.): Mental Retardation: An Annual Review I. New York, Grune & Stratton, 1970, page 207; with permission . Studies not included in original table have reference numbers noted beside investigators' names. )

J.

602

DALE MUNRO

Table 4. Prevalence of People with Mental Retardation by Socioeconomic Level per 1000 Population SOCIOECONOMIC LEVELS INV ESTIGATORS (YEAR PUBLISHED)

Lin (1953)83

Upper

Middle

Low

1.1

2.6

6.6

Richardson & Higgins (1965)1I 8 Alamance County Halifax County

34.0 49.0

60.0 56.0

172.0 108.0

Lemkau & Imre (1966)79 White Nonwhite All subjects

8.2 97.5 14.5

20.4 120.2 65.7

58.0 193.4 172.1

referred for psychodiagnostic assessments. This may not mean that there are more boys with mental retardation than girls but simply that boys are more susceptible to being labeled mentally retarded. 57 Research lends support to this argument that differences in prevalence between males and females may be more apparent than real. For instance, in the excellent study by Lemkau and Imre, 79 differences in prevalence rates between the sexes were not significant. In fact, under the age of 20, females were slightly more likely to be labeled mentally retarded . In a comprehensive agency survey, Mercer 93 found significantly more males than females were identified as mentally retarded. However, a subsequent household survey involving administering tests of IQ and adaptive behavior to a representative sample led to strikingly different results. There were no differences in prevalence between the sexes on the IQ assessment, yet the ratio of males to females who "failed" the adaptive behavior scale was almost two to one. Other studies also seem to suggest that the prevalence of mental retardation may be affected more by labeling practices than by gender. As shown in Table 3, Jones 61 found sex differences minimized in the middle teen years, whereas Fishbach and Hull 34 reported a similar finding during early adulthood . These are life stages when there is less societal pressure to have people psychometrically tested and labeled. Other empirical investigations suggest differences in prevalence between the sexes are related to functioning level. Lemkau and Imre 79 found that among persons with IQs of less than 50, females outnumbered males, possibly because of a lower life expectancy among males who are more severely retarded . At least five other studies 5· 28· 50 • 5 1· 100 have reported a higher rate of males than females among individuals who are mildly retarded, but no such differences among persons who are more severely impaired . Socioeconomic Level Research has clearly identified an inverse relationship between socioeconomic status and level of intellectual functioning. Three studies cited in Table 4 demonstrated this trend toward higher rates of mental retardation among people of lower socioeconomic background. Richardson and Higgins' study 118 showed that in urban Alamance County, the prevalence of mental

EPIDEMIOLOGY AND THE EXTENT OF MENTAL RETAR DATION

603

retardation grew as one progressed from upper to middle to lower socioeconomic levels. This pattern was similar but less conspicuous in rural Halifax County. Lemkau and lmre's 79 relatively high overall prevalence estimate of 82 persons with mental retardation per 1000 population probably reflected the fact that the investigation was conducted in a rural, predominantly lower socioeconomic area. Their observations indicated that children from the lowest socioeconomic level were nearly 13 times more likely to be mentally retarded than those from the highest level. Some reasons may be detected to explain these socioeconomic differences in prevalence. One study1 50 demonstrated that children from lower socioeconomic levels seem to lack motivation to do well on standardized IQ assessments, sometimes leading to scores in the mentally retarded range. Other evidence 17 suggested that among children with similar IQs, poor children tended to be placed in classes for persons who were educable mentally retarded, whereas middle-class children often were given preferential treatment and referred to classes for children with minimal brain injuries. Other factors reflective of low socioeconomic background also seem highly correlated with the prevalence of mental retardation. Studies assessing the occupational status of families 130· 133 · 148 show greater prevalence among families of unskilled, semiskilled, and manually skilled workers, compared with families of professional or executive background. Other investigations into the quality of home life found more individuals with mental retardation came from homes with poor living conditions 82 · 103 or from hom es characterized by overcrowding and multiple room occupancy. 101 · 130 · 148 This trend was more apparent for persons with mild retardation. Those who were more severely retarded came more equally from homes of various socioeconomic levels. Finally, Mercer93 reported that in her survey, a disproportionately high number of people with mental retardation were living in low-income housing, the majority being persons who were mildly retarded. Race

Analysis of racial influences on the prevalence of mental retardation has been confined primarily to studies conducted in the United States. As shown in Table 5, most research has reported the prevalence is noticeably higher among nonwhites than among whites. The lone exception, Wishik's 148 survey, found no racial differences. Wishik's approach demanded that a diagnosis can be made only if a clinical assessment determined the retardation constituted a real handicap in the child's present social situation. Generally speaking, however, prevalence estimates for mental retardation have been found to be higher among racial minorities. For instance, even the study by Jastak et al., 59 using separate cutoff points for diagnosing blacks and whites, reported higher prevalence rates among blacks. The investigation by Kennedy et al. 67 reported one of the highest prevalence figures ever reported . The population being surveyed was made up completely of black school children from extremely low socioeconomic backgrounds. As well, Mercer's 93 research found nearly twice as many blacks and three times as many Mexican-Am ericans diagnosed as mentally retarded

J.

604

DALE MUNRO

Table 5. Prevalence of Persons with Mental Retardation by Race per 1000 Population

INVESTIGATORS (YEAR PUBLISHED)

Lemkau et al. (1942)

80

New York State Department of Mental Hygiene (Onondaga County, 1955)97

White

Nonwhite

10- 14 20+

26.1 6.5

98.2 7.2

64.3

132.2

28. l 30.9 30.0

99.5 125.0 88.9

0- 17

Wishik (1956) 148

0-20

Kennedy et al. 67

6-14

Jastak et al. 59

Richardson & Higgins (1965)118

RACE

AGE RANGES

Predominantly nonwhite, low socioeconomic area of city Rest of city Overall urban Overall rural

34.9

34.9 184.0

10-64 2% level 9% level 25% level

3.0 13. 7 58.5

4.3 55.8 236.1

Alamance County Halifax County

65.0 62.0

141.0 85.0

0- 20

Lemkau & Imre (1966)79

1-4 5-9 10- 14 15- 19 20-34 35- 39 All ages

24.5 23.3 16.0 15.5 18.5 14.5 17.8

109.5 120.3 145.5 194.6 161. 7 209.0 157.6

Jones (1979)61

3-4 5- 13 14-17 18-21 22- 34 35-54 55- 59 60- 64 65+ All ages

2.4 4.2 5.5 4.7 5.2 3.9 3.2 1. 7 1.6 4.0

1. 3 4.5 7. 7 12.9 9.2 7.2 6.2 3.2 2.9 6.8

as should be expected from general population figures. She concluded that much of this difference was due to racial minorities living in lower socioeconomic areas. Similarly, in the Onondaga County study, 97 it was discovered that only four census tracts from the most deteriorated areas surveyed were contributing a disproportionately large share of persons with mental retardation. When racial backgrounds were compared within these four tracts, it was concluded that low socioeconomic level, not race, contributed most to the risk of mental retardation. It has been observed that agency studies, such as Lemkau et al. , 80 commonly report less pronounced differences in the prevalence of mental retardation among various races because nonwhites are less likely to be

EPIDEMIOLOGY AND THE EXTE NT OF ME NTAL RETARDATIO N

605

served by social agencies than whites. In contrast, the Lemkau and Imre 79 investigation, which combined a household survey and a clinical evaluation, found blacks nearly nine times more likely to be labeled mentally retarded than whites. This study also suggested that the prevalence among blacks tends to increase with age, while whites show the opposite trend. Jones 61 draws the conclusion that for both blacks and whites, it seems that reaching school age increases the percentage of children labeled mentally retarded, and there is little difference between the races for the 5 to 13 age range. But during the adult years, the prevalence rate for blacks is considerably higher, probably because inferior educational opportunities and limited support systems create functionally illiterate, seemingly retarded, black adults. An interesting sidelight of Jones' data is that in the 3 to 4 year age range, blacks actually have a lower prevalence rate than whites. This finding seems consistent with other studies, 3• 8• 68 which suggest that the psychomotor development of black infants up to about 18 months may be faster than that of white infants. But environmental factors-such as lower socioeconomic status, mothers with less education, inadequate prenatal care, nutritional deficiencies, and biased psychometric assessment methods-probably contribute to deficits in IQ test performance as black children grow older. It is worth noting that interpretations of prevalence results related to race frequently have been the target of controversy and passionate debate. This debate seemed to peak in the early 1970s after the release of Jensen's empirical work60 questioning the doctrine of racial genetic equality. However, one is still left with a lingering suspicion that children of poverty, especially if they are nonwhite, are assumed to be mentally retarded, whereas middle class children are treated more supportively. 119 Urban-Rural Variations

Many cross-national studies in the past6 · 20 · 82• 91 have suggested that the average IQ of people in rural areas is lower than that of urban populations. Wheeler found a similar trend in his studies of isolated Tennessee mountain children. 145 However, other empirical works 112• 129 · 131• 144 have reported no clear differences in the prevalence of mental retardation between urban and rural residents. Further confusing the picture is still other research that has found rates of mental retardation highest in urban areas. For example, Richardson and Higgins 118 observed lower rates in more rural Halifax County than in the more urban Alamance County, which rather surprisingly had a higher socioeconomic level and fewer blacks in the population. As well, in the Jastak et al. study, 59 the prevalence of mental retardation was somewhat higher in urban areas for each of the three cutoff levels. As shown in Table 5, in the Onondaga County survey, 97 little variance occurred in prevalence between the overall city (urban) rate of 30. 9 per 1000 and the ove rall rate in the rest of the county (rural) of 30. 0. However, when broken down by race and area, it becomes apparent that both white and nonwhite persons from predominantly nonwhite, lower socioeconomic areas have significantly higher rates of retardation than people from either

606

J.

DALE MUNRO

the rest of the city or rural areas. This is one of the few studies to actually concentrate on the prevalence of mental retardation in a poor urban ghetto. This investigation concluded that prevalence is a function of place of residence, race, and socioeconomic status. The high overall prevalence of mental retardation reported by Lemkau and lmre 79 in a distinctly rural population has been attributed to the low socioeconomic status and nonwhite composition of the population studied. Therefore, considerable evidence reveals differences in measured intelligence between rural and urban populations. However, the data are so confounded with racial, socioeconomic, and other variables that it is difficult to draw firm conclusions. 49 It does seem reasonable to suggest that more people from isolated agricultural and tribal communities, as well as from impoverished urban ghettos, probably score lower when tested on standardized psychometric instruments because of limited educational, occupational, and cultural opportunities. Otherwise, it is difficult to conclude that there are any significant differences in the prevalence of mental retardation between urban and rural populations. Current Living Situation Since the 1850s, institutional placement too often has been considered a final solution to the problems of people with mental retardation, their families, and society. During the past 15 years, however, something of a revolution has occurred in the development of residential services for people who are mentally retarded. The most dramatic event has been the "deinstitutionalization" of thousands of persons from large public facilities into community residential settings such as supervised residences, foster homes, group homes , and independent living situations. To illustrate these trends in more quantitative terms, let us examine the following statistics. In the United States during the 1960s, the average number of residents per institution was about 1500. 77 In contrast, by 1982, the average size was 478 residents and dropping . 120 The total number of individuals with mental retardation living in large public facilities reached its peak in 1967 with 194,650 residents. 76 By January 1983, this number was reduced to 111,311 persons, the vast majority being severely or profoundly retarded. 31 It is also interesting that one eighth of the American public institutions that existed in 1965 now have been closed. Nearly three fourths of the closures have occurred since 1982. Clearly, not only deinstitutionalization but institutional closures are emergent trends of considerable significance. 14 Some statistics from Manitoba34 help to illustrate the current state of community residential alternatives. As shown in Table 6, more than half (54.4 per cent) of the persons with mental retardation surveyed reside with family members, whereas approximately 20 per cent live in public institutions. The percentage living with family members is inversely related to their degree of intellectual deficit. Persons classified as borderline or mildly retarded were more likely to be found in independent living situations, with family, or in other types of residential programs (for example, correctional). Persons with moderate retardation were over-represented among those in foster homes and group homes. Only 1 per cent and 8 per cent,

607

EPIDEMIOLOGY AND THE EXTENT OF MENTAL RETARDATION

Table 6. Current Living Situation of Persons with Mental Retardation by Level of Intellectual Impairment Including Percentage Distribution Within Each Level BORDERLINE

Independent living Family or relative Foster home Other residences Community group home Institution

4.4 72.0 11.6 8.6 2.4 1.1

MILD

MODERATE

SEVERE OR PROFOUND

5.2 62.1 9.4 13.4 2.8 7.1

0.8 51.8 13.6 5.1 59 22.8

0.0 26.0 49 5.3 5.8 58.0

ALL LEVELS COMBI N ED

3.2 54.4 9.8 9.0 3.8 19.8

J.: Mental retardation in the Province of Manitoba: Towards establishing a data base for community planning. Can. Ment. Health, 30:16-19, 1982; with permission. ) (From Fishbach, M., and Hull,

respectively, of persons identified as borderline or mildly retarded were residing in institutions. But about 23 per cent of individuals with moderate retardation and 58 per cent of those with severe and profound retardation were institutionalized. LEVELS OF MENTAL RETARDATION

The misconception persists, in some quarters, that persons with mental retardation form a relatively homogeneous group of individuals with low intelligence . Yet, many prestigious bodies including the American Psychiatric Association (A. P.A.) and A. A. M. D . recognize there are different degrees of retardation. The A. P.A. 4 and A.A. M. D. 43 have developed similar systems of nomenclature based on the severity of symptoms using four levels of mental retardation: mild, moderate, severe, and profound. These four subtypes are defined primarily on the basis of specific IQ ranges and concurrent deficits in adaptive behavior. As shown in Table 7, most surveys have found that the more severe degrees of retardation represent a relatively small percentage of the total mentally retarded population. Research reviews 48 · 57 have suggested that the prevale nce of individuals with more severe levels of retardation is about 3 per 1000 and that they make up as little as 15 per cent of all people who are mentally retarded. People with these more severe intellectual deficits are most ofte n diagnosed during infancy or early childhood, the impairment is lifelong, and decrements in age-specific prevalence for this age group result from higher than average mortality rates . 138 Researchers generally acknowledge that the majority of persons with mental retardation fall within the mild range. Results from recent studies suggest that the proportion of persons with mild retardation appears to be decreasing. 72 Yet, it must be cautioned that there are at least two difficulties inherent in attempts to ascertain accurately the number of individuals with mild retardation. First, persons who are more severely retarded are easier . to enumerate because they tend to have more obvious physical disabilities, require intensive support services, and are more likely to live in specialized

J.

608

DALE MUNRO

Table 7. Percentage Distributions by Level of Mental Retardation from Several Studies LEVEL OF MENTAL RETARDATION INVESTIGATORS

AGE

(YEAR PUBLISH E D)

RANCES

Royal Commission (1908)112 Lewis (1929)

Matthews et al.

Borderline

Mild Moderate

Severe

Profound

all

76

all all

84 75

17

3 8

10-14 20+ all

92 67 73

6 29 25

2 4 2

Unspecified

24 13

(1937)

Lemkau et al. (1942)

Bremer (1951) New York State Department of Mental Hygiene (Onondaga County,

1- 17

80

0-20

84

20

1955)

Wishik (1956) Essen-Moller

16

all

63

24

13

all 5-20 15-19

70 87 47 44 85

25 9

4

(1956)

Akesson (1961) Levinson (1962) Kushlick (1963)

all ages Richardson & Higgins (1965) Kushlick (1966) Lemkau & Imre

0-20 15-19

71

all ages

48

1-59

88

5 53 56

5

10

29 52 12

(1966)79

Mercer A.B. + 70 IQ A.B. + 85 IQ <70 IQ >85 IQ Fishbach & Hull (1973)94

(1982)34 Baird & Sadovnick (1985)5

all 46

40 36

71 71 all

28

21 27

all

27

22

25 14

56 17 27

4 1

8

0

2 20

8

7

22

(Adapted and updated from Lapouse, R., and Weitzner, M.: Epidemiology. In Wortis, J. (ed.): Mental Retardation: An Annual Review I. New York, Gruner & Stratton , 1970, page 202; with permission. Studies not included in the original table have reference numbers noted beside investigators' names. )

institutional or community settings. In contrast, the majority of individuals with mild retardation have no additional physiologic problems and tend to live in impoverished community environments where it is difficult to track them. Second, it has been demonstrated that the people categorized as mildly retarded in school can continue to make IQ increments well into adulthood 18 · 131 · 145 and that within technologically advanced cultures mean IQs have improved significantly over those of even a few decades ago .35 • 140 These findings suggest that the IQ statistics, so frequently relied upon in epidemiologic surveys, have little predictive value concerning the future adjustment of persons with mild retardation. This may help to explain the improved prognosis for many formerly labeled persons after they leave school. Because mild retardation is found mostly among lower socioeconomic populations and more severe levels of retardation are distributed fairly

EPIDEMIOLOGY AND THE EXTENT OF MENTAL RETARDATION

609

evenly among all socioeconomic levels, it has been theorized 26 · 94 • 151 that there are two qualitatively distinct groups of persons with mental retardation. One group might be called the "functionally retarded," made up almost exclusively of people from lower socioeconomic backgrounds, with mild retardation and few physiologic problems. The etiology of the mental retardation is unknown in the majority of these cases. Another group might be referred to as the "organically retarded," characterized by more severe levels of retardation and frequently by obvious organic disorders . The cause of the mental retardation can be determined in most of these cases. Life Expectancy

Mortality rates among people with mental retardation continue to be a major concern. Unfortunately, almost all the research completed concerning this topic has been done with institutionalized persons, Nevertheless, it is obvious that the life expectancy of persons with mental retardation is correlated highly with their level of intellectual functioning. Studies 25 · 64 · 89 completed as early as the 1930s found a shorter life expectancy for people with IQs below 50 than for those of higher intellectual levels. A 197036 investigation reported that the mortality rate for individuals with mild retardation was 1. 7 times the rate for the general population at the same ages. For people with severe retardation the figure was 4. 1 times the normal rate. A 1983 study51 found that 98 per cent of persons with mild and moderate retardation reached age 20, whereas only 92 per cent of people in the severe range reached this age. For ages 1 to 19 years, this research showed the death rate among individuals with mild and moderate retardation was twice that of the general population, whereas the rates for persons with severe and profound retardation were 7 and 31 times as high, respectively. In general, modern research suggests that the increasing life expectancy found among the population at large also is being shared by people who are mentally retarded. However, although the life expectancy of persons with mild retardation is closer to normal, people who are more severely retarded and more prone to serious medical problems still have much shorter life expectancies. The youngest age groups with mental retardation continue to have the highest mortality rates, and only if they survive the first 5 years does life expectancy improve substantially. to

ASSOCIATED DISABILITIES Some individuals with mental retardation suffer from additional physical or emotional handicaps that further frustrate their educatiopal, social, and vocational adjustment. These associated handicaps range from the barely noticeable to the totally incapacitating. 21 The following discussion will consider some of the more prominent additional disabilities. PHYSICAL DISABILITIES

Vision Impairments

Most research has indicated that visual handicaps among individuals with mental retardation are clinically a heterogeneous and poorly measured

610

J.

DALE MUNRO

group of problems. 30 However, it is apparent that a far greater proportion of people with mental retardation have vision impairments than is found in the general population. Studies conducted among blind populations 24 · 149 have reported that between 21 and 25 per cent of blind children also are mentally retarded. In a study of both community and institutional settings, Levinson 81 reported that 2. 3 per cent of people with mental retardation have some form of vision defect and 0.8 per cent have severe vision impairment. Grunewald 45 found similar results indicating that 2. 7 per cent of Sweden's mentally retarded population were visually handicapped: 15 times the normal rate of prevalence. Other studies have suggested higher prevalence figures. A Canadian investigation 18 suggested about 8 per cent of all persons with mental retardation suffer vision impairments, whereas other Swedish studies 48 found 1 to 9 per cent of persons in the mild range and 10 to 15 per cent with more severe retardation had severe vision problems. Higher rates of vision impairment among persons who are more severely retarded also have been reported by others. 56 • 90 An extensive literature review by Ellis 30 concluded that among institutionalized populations, about 5 per cent of residents can be expected to be blind and 6 to 8 per cent partially sighted. Ellis cautioned that vision defects in terms of acuity or color blindness can be expected in many other institutionalized persons, although these problems are easily overlooked and seldom properly assessed. Hearing Loss Hearing impairments occur in abnormally high numbers among people who are mentally retarded. A 1963 report7 1 found hearing loss among children with mental retardation was three to four times higher than normal. Many of the hearing problems were found in persons over age 45. Other researchll 4 seems to confirm this trend toward a greater degree of hearing impairment among older persons with mental retardation. Lloyd 84 reviewed several studies of hearing impairment among institutionalized persons with mental retardation and found a prevalence rate of 10 to 15 per cent. 15 In an extensive 1975 survey of American public institutions, 7.2 per cent of residents had some degree of hearing loss and 2. 3 per cent were classified as deaf. In a 1979 study of noninstitutionalized persons with mental retardation, 114 14.6 per cent manifested some degree of hearing impairment. The prevalence of severe hearing impairments was greatest among persons who were profoundly retarded. As well, the prevalence of all levels of hearing impairment was higher for individuals who were severely and profoundly retarded than for those with mild and moderate retardation. Thus, it seems safe to say that hearing problems are found in a disproportionately large percentage of persons with mental retardation. In an excellent research summary, Stewart 135 suggests that for the entire population with mental retardation, an estimate of 10 per cent is probably closest to the true prevalence rate of hearing impairments. Yet, current figures may represent underestimates, .as Hogan 53 has reported that 80 per cent of persons with profound retardation and 51 per cent of persons with

EPIDEMIOLOGY AND THE EXTENT OF MENTAL RETARDATION

611

severe retardation were judged untestable during audiometric assessments. The increasing use of evoked response audiometry will likely result in assessing and collecting epidemiologic data on previously untestable persons . Speech and Language Problems Research has indicated that speech impairments are one of the most common handicaps found among persons with mental retardation, 137 and their prevalence appears to be a function of intelligence level. 66• 88 Most investigations regarding speech and language problems have been conducted in institutions. Prevalence estimates within institutionalized populations have ranged from 57 to 72 per cent in a 1963 study132 and from 18 to 94 per cent in a 1971 investigation. 90 A more recent study by Brindle and Dunster16 found some type of speech and language deficit in 96 per cent of the institutionalized residents surveyed. Significant language deficits were found in 81.5 per cent of the sample. Deviant or delayed articulation skills were observed in 44 per cent, voice deviations in 22.3 per cent, and stuttering in 4.4 per cent of the population. One third were not evaluated because they were nonverbal. The extremely high prevalence of communication disorders in Brindle and Dunster' s research probably reflects the lower intellectual functioning, multihandicapped nature of persons left behind in large public facilities owing to the deinstitutionalization of more able persons. In an investigation of noninstitutionalized persons with mental retardation, 114 51.2 per cent of the sample manifested some degree of speech impairment, indicating speech problems were three and one half times more common than hearing difficulties. This suggests, as might be expected, that noninstitutionalized persons with mental retardation show lower levels of speech difficulties than people in institutions. The study also found that the prevalence of moderate and severe speech impairment was greatest among persons functioning at the lowest intellectual levels. Epilepsy Only limited knowledge exists concerning the epidemiology of epilepsy among persons with mental retardation. 116 Part of the problem in measuring seizure activity is that it is sometimes difficult to differentiate epileptic seizures from nonepileptic behavior of nonverbal persons who are severely and profoundly retarded. Behaviors such as self-mutilation, episodic aggressiveness, staring, and exaggerated startle reactions can be confused easily with epileptic activity. 54 Research 116 has also shown that the mentally retarded population has more frequen t and severe seizures than the nonretarded population. In two independent studies of institutionalized persons with mental retardation,37 • 99 31 per cent of residents sampled had a history of se izures. Research involving noninstitutionalized persons with mental retardation generally has suggested lower rates of epilepsy, ranging from 1534 to 18 per cent. 142 One study123 found that 16.4 per cent of the children in the mentally retarded group had experienced seizures compared with only 1.4 per cent of the nonretarded control group.

J.

612

DALE MUNRO

Several investigations suggest that the proportion of persons with seizures definitely is related to the severity of the mental retardation. Pond 105 reported rates of epilepsy from 3 to 6 per cent for persons with mild and moderate retardation and 33 per cent for individuals with severe and profotind retardation. Swedish research 48 has found between 12 and 18 per cent of persons with mild retardation and about 36 per cent of persons who are more severely retarded experienced seizures. Similar trends related to intellectual functioning level have been shown in other studies. 37• 99 • m Cerebral Palsy In reviewing research evaluating the intelligence of children with cerebral palsy, Cardwell 19 reported that the proportion of children with mental retardation ranged between 30 and 59 per cent in various inquiries . Many of these children with cerebral palsy were also severely and profoundly retarded, as has been found in other studies. 48 Other more recent investigations seem to support Cardwell' s earlier figures. For instance, a 1967 study42 found 37 per cent of persons with cerebral palsy are also mentally retarded, whereas a 1968 report 143 indicated 66 per cent, a 1976 inquiry 23 suggested 55 per cent, and research completed in 198358 found 71 per ce'nt shared both diagnoses. It is interesting to note that the more recent studies of co-occurrence of cerebral palsy and mental retardation tend to be reporting higher rates. This likely can be attributed in large part to the use today of tighter criteria in diagnosing mental retardation. Only persons with more severe intellectual impairment are now labeled mentally retarded, and these are the individuals most prone to additional handicaps such as cerebral palsy. It has been estimated that if 45 per cent (an average from various studies) of persons with cerebral palsy are also mentally retarded, then perhaps about 2 per cent of the entire mentally retarded population suffer from this additional handicap. 21

EMOTIONAL DISORDERS

Dual Diagnosis: Mental Retardation and Mental Illness It has long been recognized that people with mental retardation ate susceptible to the full range of emotional and personality disturbances that occur in the general population. 7 • 29 • 39• 104 · 136 In fact, emotional-behavioral disturbances, after the degree of intellectual deficiency itself, have always constituted the single most important reason for institutionalizing persons with mental retardation. 9 Individuals with mental retardation are more susceptible to emotional problems than their nonretarded peers. 98 Reasons advanced for the higher rates of mental illness among individuals with mental retardation include (1) redticed capacity to withstimd the distress of society's excessive demands , constant disapproval, and exclusion from normal activities; (2) inadequate cognitive capacity to resolve emotional conflicts; (3) lack of judgmeqt leading to greater gullibility to being "led on" into trouble by others; (4) ·frequent

EPIDEMIOLOGY AND THE EXTENT OF MENTAL RETARDATION

613

sensory and central integrative disorders that hamper appropriate problemsolving; and (5) professional unwillingness to treat mental illness in persons with mental retardation. 92 • 104 Parsons et al. 98 recently have conducted a thorough review of research concerned with the frequency of the dual diagnosis of mental retardation and mental illness. They concluded that surveys rather consistently indicate that approximately 10 per cent of institutionalized adults with mental retardation demonstrate severe emotional disturbances such as psychoses. When more minor mental illnesses are included, estimates soar to as high as 60 per cent. Although it is usually assumed that estimates of mental illness among institutionalized persons with mental retardation grossly exaggerate the incidence of emotional disorders in the entire mentally retarded population, community studies do not support this assumption. Rather surprisingly, community inquiries suggest that 20 to 35 per cent of children with mental retardation suffer from some form of mental illness. Unfortunately, few studies have concentrated on adults in community settings. It is possible that some emotional disorders are greatly improved by adulthood. Psychoses Most authorities agree that persons with mental retardation are more susceptible to psychoses than are individuals of normal intelligence. Parsons et al. 98 reported that prevalence surveys suggest that people with mental retardation account for approximately 3 per cent of all cases of psychoses and that approximately 5 to 12 per cent of institutionalized adults with mental retardation demonstrate psychotic symptoms. Schizophrenia is the type of psychosis most frequently associated with mental retardation. 9 · 102 In one study of a community-based mentally retarded population, 21 per cent of the people referred for psychiatric consultation were diagnosed as schizophrenic. 117 The prevailing opinion is that verbal individuals with mildto-moderate retardation who are also schizophrenic show only slight variations from traditional clinical symptoms of schizophrenia. There is considerable disagreement over the nature of schizophrenic symptomatology in people who are more severely retarded. There is a common belief that unipolar depression is rare in persons with mental retardation. It is probable that most cases of depression in individuals with mental retardation go unnoticed because of the nondisruptive nature of depressive symptoms for family members or direct-care staff. 98 Frequently noted in the literature are manic-depressive psychoses in mentally retarded populations. 55 Paranoid psychoses also occasionally have been mentioned in the mental retardation literature, 110 as has catatonic schizophrenia. 29 Finding reliable prevalence data on the various subgroups of psychoses seems very difficult at this time. Neuroses Neuroses in persons with mental retardation have been less frequently reported than the psychoses. 98 For instance, one study29 found only one case of neurosis out of the 168 persons with mental retardation referred for psychiatric assessment. It has been noted that neuroses may be under-

614

J.

DALE MUNRO

reported in prevalence surveys because these disorders do not interfere appreciably with the social and vocational adjustment of individuals with mental retardation and generally are considered minor mental illnesses. Menolascino92 has found a higher frequency of neuroses in persons with mild and moderate retardation than in individuals with more severe retardation. The research review by Parsons et al. 98 suggested that the prevalence of neuroses in people with mental retardation is estimated as being between 4 and 6 per cent. This is a rate greater than that found in the general population . Conversion disorders are reported as one of the most common neuroses in individuals with mild and moderate retardation. Anxiety reactions, obsessive-compulsive states, and phobic conditions also have been reported regularly in studies of mentally retarded populations. Neurotic depression is reported infrequently in the literature, even though some evidence 109 suggests that its prevalence may be higher in individuals with mental retardation than in nonretarded persons. One study65 reported that 9.4 per cent of persons with mental retardation referred to a community psychiatric clinic were clinically depressed. Other recent community-based researchrn has reported a relatively high level of demand for psychotherapy to treat depression in adults with mental retardation. Personality Disorders DSM-1114 indicates that personality disorders are characterized by chronically maladaptive behavioral patterns (for example, antisocial, passiveaggressive, and dependent) that are qualitatively different from psychotic or neurotic disorde rs. Personality disorders rarely have been reported in psychiatric literature concerning persons with mental retardation, although the few studies attempting to study this topic have reported a relatively high rate. For instance, Eaton and Menolascino 29 found 27. l per cent of the psychiatric problems identified in their community-based mentally retarded population were diagnosed as personality disorders . Craft22 reported that nearly 25 per cent of the inpatients with mental retardation surveye d were diagnosed as personality disorders . Unfortunately, during the first three decades of this century, it was believed that virtually every person who was mentally retarded was a potential criminal. 122 Beier, 9 in reviewing the literature, found early estimates of the frequency of mental retardation in criminal offenders ranged from 0.5 to 55 per cent, with most early studies indicating figures in the higher percentage range. It has been suggested2 that a disproportionately high number of adults with mental retardation are found in correctional facilities. Research also reveals that persons with mental retardation demonstrate higher rates of crimes against the person, such as homicide, 1 and are implicated in a relatively high number of sex-related offenses. 41 However, caution must be shown when interpreting these findings , as it must be remembered that criminal offenders make up only a tiny fraction of the entire mentally retarded population. As well, it often is suggested that intellectual deficits make persons with mental retardation more prone to being caught and convicted and that many of the criminal offenders who

EPIDE MIOLOGY AN D THE E XT E NT O F M E NTAL RETAHDATION

615

are mentally retarded come from socioeconomically deprived environments that have always contributed a disproportionately high number of prison inmates. Knowledge concerning the prevalence of suicides or attempted suicides in persons with mental retardation is sketchy. The most extensive study on the issue conducted among an institutionalized population found a prevalence of attempted suicide of nine per thousand persons, a rate closely approximating the estimate for the general population. Although suicidal behavior is not regarded generally as a problem among mentally retarded populations, some reports 115 suggest that suicides do occur and suicide threats should be taken seriously. Very little is known regarding alcoholism and drug addiction in people with mental retardation. What evidence is available 116 suggests that these conditions are rare in mentally retarded populations. This may be partially due to the fact that alcohol and illicit drugs traditionally have not been available to many persons with mental retardation owing to institutionalization or family overprotection. 98 Another disorder, once commonly attributed to people with mental retardation, has been referred to as the "epileptic personality." One review of this concept 46 indicates that the prevailing viewpoint in the late nineteenth and early twentieth centuries was that persons with epilepsy manifested a disturbed personality pattern characterized as egocentric and seclusive with unpredictable outbursts of emotion. In recent decades, this notion of a specific epileptic personality disorder has been discredited. 141 Yet, higher rates of behavioral disorders have been associated with people with mental retardation who have convulsive disorders, including feigning seizures or actually inducing seizures to get attention or to avoid certain responsibilities. 98 Severe Disorders of Childhood Most professionals no longer believe that infantile autism is caused by emotionally cold parenting. CNS dysfunction is considered the probable etiologic factor. Although Kanner 63 in his pioneering work felt impaired intellectual functioning in children with autism was secondary to their inability to form basic relationships, there are now indications that autism can be identified at all intelligence levels. In fact, considerable literature 58 • 85 • 146 has suggested that approximately 70 per cent of persons with autism are also mentally retarded. Bhatara 11 reports that 30 per cent of persons with autism are mildly retarded, and 40 per cent are moderately to profoundly retarded. Within the field of autism, an interesting development is the growing recognition that children with autism grow up to become adults with autism requiring specialized treatment and services. 128 There is more and more pressure to diagnose autism in adults with mental retardation and provide them with services that appropriately meet their needs. A preliminary study by Munro and Duncan96 recently suggested that almost 10 per cent of one institutionalized mentally retarded population could be diagnosed as autistic. The term childhood schizophrenia is sometimes used interchangeably

J.

616

DALE MUNRO

with infantile autism . Yet, among professionals who differentiate between these conditions, a key consideration is the severe amount of personality regression and thought disorder noted in childhood schizophrenia. Although some individuals with mental retardation have been diagnosed as childhood schizophrenics, accurate prevalence figures have never been determined. Idiot savantism, a condition in which persons who are otherwise mentally retarded demonstrate phenomenal skills in a particular area (such as music), is periodically reported in the literature. 95 However, this is considered a rare syndrome.

MULTIPLE HANDICAPS

Any person with mental retardation suffering from an additional physical or emotional disorder is, by definition, multihandicapped. Quite frequently, individuals with mental retardation suffer from two or more additional disabilities. 21 Stewart 135 observed that such combinations of disabilities are not additive, but multiplicative, and their effect, devastating. Failure to understand clearly the nature of multiple handicaps in persons with mental retardation can hinder treatment and lead to confusion when trying to interpret data from epidemiologic research. One study, noteworthy for the fact it tried to grapple with this issue, is that of Jacobson and Janicki. 58 They carried out a comprehensive investigation into the cooccurrence of mental retardation, autism, cerebral palsy, and epilepsy within the large developmentally disabled population of New York State. Table 8 shows the observed frequencies of each disability condition and combination of conditions reported in their research. Jacobson and Janicki reported that the average number of conditions reported per case was 1.32. Seventy-three per cent of cases had one reported condition, 23 per cent had two conditions, 4 per cent had three conditions, and less than 1 per cent had all four conditions. Persons with mental retardation accounted for 68 per cent of instances of autism, 71 per cent of cerebral palsy, and a surprisingly high 91 per cent of epilepsy. Other studies have indicated an overlap of additional disabilities with mental retardation. For example, a British study 115 concluded that within their sample of children with mental retardation, 22.5 per cent had multiple physical impairments. Wolf1 49 found that two thirds of the students attending special classes for the mentally retarded had three or more additional emotional or physical handicaps . Reports on the prevalence of persons who are blind-deaf-retarded range from 1.6 per cent of residents in American institutions 126 to 2 per cent of the entire mentally retarded population of Manitoba. 34 Other research has linked the prevalence of multiple handicaps with lower levels of intellectual functioning. Multiple disabilities repeatedly have been shown to occur frequently among persons with severe and profound retardation and much less often among individuals with mild and moderate retardation. 5 • 28 • 34• 51 • 73 Most people with multiple disabilities live the greater part of their lives in institutional settings. In summary, overlap among various disabling conditions must be considered when estimating the prevalence of any disabled population. 58

617

EPIDEMIOLOGY ANO THE EXTENT OF MENTAL RETARDATION

Table 8. Observed Frequencies and Percentages of Developmental Disability Conditions and Combinations of Conditions by Age Range NUMBER AND PERCENT OF CASES

Age Birth-21 COMBINATIONS OF CONDITIONS

Specific combinations of conditions Autism (AUT) only Cerebral Palsy (CP) only Epilepsy (EP) only Mental Retardation (MR) only AUT & CP AUT & EP AUT & MR CP & EP CP&MR EP &MR AUT, CP, & EP AUT, CP, & MR AUT, EP, & MR CP, EP, & MR AUT, CP, EP, & MR

N

Age 22+

N

%*

%

All Cases

N

%

All Cases

.024 36 .001 384 .009 .048 383 .013 985 .023 .021 384 .013 691 .016 .590 21305 .731 29879 .684 .000 0 .000 5 .000 .001 5 .000 21 .000 .037 174 .006 706 .016 .006 55 .002 138 .003 .075 1531 .053 2616 .060 .128 4609 .158 6506 .149 .000 0 .000 0 .000 .001 5 .000 18 .000 .006 35 .001 116 .003 .067 630 .022 1609 .037 .001 8 .000 18 .000 14532 29160 43692

Total instances of each condition Cases with AUT Cases with CP Cases with EP Cases with MR

1005 3400 3373 13171

Total Conditions Conditions/Case

20949 1.44

348 602 307 8574 5 16 532 83 1085 1897 0 13 81 979 10

.069 263 .233 2612 .232 5726 .906 28297

.009 1268 .090 6012 .196 9099 .969 41484 36898 57863 1.29 1.32

.029 .138 .208 .949

ESTIMATED CASES IN 100,000 GENERAL POPULATION

9 24 17 720 0 1 17 3 63 157 0 0 3 39 0 1053 31 145 219 1000 1395

*Column percent of cases for specific combination reflects joint probability for occurrence within the survey population. (From Jacobson, J. W., and Janicki, M. P.: Observed prevalence of multiple developmental disabilities . Ment. Retard., 21 :87-94, 1983; with permission.)

When multiple handicaps within the same target population are not considered, estimates of the number of individuals with disabilities become artificially inflated.

SUMMARY Drawing firm conclusions from various findings of epidemiologic research in the mental retardation field is fraught with seemingly endless frustration and confusion. There have been so many studies in different countries on various sized populations with dissimilar methodologies, and con~epts and conditions often have been defined differently. Results from various surveys too often lack congruity, and sometimes findings are not reported until several years after the data are collected. However, these criticisms of epidemiologic studies are not meant to suggest that they are worthless or that investigators have been incompetent.

J.

618

DALE MUNRO

Each was attempting to examine different features of particular populations under diverse circumstances, with varying resources and measurement tools , and with distinct goals in mind. It is safe to say that attempting to compare or generalize information collected in one geographic location with another can be very difficult and often unwise. 49 If one intends to plan rationally the services for a particular geographic area, then one must use data that are derived from the population in that area. 34 Finally, results from epidemiologic surveys have been useful in educating professionals and the general public about the extent of problems presented and experienced by persons with mental retardation. As well, this information has proved essential for properly planning and prioritizing the need for services of a diagnostic, treatment, prevention, and research nature . To improve epidemiologic research in the future, it is hoped that coordinated, multipurpose, nationwide, and international data reporting systems for the collection of uniform mental health-mental retardation data can be developed. 121

REFERENCES 1. Allen, R. C.: Law and the mentally retarded. In Menolascino, F. J. (ed. ): Psychiatric Approaches to Mental Retardation. New York, Basic Books, 1970, pages 585-611. 2. Allen, R. C.: The re tarded offender: Unrecognized in court and untreated in prison. Fed. Prob., 32:22-27, 1968. 3. Alley, G. R. , and Snider, B.: Comparative perceptual motor performance of Negro and white young mental retardates. Dev. Psycho!., 2:100-114, 1970. 4. American Psychiatric Association: DSM-III Diagnostic and Statistical Manual of Mental Disorders. Ed. 3. Washington, D.C., A.P.A., 1980. 5. Baird, P. A., and Sadovnick, A. D.: Mental retardation in over half-a-million consecutive livebirths: An epidemiological study. Am. J. Ment Delle., 89:323-330, 1985. 6. Baldwin, B. T., Fillmore, E. A., and Hadley, L.: Farm Children. New York, AppletonCentury-Crofts , 1930. 7. Balthazar, E. E., and Stevens, H. A.: The Emotionally Disturbed Mentally Retarded: A Historical and Contemporary Perspective. Englewood Cliffs, New Jersey, PrenticeHall, 1975. 8. Bayley, N.: Comparisons of mental and motor test scores for ages 1-15 months by sex, birth order, race, geographic location, and education of parents. Child Dev., 36:379411, 1965. 9. Beier, D. C.: Behavioral disturbances in the mentally retarded. In Stevens, H. A., and Huber, R. (eds.): Mental Retardation. Chicago, University of Chicago Press, 1964. 10. Balakrishnan, T. R., and Wolf, L. C.: Life expectancy of mentally retarded persons in Canadian institutions. Am. J. Ment. Delle., 80:650-662, 1976. 11. Bhatara, V.: Overlap of autism, mental retardation, and CNS dysfunction. Psychiatr. Ann., 10:31-40, 1980. 12. Birch, H. G., Richardson, S. A., Baird, D ., et al.: Mental Subnormality in the Community. A Clinical and Epidemiologic Study. Baltimore, Williams & Wilkins, 1970. 13. Book, J. A.: A genetic and neuropsychiatric investigation of a north Swedish population with special regard to schizophrenia and mental deficiency. Part I and II. Acta Genet., 4:1-100, 345-414, 1953. 14. Braddock, D., and Heller, T.: The closure of mental retardation institutions. I: Trends in the United States. Ment. Retard. (U.S.A.), 23:168-176, 1985. 15. Brannan, A. C., Sigelman, C. , and Bensberg, G.: The Hearing Impaired Mentally Retarded: A Survey of State Institutions for the Retarded. Lubbock, Texas Tech University, 1975.

EPIDEMIOLOGY AND THE EXTENT OF MENTAL RETARDATION

619

16. Brindle, B. R., and Dunster, J. R.: Prevalence of communication disorders in an institutionalized mentally retarded population. Hum. Commun. (Can.) 8:72-80, 1984. 17. Broadhead, G. D.: Socioeconomic traits in mildly retarded children of differential diagnosis. Rehab. Lit., 34:104-107, 1973. 18. Clarke, A. D. B., Clarke, A. M., and Reiman, S.: Cognitive and social changes in the feebleminded: Three further studies. Br. J. Psycho!., 49:144- 157, 1958. 19. Cardwell, V.: Cerebral Palsy. New York, Association for the Aid of Crippled Children, 1956. 20. Carlssen, G.: The regional distribution of intelligence. Stat. Rev., 3:313-323, 1954. 21. Conley, R. W.: The Epidemiology of Mental Retardation. In The Economics of Mental Retardation. Baltimore, John Hopkins University Press, 1973, pages 6-49. 22. Craft, M.: Mental disorder in the defective: A psychiatric survey among inpatients. Am. J. Ment. Delle., 63:829-834, 1959. 23. Cruickshank, W. M., Hallahan, D. P., and Bice, H. V.: The evaluation of intelligence. In Cruickshank, W . M. (ed.): Cerebral Palsy: A Developmental Disability. Ed. 3. Syracuse, New York, Syracuse University Press, 1976. 24. Cruickshank, W. M., and Trippe, M. J.: Services to Blind Children in New York State. Syracuse, New York, Syracuse University Press, 1959. 25. Dayton, A., et al. : Mortality and expectation of life in mental deficiency in Massachusetts: Analysis of the fourteen-year period 1917-1930. N. Engl. J. Med., 202:120-132, 1932. 26. Dingman, H. F., and Tarjan, G.: Mental retardation and the normal distribution curve. Am. J. Ment. Defic., 64:991-994, 1960. 27. Doll, E. A.: Community control of mental deficiency in the United States. J. Ment. Sci., 79:578-589, 1933. 28. Drillien, C . M., Jameson, S., and Wilkinson, E. M.: Studies in mental handicap. Part I: Prevalence and distribution by clinical type and severity of defect. Arch. Dis. Child., 41:528-538, 1966. 29. Eaton, L. F., and Menolascino, F. J.: Psychiatric disorders in the mentally retarded: Types, problems, and challenges. Am. J. Psychiatry, 139:1297-1303, 1982. 30. Ellis, D.: Visual handicaps of mentally handicapped people. Am. J. Ment. Defic., 83:497- 511, 1979. 31. Epple, W. A., Jacobson, J. W., and Janicki, M. P.: Staffing ratios in public institutions for persons with mental retardation in the United States. Ment. Retard. (U.S.A. ), 23:115-124, 1985. 32. Facts and Figures on the Mentally Retarded and Their Living Conditions in Sweden. Stockholm, Socialstyreison Redovisa, 1979, page 5. 33. Farber, B.: Mental Retardation: Its Social Context and Social Consequences. Boston, Houghton-Mifflin, 1968. 34. Fishbach, M., and Hull, J. T.: Mental retardation in the province of Manitoba: Towards establishing a data base for community planning. Can. Ment. Health, 30:16-19, 1982. 35. Flynn, J. R.: Lynn, the Japanese, and environmentalism. Bull. Br. Psycho!. Soc., 35:409413, 1982. 36. Forss man, H., and Akesson, H. 0.: Mortality of the mentally deficient: A study of 12,903 institutionalized subjects. J. Ment. Defic. Res., 14:276-296, 1970. 37. Freeman, J. M., Holder, K. R., and Schoenfeld, E.: Comprehensive Care of Institutionalized Persons with Epilepsy. Paper presented at the meeting of the Epilepsy International Symposium. Vancouver, September 14, 1978. 38. Fremming, K. H.: The Expectation of Mental Infirmary in a Sample of the Danish Population. London, Eugenics Society, 1951. 39. Garfield, S. L.: Abnormal Behavior in Mental Deficiency. In Ellis, N. R. (ed. ): Handbook of Mental Deficiency: Psychological Theory and Research. New York, McGraw-Hill Book Co., 1963. 40. Goodman, N., and Tizard, J.: Prevalence of imbecility and idiocy among children. Br. Med. J., 1:216-219, 1962. 41. Greenland, C . : Mentally retarded sex offenders. In Zarfas, D. E ., and Goldberg, B. (eds.): Clarence M. Hincks Memorial Lectures 1978. London, Ontario, Department of Psychiatry, University of Western Ontario, 1980, pages 206-213. 42. Griffiths, M. I., and Bassett, N. M.: Cerebral Palsy in Birmingham. Dev. Med. Child Neurol. , 9:713- 731, 1967.

620

J.

DALE MUNRO

43. Grossman, J. H.: Manual on Terminology and Classification in Mental Retardation (1977 Revision). Washington, D.C ., A.A.M.D., 1977. · 44. Gruenberg, E. M.: Epidemiology. In Stevens, H. A., a~d Heber, R. (eds. ): Mental Retardation: A Review of Research. Chicago, University of Chicago Press, 1964, pages 259-306. 45. Grunewald, K.: Blind, deaf and physically handicapped mentally retarded. An epideiniological study as a base for action. Proceedings of the 3rd Congress of the International Association for the Scientific Study of Mental Deficiency, 1 :349~352, 1975. 46. Guerrant, J., Anderson, W. W., Fischer, A., et al.: Personality in Epilepsy. Springfield, Illinois, Charles C Thomas, 1962. 47. Hagberg, B., Hagberg, G., Lewerth, A., et al.: Mild mental retardation in Swedish school children. I: Prevalence. Acta Paediatr. Scand., 70:441-444, 1981. 48. Hagberg, B., and Kyllerman, M.: Epidemiology of mental retardation: A Swedish survey. Brain Dev., 5:441-449, 1983. 49. Heber, R.: Epidemiology of Mental Retardation. Springlleld, Illinois, Charles C Thomas, 1970. 50. Henderson, S., Pate, G., Wegman, S., et ai.: The preva,lence of mental retardation in Tasmania. Med. J. Aust., 1:1291-1296, 1972. 51. Herbst, D. S., and Baird, P. A.: Nonspecific mental retardation in British Columbia as ascertained through a registry. Am. J. Ment. Defic., 87:506-513, 1983. 52. Herbst, D. S., and Miller, J. R.: Nonspecific X-linked mental retardation. II. The frequency in British Columbia. Am. J. Med. Genet., 7:461-469, 1980. 53. Hogan, D. D.: Errors in computation of hearing loss in studies of large populations. Ment. Retard. (U. S.A. ), 11:15-17, 1973. 54. Holmes, G. L., McKeever, M., and Russman, B. S.: Abnormal behavior or epilepsy? Use of long-term EEG and video monitoring with severely to profoundly mentally retarded patients with seizures. Am. J. Ment. Defic., 87:456-458, 1983. 55. Hucker, S. J.: Pubertal manic-depressive psychosis and mental subnormality: A case report. Br. J. Ment. Sub., 21:34-37, 1975. 56. Iivanainen, N. B.: A Study on the Origins of Mental Retardation Clinics in Developmental Medicine (No. 51). London, Spastics' International Medical Publishers & Heinemann Medical, 1974. 57. Ingalls, R. P. : Mental Retardation: The Changing Outlook. New York, John Wiley & Sons, 1978. 58. Jacobson , J. W ., and Janicki, M. P.: Observed prevalence of multiple developmental disabilities. Ment. Retard. (U .S.A. ), 21:87-94, 1983. 59. Jastak, J. F., MacPhee, H. M., and Whiteman, M.: Mental Retardation: Its Nature and Incidence. Newark, Delaware, University of Delaware Press, 1963. 60. Jensen , A. R.: How much can we boost IQ and scholastic achievement? Harvard Ed. Rev., 39:1-123, 1969. 61. Jones , L. A.: Census-based prevalence estimates for mental retardation. Ment. Retard. (U .S.A. ), 17:199-201, 1979. 62. Kanner, L.: A History of the Care and Study of the Mentally Retarded. Springfield, Illinois, Charles C Thomas, 1964, 63. Kanner, L.: Autistic disturbances of affective contact. Nerv. Child, 2:217-250, 1943. 64. Kaplan, 0.: Life expectancy oflow grade mental defectives. Psycho!. Bull., 36:513, 1939. 65. Kazdin, A. E., Matson, J. L., and Senatore, V.: Assessment of depression in mentally retarded adults. Am. J. Psychiatry, 140:1040-1043, 1983. 66. Keane , V. E.: The incidence of speech and language problems in the mentally retarded. Ment. Retard. (U.S.A. ), 10:3-5, Hi72. . 67. Kennedy, W. A., Van Do Riet, V., and White, J. C.: A Normative Sample oflntelligence and Achievement of Negro Elementary School Children in the Southeastern United States. Monographs of the Society for Research in Child Development. Yellow Springs, Ohio, The Antioch P~ess , 1963. 68. King, W. L., and Seegmiller, B.: Performance of 14- to 22-month-old black, firstborn male infants on twci tests of cognitive development. Dev. Psycho!. 8:317-326, 1973. 69. Kirk, D.: A community prevalence study of mental retardation in New York State. J. Spec. Educ., 12:83-88, 1978. 70. Klineberg, 0.: Social Psychology. New York, Henry Holt, 1940.

EPIDEMIOLOGY AND THE EXTENT OF MENTAL RETARDATION

621

71. Kodman, F.: Sensory processes and mental deficiency. In Ellis, N. R. (ed.): Handbook of Mental Deficiency, New York, McGraw-Hill Book Co., 1963. 72. Kushlick, A., and Blunden, R.: The epidemiology of mental subnormality. In Clarke, A. M., and Clarke, A. D. B. (eds.): Mental Deficiency: The Changing Outlook. Ed . 3. London, Methuen & Company, 1974, pages 31-81. 73. Kushlick, A., and Cox, G. R.: The epidemiology of mental handicap. Dev. Med. Child Neurol., 15:748-759, 1973. 74. Kushlick, A. : Subnormality in Salford. In Susser, M. W., and Kushlick, A.: A Report on the Mental Health Services of the City of Salford for the Year 1960. Salford, England, Salford Health Department, 1961. 75. Labrisseau, A., Jean, P., Messier, B., and Richer, C.: Fragile X chromosome and Xlinked mental retardation. Can. Med. Assoc. J., 127:123-126, 1982. 76 . Lakin, K. C., Bruininks, R. H., Doth, D., et al.: Sourcebook on Long-term Care for Developmentally Disabled People. Minneapolis, University of Minnesota, Department of Educational Psychology, 1982. 77. Lakin, K. C. : Demographic Studies of Residential Facilities for the Mentally Retarded: An H istorical Review of Methodologies and Findings. Minneapolis, University of Minnesota, Department of Psychoeducational Studies, 1979. 78. Lapousse, R., and Weitzner, M. : Epidemiology. In Wortis, J. (ed.): Mental Retardation: An Annual Review I. New York, Grune & Stratton, 1970, pages 197-223. 79. Lemkau, P. V., and Imre, P .: Preliminary Results of a Field Epidemiological Study. A paper presented at the Scientific Symposium for the Dedication ofJoseph P. Kennedy, Jr., Memorial Laboratories, March 15, 1966. 80. Lemkau, P., Tietze, C., and Cooper, M .: Mental-hygiene problems in an urban district. Ment. Hyg., 26:275-288, 1942. 81. Levinson, E. J.: Retarded Children in Maine. Orono, Maine, University of Maine Press, 1962. 82. Lewis, E. 0 .: Report on an Investigation into the Incidence of Mental Deficiency in Six Areas, 1925-1927. Part IV of Report of the Mental Deficiency Committee, being a Joint Com mittee of the Board of Education and Board of Control. London, His Majesty's Stationery Office, 1929. 83. Lin, T.: A study of the incidence of mental disorders in Chinese and other cultures. Psychiatry, 16:313- 336, 1953. 84. Lloyd, L. L. : Audiological aspects of mental retardation. In Ellis, N. R. (ed.): International Review of Research in Mental Retardation. Vol. 4. New York, Academic Press, 1970. 85. Lotter, V.: Epidemiology of autistic conditions in young children : I. Prevalence. Soc. Psychiatry, 1:124-137, 1966. 86. Luckey, R. E., and Neman, R.: Practices in estimating mental retardation prevalence. Ment. Retard. (U.S.A.), 14:16-18, 1976. 87. Lundy, L. A.: H ow Many Developmentally Handicapped Citizens Does Ontario Have? A paper presented at the Annual Conference of the Ontario Association for the Mentally Retarded, Ottawa, April 28, 1983, page 9. 88. Martyn, M. M., Sheehan, J., and Slutz, K.: Incidence of stuttering and other speech disorders among the retarded. Am. J. Ment. Defic., 74:206-211, 1969. 89. Martz, E. W.: Mortality among the mentally deficient during a twenty-five year period. Training School Bull., February: 189-193, 1934. 90 . Matthews, J.: Communication disorders in the mentally retarded. In Travis, L. E. (ed.): Handbook of Speech Pathology and Audiology. Englewood Cliffs, New Jersey, Prentice-Hall, 1971. 91. McNemar, Q .: The Revision of the Stanford-Binet Scale. Boston, Houghton-Mifflin, 1942. 92. Menolascino, F.: Challenges in Mental Retardation: Progressive Ideology and Services. New York, Human Sciences Press, 1977. 93. Mercer, J. R.: Labeling the Mentally Retarded. Berkeley, University of California Press, 1973. 94. Mercer, J. R.: The myth of 3 per cent prevalence. In Tarjan, G., Eyman, R. K., and Meyers, C. E. (eds .): Sociobehavioral Studies in Mental Retardation. Washington, D.C., Monographs of A.A.M.D., 1973, pages 1-18.

J.

622

DALE MUNRO

95. Morishima, A., and Brown, L. F.: A case report on the artistic talent of an autistic idiot savant. Ment. Retard. (U.S.A.), 15:33-36, 1977. 96. Munro, J. D., and Duncan, H. G.: Preliminary Identification of Adult Autistic Persons

97.

98.

99.

100. 101.

102. 103. 104.

in a Mental Retardation Facility. Presented at the Annual Conference of the Ontario Chapter of A. A. M. D., March 28, 1985, Toronto, Canada. New York State Department of Mental Hygiene, Mental Health Research Unit. A Special Census of Suspected Referred Mental Retardation, Onondaga County, New York. Technical Report of the Mental Health Research Unit. Syracuse, New York, Syracuse University Press, 1955. Parsons, J. A., May, J. G., and Menolascino, F. J.: The nature and incidence of mental illness in mentally retarded individuals. In Menolascino, F. J., and Stark, J. A. (eds. ): Handbook of Mental Illness in the Mentally Retarded. New York, Plenum Press, 1984, pages 3-43. Payne, D., Johnson , R. C., and Abelson, J.: A Comprehensive Description of Institutionalized Retardates in the Western United States. Boulder, Colorado, Western Interstate Commission for Higher Education, 1969. Peckham, C.: Mentally Handicapped Children. National Children's Bureau Tenth Annual Review. London, National Children's Bureau, 1974. Penrose, L. S.: A Clinical and Genetic Study of 1280 Case of Mental Defect. Special Report Series, Medical Research Council, No. 229. London, His Majesty's Stationery Office, 1938. Penrose, L. S.: Biology and Mental Defect (Revised Edition). New York, Grune & Stratton, 1962. Penrose, L. S.: The Biology of Mental Defect. London, Sidgwick, & Jackson, 1949. Pollock, H. M.: Mental Disease among Mental Defectives. Am. J. Psychiatry, 101:361-

363, 1944. 105. Pond, D.: Epilepsy and Mental Retardation. In Craft, M. (ed. ): Tredgold's Mental Retardation. Ed. 12 London, Bailliere Tindall, 1979. 106. President's Committee on Mental Retardation and Bureau of Education of the Handi-

capped: The six-hour retarded child. Report on a Conference of Problems of Education of Children in the Inner City, August 10-12, 1969. Washington, D .C., 1969. 107. President's Panel on Mental Retardation: A Proposed Program for National Action to Combat Mental Retardation. Washington, D .C., U.S. Government Printing Office, 1962. 108. Price, J.: A note on mental retardation in the Cook Islands. Aust. J. Dev. Dis., 6:3538, 1980. 109. Prout, H. T., and Schaefer, B. M.: Self-reports of depression by community-based mildly mentally retarded adults. Am. J. Ment. Defic., 90:220-222, 1985.

110. Reid, A. H.: Psychoses in adult mental defectives: II. Schizophrenia and paranoid psychoses. Br. J. Psychiatry, 120:213-218, 1972. 111. Reiss, S., and Benson, B. A.: Psychosocial correlates of depression in mentally retarded adults: I. Minimal social support and stigmatization. Am. J. Ment. Defic., 89:331337, 1985.

112. Report of the Royal Commission on the Care and Control of the Feeble-minded. London, His Majesty's Stationery Office, 1908. 113. Reynolds, A. R.: The prevalence of known mental retardation in Queensland. Aust. J. Ment. Retard., 4:69-73, 1976. 114. Reynolds, W. M., and Reynolds, S.: Prevalence of speech and hearing impairment of noninstitutionalized mentally retarded adults. Am. J. Ment. Defic., 84:62-66, 1979. 115. Richardson, S. A., Katz, M., Koller, H., et al. : Some characteristics of a population of mentally retarded young adults in a British city. A basis for estimating some service needs. J. Ment. Defic. Res. , 23:275- 285, 1979. 116. Richardson, S. A., Koller, H., Katz, M., et al.: A functional classification of seizures and its distribution in a mentally retarded population. Am. J. Ment. Defic., 85:457-466, 1981.

117. Richardson, S. A., Koller, H ., Katz, M., et al.: Career paths through mental retardation services: An epidemiological perspective. Appl. Res. Ment. Retard., 5:53-67, 1984. 118. Richardson, W. P., and Higgins, A. C .: The Handicapped Children of Alamance County, North Carolina. Wilmington, Delaware, Nemours Foundation, 1965.

EPIDEMIOLOGY AND THE EXTENT OF MENTAL RETARDATION

623

119. Robinson, N. M., and Robinson, H. B.: The Mentally Retarded Child. Ed. 2. New York, McGraw-Hill Book Co., 1976. 120. Rotegard, L. L., Bruininks, R. H., and Krantz, G. C.: State operated residential facilities for people with mental retardation July 1, 1978-June 30, 1982. Ment. Retard. (U.S.A.), 22:69-74, 1984. 121. Rowitz, L.: Guidelines for the development of a uniform mental health data reporting system. J. Ment. Health Admin. , 9:8-10, 1982. 122. Ruedrich, S., and Menolascino, F. J.: Dual diagnosis of mental retardation and mental illness: An overview. In Menolascino, F. J., and Stark, J. A. (eds.): Handbook of Mental Illness in the Mentally Retarded. New York, Plenum Press, 1984, pages 4581. 123. Rutter, M., Tizard, J., and Whitmore, K.: Education, Health and Behaviour. London, Longman, 1970. 124. Scally, B. G., and MacKay, D. N. : Mental subnonnality and its prevalence in Northern Ireland. Acta Psychiatr. Scand., 40:203-211, 1964. 125. Scarbrough, P. R., Cosper, P., Finley, S. C., et al.: Fragile-X syndrome: An overview. Ala. J. Med. Sci., 21 :68-72, 1984. 126. Scheerenberger, R. : Public Residential Services for the Mentally Retarded. Madison, Wisconsin, National Association of Superintendents of Public Residential Facilities fo r the Mentally Retarded, 1976. 127. Scheerenberger, R. C.: Mental retardation: Definition, classification and prevalence. Ment. Retard. Abstr., 1:432-441, 1964. 128. Schopler, E., and Mesibov, G. B.: Autism in Adolescents and Adults. New York, Plenum Press, 1983. 129. Scottish Council for Research in Education: Educational and Other Aspects of the 1947 Scottish Mental Survey. London, University of London Press, 1958. 130. Scottish Council for Research in Education : Social Implications of the 1947 Scottish Mental Survey. London, University of London Press, 1953. 131. Scottish Council for Research in Education: The Intelligence of Scottish Children: A National Survey of an Age Group. London, University of London Press, 1933. 132. Spradlin, J.E.: Language and communication of mental defectives. In Ellis, N. R. (ed. ): Handbook of Mental Deficiency: Psychological Theory and Research. New York, McGraw-Hill Book Co., 1963. 133. Stein, Z. A., and Susser, M. W .: The social distribution of mental retardation. Am. J. Ment. Defic. , 67:811-821, 1963. 134. Stevens, H . A.: Overview. In Stevens, H. A., and Heber, R. (eds.): Mental Retardation: A Review of Research. Chicago, University of Chicago Press, 1964. 135. Stewart, L. G.: Hearing impaired/developmentally disabled persons in the United States: Definitions, causes, effects, and prevalence estimates. Am. Ann. Deaf, 123:488-495, 1978. 136. Szymanski, L. S., and Tanguay, P. E. (eds.): Emotional Disorders of Mentally Retarded Persons. Baltimore, University Park Press, 1980. 137. Tarjan, G., Dingman, H. F ., and Miller, C. R.: Statistical expectations of select handicaps in the mentally retarded. Am. J. Ment. Defic., 65:335-341, 1960. 138. Tarjan, G., Wright, S. W ., Eyman, R. K., et al.: Natural history of mental retardation: Some aspects of epidemiology. Am. J. Ment. Defic., 77:369- 379, 1973. 139. Taylor, J. L., et al.: Mental Retardation Prevalence in Oregon, A Survey by the Oregon State Board of Health. Portland, Oregon State Board of Health, 1965. 140. Terman, L. M., and Merrill, M. A.: Stanford-Binet Intelligence Scale: 1972 Norms Edition. Boston, Houghton-Mifflin, 1973. 141. Tizard, B.: The personality of epileptics: A discussion of the evidence. Psycho!. Bull. , 59:196- 210, 1962. 142. Tizard, J., and Grad, J. C.: The Mentally Handicapped and Their Families. Maudsley Monograph No. 7. London, Oxford University Press, 1961. 143. U.C.P.A.: Annual Report of 1968. New York, United Cerebral Palsy Association, 1968. 144. U.S. Department of Health, Education, and Welfare, Health Services and Mental Health Administration: Intellectual Development of Children, Series 11, December 1971, page 41. 145. Wheeler, L.: A comparative study of the intelligence of east Tennessee mountain children. J. Educ. Psycho!. , 3:22, 1942.

624

J.

DALE MUNRO

146. Wing, L.: Children Apart. Albany, National Society for Autistic Children, 1974. 147. Wing, L.: Seve rely retarded children in a London area: Prevalence and provision of services. Psycho!. Med., 1 :405, 1971. 148. Wishik, S. M.: Handicapped children in Georgia: A study of prevalence, disability, needs, and resources. Am. J. Public Health, 55:195- 203, 1956. 149. Wolf, J. M.: The Blind Child with Concomitant Disabilities. New York, American Foundation for the Blind, 1967. 150. Zigle r, E., and De Labry, J.: Concept-switching in middle-class, lower-class, and retarded children. J. Abnorm. Soc. Psycho!. , 65:267-272, 1962. 151. Zigler, E.: Familial mental retardation: A continuing dilemma. Science, 155:292-298, 1967. Oxford Regional Centre Box 310 Woodstock, Ontario, Canada N4S 7X9