Birth order and health: Major issues

0277-9536192 $5.00+ 0.00 Copyright 0 1992Pergamon Press Ltd

Sot. Sri. Med. Vol. 35, No. 4, PP. 443452, 1992 Printed in Great Britain. All rights reserved

SECTION G BIRTH

ORDER

AND HEALTH:

MAJOR

ISSUES

BARBARAA. ELLIOTT Department of Behavioral Science, University of Minnesota-Duluth, Duluth, MN 55812, U.S.A. and Director of Behavioral Science and Research, Duluth Family Practice Residency Program, Duluth, Minnesota, U.S.A. Abstract-Birth Order has been described as a variable with a complex relationship to child and adult outcomes. A review of the medical literature over the past 5 years identified 20 studies that iuvestigated the relationship between Birth Order and a health outcome. Only one of the studies established a relationship between Birth Order and a health outcome: third and fourth-born children have a higher incidence of accidents that result in hospitalization. The other demonstrated relationships are each explained by intervening variables or methodological limitations. Although Birth Order is not a strongly independent explanatory factor in understanding health outcomes, it is an important marker variable. Statistically significant relationships between Birth Order and health outcomes yield insights into the ways a family influences an individual’s health. Key words-birth

order, family status and health, inter-family relationships

INTRODUCTION Birth Order has long been investigated as a variable in trying to understand child and adult outcomes. It has been evaluated in the disciplines of psychology, economics and sociology for its relationships to intelligence, education and occupational achievement, personality, psychopathology and other indicators of health and development. This paper provides a comprehensive review of recent studies in the medical literature which have examined the relationship between the concepts of Birth Order and health. Implications for further research are included. Theoretical background

In psychology, the past decade has brought changes in the theory about the influence of genetics and the environment on human behavior. The evidence is based on twin and adoption studies which use naturally-occurring experimental designs. These studies document that three major categories of variables are involved in the development and explanation of human phenotypes: genetic, shared experiences, and non-shared experiences. The research has shown that the majority of the measured variance in intelligence, personality and psychopathology can be explained by non-shared experiences [ 1,2]. In non-identical twins, approx 25% of the variance in behaviors can be explained genetically; this compares to about 50% of the variance in the behaviors of identical twins. Thus, psychologists conclude that the majority of human behavior is explained by (shared and non-shared) experiences rather than genetic influences [3-51. Of this amount, the evidence suggests that the largest portion of the variance is due to non-shared experi-

ences [6-191. This finding presents a new focus for psychological theory, and is redirecting psychological research and interventions [2]. Within the psychological perspective, Birth Order is a non-shared environmental influence, since it is an experience unique to each individual within a nuclear family [ 11. Many social changes have occurred while psychology has assumed this new perspective. These changes are based in population, economic and ecological events. Globally, the population changes are reflected in the increase in the number of people alive and the density of their living situations [20,21]. Economically, there has been a general decline in the standard of living and now more adults are working outside the home or needing public assistance to meet their economic needs [22-241. In turn, communal child care is evolving [25,26]. The ecology of our plant is also setting new limits now: it is defining the extent of economic and social growth that is possible [27-291. Separations, divorces, remarriages and a variety of child care settings have been a part of these social changes as well [30,31]. With these changes, the term Birth Order becomes more difficult to define and perhaps changes to a different independent variable when explaining human behavior. Within the field of sociology, these macro-level changes have yet to be formally incorporated by social theory. The Family Development scholars have begun to recognize the need for family theory to account for these transitions and changes [32]. As new propositions evolve to address these events, social theory will be based on the discipline’s assumptions about families: (1) families develop over time, (2) families provide systematic developmental experiences for their members, and (3) families fulfil a predictable role within the society. 443

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444

Concurrently, the bio-psycho-social theory of illness and disease has evolved in the medical and social sciences [33,34]. This perspective assumes that illness and disease are expressions of biologic as well as psychologic and social experience. At its most conservative, this theory recognizes that biological illnesses are experienced in psycho-social contexts. At its other extreme, biologic, psychologic, and/or social experiences are recognized as causes and sequelae of medical illness. Within this theoretical context, health refers to a condition of psychological, social, and biological well-being. This definition of health is endorsed by the World Health Organization [35] and enables research into the relationships among bio-psychosocial variables. The disciplines of medicine, psychology, and sociology each provide a different theoretical insight for conceptualizing relationships between Birth Order and health. From the bio-psycho-social theory, we know that a relationship between a contextual and outcome variable can be researched. Within this model, Birth Order is a contextual variable that may be related to health outcomes. A second perspective comes from psychology: Birth Order is identified as one variable in the category of non-shared (within family) experiences which, in turn, explains the majority of human behavior. Psychologists work exclusively at the individual level, however, which makes generalization difficult. The sociological perspective views Birth Order from a third perspective: Birth Order is considered a shared experience by first borns, second borns, etc. Family theory in sociology provides the bridge between medical and psychological theories; its assumptions and methodologies make analyses of relationships between Birth Order and health outcomes reasonable and valid. Methodology In this paper, a comprehensive review of recent articles in the medical literature is done to describe what is known of the relationship between Birth Order and health variables. All the studies in the Index Medicus since 1986 (within the past 5 years) under the listing of Birth Order are included in this review. First, the variables of Birth Order and health are defined as they have been operationalized in these studies. Secondly, the extent of any significant findings in these studies among the operationalized variables is reviewed. Thirdly, other variables and methodological issues that are part of the research findings are discussed. Finally, a summary of the findings regarding relationships between Birth Order and health is included, with implications and directions for further study. DEFINITIONS

Birth Order Birth Order has been operationalized several ways. Most classically it refers to the order in which children

are born from one set of parents into a family setting. Children are described as first born, second born, third born and so on. Categories of Birth Order have also been used: first born children, only or singleton children, middle children, early- and later-born children, and last-born children. There are limitations to this operationalization. Clearly the specific sensitivity of any category is dependent on the size of the sibship group [36]. In two families, one with 7 children and another with 2 children, the only consistent measurement references the first born. All other terms may make unreasonable assumptions about common Birth Order circumstances and their relationship to the dependent variables being considered. A second limitation with the use of these categories is a consequence of changing family structures [37,38]. Since parent partnerships change, often a sibship includes children birthed in a particular chronologic order that are from several parent partnerships. Half-brothers and sisters expand the sibship and complicate any understanding of the effects of Birth Order. A third limitation of these categories of Birth Order is the insensitivity to birth intervals [39]. A first-born child may be 20-25 years older than the second-born sibling. Also, a fourth limitation is the inability to distinguish the effect of gender [36]. A Birth Order that begins with 3 daughters before a fourth-born son masks the unequal resources that are commonly focused on the fourth-born in these circumstances. These limitations indicate some of the methodological difficulties in working with Birth Order variables. For purposes of this study, Birth Order refers to the order in which children have been born to one set of parents. Other confounding issues that are nested in the measurement of Birth Order are addressed when they may have impacted the findings of the studies included in the present review. Health Health is a concept with biological, psychological and social dimensions. However for purposes of this paper, a conservative approach is taken, and health outcomes are operationalized to include traditionally medical or biological indicators. Thus, the indicators of health used in this review include mortality [40-421 and morbidity indicators, which focus on developmental [38,40,43-451, psychopathological [46-521, accidental [53], and specific diagnostic conditions [54-571. Some of these conditions are evident at or near the time of birth, others within the first 5 years of life, still others during youth, and others during adulthood. The major limitation of this operationalized definition of health is that it does not acknowledge the psycho-social indicators of health inherent in the biopsycho-social theory. The indicators excluded here are those that measure an individual’s psychological and social health. Outcome variables of this type include obtaining gainful employment occupational and educational achievement, intelligence, personality,

Birth order and health avoiding legal incarceration, having non-abusive relationships, and accomplishing successful parenting. Studies with these outcome indicators were not included in the Index Medicus listings between 1986 and 1991. A second limitation of this operationalized definition of health is its insensitivity to the effects of economic development and/or urban experiences. Differences in nutrition, environmental exposures, cultural expectations, and economic realities are basic to understanding health outcomes. Methodologically, distinctions need to be made between developed and developing settings, as well as between urban and rural experiences, in order to control for these types of bias. For purposes of this paper, health is operationalized to include physical and mental conditions that lead to death or disability. Confounding variables are acknowledged when known, and distinctions between samples (urban-rural, developed-developing) are clarified. RESULTS

Birth Order has been investigated as a variable to explain health conditions over many years. In the past 5 years, few positive correlations have been demonstrated. There are several methodological issues that complicate these findings, but the majority of recent published works do not describe Birth Order as an important explanatory factor in health conditions. The findings of the studies that have been done can be best understood when displayed along two axes: types of health conditions and the ages of the samples that were studied (See Table 1). The evidence in Table 1 suggests that studies done with younger samples are more likely to document a relationship between Birth Order and health outcomes than studies done with older samples. Birth Order seems to have a stronger relationship with health indicators at younger ages. Mortality

studies

The studies that have investigated the relationship between Birth Order and death have all involved very young samples. They have also included measurements of the mother’s health, with the hypothesis that Birth Order may be related to infant mortality through maternal factors. The mortality studies include samples from developed countries (United States [40] Table I. Number of studies listed in the Index Medicus since 1986 investigating Birth Order and health (percentages are the proportion with significant findings) Age of sample in study Health outcome

Perinatal

Mortality Developmental conditions Accidents Psychopathology Specific diseases

3 (66%)

l(loo%)

4 (50%)

I (0%) I (lOO%) 1(loo%)

<5yr

5-20 yr

3 (33%)

Adult

6 (33%) 4 (25%)

445

and Norway [42]) and from a less developed setting (Bangladesh [41]). The sample that included the older children was also from Bangladesh [41]. There were two statistically significant findings: (1) Birth Order was inversely related to infant mortality in the United States; that is, later-born children were more likely to survive, and (2) Birth Order is related in a curvilinear fashion with infant mortality in Bangladesh; that is, fourth- and fifth-born children were most likely to survive. These studies both present findings that are contrary to a large number of previous works (for examples, see Refs [58-611 for U.S. studies and Refs [62-631 for studies in the developing world). Both of these recent studies propose that the biological maturity of the mother’s body explains the deaths of the first and early-born children. Likewise, the curvilinearity observed in the study with the sample from Bangladesh occurs because of the mother’s biologic maturity with the middle children; maternal aging explains the death of latest born children. Thus in these studies, significant relationships between Birth Order and infant mortality are explained through the intervening variable of the mother’s biologic maturity. The study [42] with the Norwegian sample did not document a statistically significant relationship between Birth Order and infant mortality. The study involving the sample of infants in Bangladesh [41] also investigated the relationship between Birth Order and child mortality in a sample of children ages 1-5. It established that an optimal birth interval, maternal education and urban residence were statistically significant variables in predicting improved child survival; Birth Order was not a statistically significant variable in predicting survival in this age group. Developmental

health conditions

Since 1986, five studies have been listed in the Index Medicus that investigated the relationship between Birth Order and developmental health conditions. Two of them, or 40% (one was done with a sample from the United States [40], the other was done with a sample from Saudi Arabia [45]) found statistically significant relationships between Birth Order and infant birth weight: they each demonstrated that infant birth weight increased with birth order, through the twelfth born. The findings of the studies are consistent with data from several countries [64-671. However, other studies have documented a curvilinear relationship between Birth Order and birth weight [68,69]. These studies indicated increasing birth weights through the fourth (sixth or seventh) births, followed by a decline in birth size. The differences between the two sets of literature may be explained by the health of the mother when she is pregnant with later gestations; the samples documenting the curvihnearity did not investigate the role of maternal variables. The other study in the developmental conditions group was done with older children. It did not document a relationship between Birth Order and

446

BARBARAA.ELLIOTT

speech impairment [38]. This study’s findings differ from earlier studies done by speech pathologists which have found statistically significant relationships [70-751. A difference between the current study and previous ones, however, was that the current study statistically controlled sibship size, age, and social class in its analysis. These variables are important to the understanding of the development of speech impairments. Two other studies investigated relationships between Birth Order and measures of physiology in multiple births. They investigated differences in the relationship between Birth Order and respiratory distress [43], arterial and venous pH, r02, &O,, lactate and base deficit [44]. No statistically significant differences could be demonstrated. The studies were done to investigate previous clinical observations, so they provide the first statistical evidence regarding these relationships. Accidents

Before 1986, the studies about accidents published in the literature had examined the relationship between family characteristics and accidents, but they had not reached consensus about any of the relationships [76-821. None of the studies had considered the relationship between Birth Order and accidental injuries. In 1988, a British study was published that used a national birth cohort (England, Scotland and Wales) to investigate this relationship [53]. They demonstrated statistically significant results: (1) for later born preschool children, the risk of accidental injury requiring hospitalization increased with three or more older children in the household; and (2) for school-age children, being later born in a setting with 4 or more older children put the younger child at greatest risk for accidental injury requiring hospitalization. Thus, being later born into a larger sibship (being third, fourth or higher in birth rank) does increase risk of accidental injury for younger children in families (in a developed country). Social class, gender and number of accidents needing hospitalization were statistically controlled in the study, so they are not intervening variables. Birth Order is the remaining explanatory variable. The authors suggest that inadequate parental supervision or family dynamics (younger children wanting to keep up with the older ones) may result in the increased incidence of these accidents. Psychopathology

Within the psychology and medical literature, the relationship between Birth Order and particular types of mental pathology have long been studied. Since 1986, seven studies have been listed in the Index Medicus [46-521 that have investigated these relationships in youth and adult samples. One third of them have reported statistically significant relationships between Birth Order and indicators of psychopathology [48,49, 521. All but one of the studies were done in developed countries.

In one study done with a youthful sample, first borns were found to be more psychologically healthy (to have less psychopathology) than later-born children [48]. These findings add to the multitude of studies that have not yet reached consensus about the relationship between Birth Order and mental health [83-891. The study used a convenience, public school sample in the United States. The studies published since 1986 that have worked with adult samples investigated the relationship between Birth Order and a variety of psychopathologic diagnoses: somatization disorders [46], obsessive/compulsive disorders (47,511, alcoholism [49], anorexia nervosa [50], and sexual delinquency [52]. Two of these studies reported statistically significant linear relationships: (1) first-born children of an alcoholic parent are less likely to develop associated psychopathologies with their alcoholism, compared to their later-born siblings; and (2) sexual delinquents (pedophiles and homosexual delinquents) are more frequently the last of three or more children. The other papers reported non-significant findings, adding to a long list of studies that have yielded no consensus about the role of Birth Order in their etiology (somatization [90-1021; obsessive/compulsive disorder [103-l lo]; anorexia nervosa [ll l-l 161). Studies that investigate the relationship between Birth Order and alcoholism have been widely described in the alcoholism literature, but the extent of any psychopathology that accompanies the development of alcoholism had not been previously reported. This report demonstrates that the last born in a family is most likely to become alcoholic and adds that the youngest is also most likely to be more disabled by any psychopathology that can accompany it. The first born are less likely to have accompanying psychopathology and are described as more resilient than later borns. The authors explain this finding with the family dynamics of an alcoholic home: later-born children are exposed to a progressively more disruptive home life because of the alcoholism and also experience little parental protection or support. The other study with statistically significant findings in this group reported that sexual delinquency is also associated with later Birth Order (in families of three or more children). No explanation of these findings was proposed, and no previous research has investigated this relationship. Specific diseases

Since 1986, four original papers have been listed in the Index Medicus that investigate the relationship of Birth Order and the particular disease conditions of multiple sclerosis [54-561 and irritable bowel syndrome [57]. Three of the four studies document curvilinear relationships between Birth Order and development of the diseases: youngest and oldest children are most likely to get either disease. The multiple sclerosis data were gathered from people

Birth

order

and

health

441

Table 2. Methodologies Studies

Data collection

Design

Sampling

N

Control group

Mmality

Majumder [41] Skjaerven et al. [42] Sweeney [40]

Survey survey Medical record

Multivariable Repeated measures Repeated measures

National survey National registry Population (local)

14,013 278,47 I 407

None Self controls Self controls

Medical record Medical record Medical record Survey Survey

Case description Case control Repeated measures Case control Population description

Population (local) Convenience Population (local) Population (local) Random (local)

21 221 407 352 4498

Co-triplets Matched Self controls Yes No

Survey

Case control

Cohort

Medical record, interview Psych inventory Medical record Survey Survey Medical record, interview Medical record, interview

Description Description Description Description Description Description Description

Convenience Convenience Convenience Convenience Convenience Convenience Convenience

Koch-Henriksen’s data Medical record, interview Medical record, interview Medical record, interview

Case Case Case Case

National statistics Population (local) Population (local) National register

Developmental

Antoine et al. [44] Arnold etal.[43] Sweeney [40] Tomblin [38] Wong [45] Accidents

Bijur er al. [53]

10,394

Yes

Psychopathology

Brown eta/. [46] Gates et al. [48] Gowers t-l al. [50] Khanna er a/. [5l] Keltner et al. [49] Pollard ef al. [47] Raboch et al. [52] Specifc

143 404 252 412 90 214 1581

No No No No No No No

diseases

James [56] Koch-Henriksen [55] McAllister et al. [57] Zilber et al. [54]

control control control control

living in Israel and Denmark; the people who were studied with irritable bowel lived in Ireland. The authors of the studies about multiple sclerosis performed additional analyses to explain the curvilinearity. They discovered that the first borns with multiple sclerosis were native Israeli and the later borns who developed multiple sclerosis were immigrants to Israel. This finding supports a controversial theory about the etiology of multiple sclerosis [I 171231. This theory purports that some individuals have a genetic predisposition to the disease which is activated by viral exposures occurring later in life than usual. This epidemiological pattern and theory is also described in Hodgkin’s disease [124]. The study regarding inflammatory bowel disease is the first one to investigate any relationship

648 648 200 555

Matched Matched Matched Matched

between birth Order and the development of this condition. In addition to describing the curvilinearity (first and later-born children are more Iikely to develop inflammatory bowel disease), the majority of the patients in this sample had only female siblings. Family dynamics and psychopathologic explanations for these findings were proposed by the authors. Methodological comparisons

Tables 2, 3 and 4 compare the methodologies, statistics, and confounding variables of the studies included in this review. It is evident from these tables that there is great variation among the studies, but that the differences are largely between the topical groupings of studies, rather than within them.

Table 3. Statistics Studies

Outcome measures

Approach

Curvilinearity

Interaction

Mortality

Majumder [4l] Skjaerven et al. [42] Sweeney [40]

Odds ratios Distributions (%) x’ correlation

Maximum likelihood Actuarial life tables ANOVA

Yes NO No

Yes No Yes

t-test Odds ratios x2 correlation x2 t-test Distributions (%)

0.05 Logistic regression ANOVA ANOVA Frequencies

No No No No No

No No Yes No No

Odds ratios

Logistic regression

No

t-test t-test r-test X2 X2 x2 t-test XZ

Slater’s index Means scores Slater’s index Slater’s index ANOVA X-TAB, frequencies X-TAB, frequencies

No No No No No No No

No No No NO No No No

X2

X-TAB, Logistic X-TAB, X-TAB.

Yes No Yes Yes

No No No No

Developmental

Antoine er al. [44] Arnold et al. [43] Sweeney [40] Tomblin [38] Wong [45]

P <

Accidents

Bijur et al. [a] Psychopathology

Brown et a/. [46] Gates et al. [48] Gowers et al. [50] Khanna et al. [Sl] Keltner et al. [49] Pollard et al. [47] Raboch er al. [52] Specific diseases

James [56] Koch-Henriksen [55] McAllister ef al. [57] Zilber et al. 1541

Odds ratio ,y2 2

correlation

frequencies regression frequencies freauencies

448

BARBARAA.ELLIOTT Table 4. Confounding

Studies Mortality Majumder [41] Skjaerven ef al. [42] Sweeney [40] Dewlopmenfol Antoine ef al. [44] Arnold ef al. 1431 Sweeney [40] Tomblin [38] Wang [45] Accidents Bijur et al. [53] Psychopathology Brown et al. [46] Gates et al. [48] Cowers er al. [50] Khanna ef al. [Sl] Keltner et al. [49] Pollard ef al. [47] Raboch et al. [52] Specific diseases James [56] Koch-Henriksen [55] McAllister et al. 1571 Zilber et al. [54]

variables Explanatory

variable

Urban/rural

Gender

Bangladesh Norway United States

Both Both Urban

Both Both Both

Maternal development, Birth weight Maternal development,

United States Canada United States United States Saudi Arabia

Urban Urban Urban Both Urban

Both Both Both Both Both

Multiple gestation Birth presentation Maternal development, Sibship size and SES Maternal development,

England, Scotland, Wales

Both

Both

Birth Order

United States United States England India United States United States Czechoslovakia

Both Both Urban Urban Urban Urban Urban

Both Both Both Both Both Both Male

Random occurrence Birth Order Small sample size Cross-cultures, family typologies Birth Order Family expectations Birth Order

Denmark Denmark Ireland Israel

Both Both Both Both

Both Both Both Both

Viral exposure Random occurrence Psycho dynamics, Birth Order Viral exposure

Site

Two of the three studies in the mortality group are large, sophisticated epidemiologic studies [41,42]. The third is a controversial study from the United States [40] that documents an inverse relationship between Birth Order and infant mortality, explaining that maternal biological immaturity is responsible for more deaths in first born children than in second borns. All three studies are well designed, executed, and reported. Their findings can be considered reliable and valid. The studies included in the developmental conditions section are not as large or as strong methodologically as those in the mortality group. Nonetheless, the four studies with samples of less than 500 people do include control groups and use appropriate statistics [38,40,43,44]. The study with the sample of 4498 people is the only one in this entire review with a randomly-selected sample; although they are descriptive statistics, the reported frequency distributions and percentages are valuable because of the sampling [45]. Only one study in this group includes rural and urban children in the sample [38]; the others are all urban samples. This group of studies also includes samples from both developed and lessdeveloping countries. The findings of this group of studies can also be considered reliable, given the methodologies used. The study in the accident group represents the strongest research methodology among all of the studies in this review [53]. It is a cohort study, with urban and rural representation from a developed country. The methodology and statistics are sophisticated, with appropriate investigation of interactions and curvilinearity. Its findings (that Birth Order is an important risk factor for accidents that result in hospitalization) are statistically powerful. The many studies listed in the psychopathology grouping are published in the psychiatric literature

Birth Order Birth Order

Birth Order Birth Order

and are much weaker methodologically [46-521. All of them were done with convenience samples, no control groups, and simple statistics. They all continue the controversy about relationships between Birth Order and mental health, since none of them are designed with the sophistication needed to address the differences of opinion. The four studies listed in the section called specific diseases are based on research that used samples from Denmark, Israel and Ireland [54-571. Appropriately sophisticated sampling, analysis, and statistics have been used and reported. The findings of curvilinearity regarding both diseases are intriguing and are resulting in further epidemiologic investigation. DISCUSSION

AND IMPLICATIONS

As discussed above, theoretical background in the disciplines of medicine, psychology, and sociology provide the rationale for further investigation of potential relationships between Birth Order and health outcomes. Although no specific theories about the nature of relationships between Birth Order and health outcomes have been proposed, social scientists have suggested three hypotheses to explain observed relationships between Birth Order and other child or adult outcomes [39]. The first hypothesis (H-l) is the null hypothesis: there is no causal relationship between Birth Order and child or adult outcomes; any apparent relationship is spurious. The second hypothesis (H-2) is a zero-sum hypothesis applied to family resources; it proposes that relationships between Birth Order and outcomes occur because there is a reduction in the time and resources available to each child as the number in the sibship increases. The third hypothesis (H-3) is generated by systems thinkers and proposes that although the increase in sibship size lessens available parental resources

Birth order and health for each family member, it also increases the total resources available to family members because of the additional siblings. These three general explanations about how Birth Order affects family outcomes can be adapted to explain relationships between Birth Order and health outcomes. The first hypothesis (H-l) does not change; it is the null hypothesis. The second hypothesis (H-2) explains that the available parental resources for each child are reduced as the sibship expands. This has health implications for families: for each successive child, there are fewer maternal resources for gestation and lactation, less preventive care (immunizations and well-child care), less available nutrition, less water, fewer direct parental interactions, and less parental supervision of activities. The third hypothesis (H-3) proposes that although the parental resources drop with more children, there is an expanded resource base at the sibship level. From a health perspective, an expanded sibship brings more communicable disease into the family, introduces developmentally inappropriate activities to younger siblings, and provides parents with a health reference group that keeps them from accessing medical care for a child until a threshold is reached. The studies reviewed in this paper lend support to each of these three hypotheses and suggest a change in H-2. Of the 20 studies reviewed here, half documented statistically significant relationships between Birth Order and a health outcome [40,41,45, 48, 49, 52-54, 56, 571. Five of these demonstrated other variables intervening in the relationships that better explained the findings; the intervening variables are actually variations of H-2 [40,41, 451 or H-3 [54,56]. Four of the remaining 5 studies with statistically significant findings were not methodologically rigorous [48,49, 52, 571; their findings add to the data base but cannot adequately address the theoretical issues. There is only one study that clearly documents a relationship between Birth Order and a health outcome: the British accident study [53]. It is a well done cohort study that described increased risk of hospitalization due to accidents for preschoolers who are third or more in a Birth Order and comparably increased risks for school-aged children who are fourth our more in their sibship order. These findings may support either the second or third hypotheses (H-2 or H-3) described above. Parents may be unable to supervise these larger sibship groups, which results in accidents to the children; this explanation supports (H-2). Another plausible explanation is that the older siblings introduced developmentally appropriate activities (for themselves) which were then attempted by the younger siblings, resulting in accidents. This idea supports the third hypothesis, (H-3). Four studies with significant findings actually demonstrated curvilinear relationships between Birth Order and mortality [41], and specific diseases [54,56,57]. Three of these studies described interven-

449

ing variables that helped explain the findings [41,54, 561. The mortality study explained its findings with maternal biological development. This explanation is a variation of H-2 which proposes there are fewer parental resources available to each child as sibship size increases. Since this study documented more deaths among children born of the youngest and oldest mothers, it seems the maternal gestational resources are at their best during the middle years of a woman’s fertility. This suggests that H-2 be changed to read: There are different parental resources available to offspring as sibship size increases. This statement allows for the curvilinear relationship documented by the mortality studies and still maintains the zero-sum hypothesis. The three other studies with curvilinear findings are in the specific diseases category. Two of the three support H-3: the increased sibship group brings communicable conditions into the household at earlier ages which triggers later onset of multiple sclerosis. The final study with curvilinear findings did not pursue further environmental explanation of its results. The remaining 10 studies support H-l: there is no statistically significant relationship between Birth Order and health variables. In fact, 9 of the 10 studies reporting statistically significant results can also be considered supportive of the null hypothesis. They have found intervening variables that explain their statistically demonstrated relationships, or they are methodologically suspect. Only the British accident study remains. There are plausible explanations for the limited statistical support of the role of Birth Order in health outcomes. First, it is possible that the methodological and statistical sophistication of current research efforts may be validly documenting what is-and is not-to be found. Earlier studies may not have been able to document these findings with the same validity. Despite theoretical implications, it may indeed be true that no relationship exists between these variables. Alternatively, it is also possible that Birth Order continues to be a subtle, systems-level variable that does impact health outcomes. Only investigations of interactions, curvilinearity and other sophisticated statistics will reveal its importance. Certainly in this review, the intervening variables and curvilinear relationships demonstrated by several studies provided the most insight into the relationships between family and health outcomes in general. A third explanation for the limited statistical support may be based in evolving social and family settings. As described above, social ecology (population demographics, economic realities, and ecological limits) is changing the ways children are being raised. Consequently, Birth Order may have an increasingly shallow impact on all outcomes, including health. Instead, communal child care settings may have more impact than nuclear family characteristics on health outcomes. In order to better understanding what is happening in the relationship between Birth Order and health,

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450

future research needs to consider three other factors that have become evident through this review: (1) Birth Order may be related to health outcomes in younger age groups, but not in adulthood. Is this because Birth Order has real impact on health at young ages? Does this change when there is communal child care available? Or is it that maternal factors intervene and actually explain the relationship? (2) There is a need for well designed, rigorous evaluations of the relationship between Birth Order and mental health. The research in this area has not been helpful because of its methodological limitations, and there is much to learn. (3) Further investigations of Birth Order and the incidence of specific diseases is likely to reveal insights into more social and biological epidemiology. Since the relationships that have been recently documented in this area are all curvilinear, there is the clear implication that other environmental explanations are involved in the relationships. Solving these puzzles will add to our medical and social sciences. (4) Research that investigates relationships between Birth Order and health needs to also consider evidence of intervening variables, interactions between variables, and curvilinearity in the relationships. A question remains whether Birth Order is an independent variable in understanding health outcomes. From this review, studies that report relationships between Birth Order and health outcomes help us to better understand the relationship between families and health. Birth Order does not seem to be an independent explanatory factor in understanding this relationship. REFERENCES 1. Plomin R. and Daniels D. Why are children in the same family so different from one another? Behau. Brain Res. 10, l-60, 1987. 2. Reiss D., Plomin R. and Hetherington E. M. Genetics and psychiatry: an unheralded window on the environment.-& J. isychiat. 148, 283-291, 1991. 3. Loehlin J. C. and Nichols R. C. Heredity, Environment and Personality. University of Texas. Press, Texas, 1976. 4. Goldsmith H. H. Genetic influences on personality from infancy to adulthood. Child Dm. 54, 331-355, 1983.

5. Floderus-Myrhed B., Pederson N. and Rasmuson S. Assessment of heritability for personality based on a short form of the EPI. Behnu. Gener. 10, 153-162, 1980. 6. Crook M. N. Intra-family relationships in personality test performance. Psychol. Rec. 1, 479-502. 1937. 1. Ahern F. M., Johnson R. C., Wilson J. R., McClern G. E. and Vandenberg S. G. Family resemblances in personality. Behau. Genet. 12, 261-280, 1982. 8. Gehlin J. C.. Horn J. M. and Willerman L. Personality resemblance’ in adoptive families. Behav. Genet. 11, 309-330, 1981.

9. Loehlin J. C., Willerman L. and Horn J. M. Personality resemblance in adoptive families when the children are late adolescents and adults. J. Pets. Sot. Psychol. 48, (2) 376-392,

1985.

10. Starr S., Webber P. I., Weinberg R. A. and Wittig M. A. Personality resemblance among adolescents and their parents in biologically related and adoptive families. J. Pets. Sot. Psychol. 40, 885-898, 1981. 11. Daniels D. Understanding the family environment: a study of adoptive and non-adoptive infant siblings. Unpublished dissertation, University of Colorado, 1985. 12. Macoby E. E. and Martin J. A. Socialization in the context of the family: parent-child interaction. Handbook of Child Psychology (4th edn) Socialization, Personality and Social Development (Edited by P. H.

Mussen). Wiley, New York, 1983. 13. Gottesman I. I. and Shields J. Schizophrenia: the Epigenetic Puzzle. Cambridge University Press, Cambridge, MA, 1982. 14. Plomin R., DeFries J. C. and McClern G. E. Behavioral Genetics: A Primer. Freeman, San Francisco, 1980. 15. Fuller J. L. and Thompson W. R. Foundations of Behavior Genetics. Mosby, St Louis, 1978. 16. Bouchard T. J. Jr and McGue M. Familial studies of intelligence: a review. Science 212, 1055-1059, 1981. 17. Starr S. and Weinberg R. A. The influence of “family background” on intellectual attainment. Am. Sot. Rev. 43, 674-692,

1978.

18. Kent J. Genetic and environmental contributions to cognitive abilities as assessed by a telephone test battery. Unpublished dissertation, University of Colorado, 1985. 19. DeFries J. C. and Fulker D. W. Multiple regression of twin data. Behav. Gene!. 15. 467-473, 1985. 20. The State of the World’s ‘Children 1990. Published for UNICEF, Oxford University Press, Oxford, 1990. 21. The Right fo Housing. Institute for Policy Studies, Working Group on Housing, Washington, DC, 1989. 22. U.S. Census Bureau, Current Population Reports. Money, income and poverty status, publication no. 149, 1984; and Money, income and poverty status, publication no. 154, 1985. 23. The World Bank Atlas 1990. International Bank for Reconstruction and Development/The World Bank, Washington, DC, 1990. 24. U.S. Department of Labor, Women’s Bureau. “Facts on U.S. working women: women and workforce 2000.” Fact sheet no. 88-1, 1988. Book and 25. Children 1990: A Report Card-Briefing Acrion Primer. Children’s Defense Fund, Washington, DC, 1990. 26. League of women voters. Child care: an issue’s coming of aae. Naln. Voter 38. (3). 4-9, 1988. 27. Brown L. State of the’ World 1989. World Watch Institute, Washington DC, 1989. 28. Wijkman A. and Timberlake L. Natural Disasters: Acta of God or Acts of Man. International Institute for Environment and Development, London, 1984. 29. Attwood D. Power and Poverty: Development and Derrelopment Projects in the Third World. Westview Press, Boulder, CO, 1988. 30. The Health of America’s Children. Children’s Defense Fund, Washington, DC, 1989. 31. Associated Press. “American marriage trends reflect change.” August 26, 1991. (Source: National Center for Health Statistics.) 32. Klein M. and Mederer H. Family development theory session. Theory construction and research methadology workshop. National Council of Family Relations Annual Conference, Denver, 15 November 1991.

Birth order and health 33. Engel G. L. The need for a new medical model: a challenge for biomedicine. Science 196, 129-136, 1977. 34. Smilkstein G. The unity of biomedical and psychosocial issues in individual and family health care. In Behavioral Science in the Practice of Medicine (Edited by Carr J. E. and Dengeriuk H. A.), pp. 168-183. Elsevier Biomedical. New York. 1990. 35. Mahler H. The philosophy underlying the Alma-Ata conference of 1978. In Basch P. F. Textbook of International Health, pp. 232-233. Oxford University Press, New York, 1990. 36. Taubman P. and Behrman J. R. Effect of number and position of siblings on child and adult customs. Sot. Biol. 33, (l-2), 22-34, 1986. 37. Parish T. Evaluations of family by youth: do they vary as a function of family structure, gender and birth order? Adolescence 25, (98), 353-356, 1990. 38. Tomblin J. B. The effect of birth order on the occurrence of developmental language impairment. Br. J. Disord. Commun. 25, 77-84,

1990.

39. Heer D. M. Effect of number, order and spacing of siblings on child and adult outcomes: an overview of current research. Sot. Biol. 33, (I-2) l-4, 1986. 40. Sweeney P. J. A comparison of low birth weight, perinatal mortality, an infant mortality between first and second births to women 17 years old and younger. Am. J. Obstet. Gynecol. 160, (6), 1361-1370, 1989. 41. Majumder A. K. Maternal factors and infant and child mortality in Bangladesh. J. Biosoc. Sci. 20,89-98, 1988.

55. Koch-Henriksen N. An epidemiological study of multiple sclerosis. Acta neurol. scand. Suppl. 124, I-123, 1989. 56. James W. H. Further evidence in support of the hypothesis that one cause of multiple sclerosis is childhood infection. Neuroepidemiology 7, 130-133, 1988. 57. McAllister C. and Fielding J. F. The irritable bowel syndrome, single sex representation and birth order. Br. J. c/in. P&t. 41, (6, 869-871, 1987. 58. Babson S. G. and Clark N. G. Relationshio between infant death and maternal age. J. Pediatr. 103, (3), 391-393,

1983.

59. Elster A. B. The effect of maternal age, parity, and prenatal care on perinatal outcome in adolescent mothers. Am. J. Obstet. Gynecol. 149, (8), 845-847, 1984. 60. Jekel J. F., Harrison J. T., Bancroft D. R. E. et al. A comparison of the health of index and subsequent babies born to school-age mothers. Am. J. Publ. Hlth 65, (4), 370-374,

1975.

61. Zuckerman B. S., Walker D. K., Frank D. A. et al., Adolescent pregnancy: biobehavioral determinants of outcome. J. Pediatr. 105, (6), 857873, 1984. 62. Gubhaju B. B. Demographic and social correlates of infant and child mortality in Nepal. Unpublished dissertation, Australian National University, Canberra. 63. Hull T. and Gubhaju B. Multivariate analysis of infant and child mortality-in Java and Bali. J. Bibsoc. Sci. 18, 109-I 18, 1986. 64. Mills J. and Seng Y. P. The effect of age and parity of the mother and birth weight of the offspring. Ann. Hum. Genet. 18, 58-67,

42. Skjaerven R., Wilcox A. J. and Russel D. Birthweight and perinatal mortality of second births conditional on weight of the first. Znt. J. Epidemiol. 17, (4), 830-838, 1988.

43. Arnold C., McLean F. H., Kramer M. S. and Usher R. H. Respiratory distress syndrome in second-born vs first-born twins. N. Engl. J. Med. 317, (18) 1121-I 125, 1987. 44. Antoine C., Kirshenbaum N. W. and Young B. K. Biochemical differences related to birth order triplets. J. Reprod. Med. 31, (5), 330-332,

1986.

45. Wong S. S. Birth order and birth weight of Saudi newborns. J. R. Sot. Hlth 110, (2) 64-66, 1990. 46. Brown F. W. and Smith G. R. Jr Birth order of patients with DSM-III-R somatization disorder. Am. J. Psychiat. 146, (9) 1193-1196, 1989. 47. Pollard C. A., Wiener R. L., Merkel W. T. and Enseley C. Reexamination of the relationship between birth order and obsessive-compulsive _disorder. Psvchopathologv 23. 52-56.

1990.

48. Gates L., Lineberger M. R:, Crockett J. and Hubbard J. Birth order and its relationship to depression, anxiety and self-concept test scores in children. J. Genet. Psychol. 149, (I), 29-34, 1988. 49 Keltner N. L., McIntyre C. W. and Gee R. Birth order effects in second generation alcoholics. J. Stud. Alcohol. 47, (6), 495-497,

1986.

50 Gowers S.. Kadambari S. R. and Criso A. H. Familv structure and birth order of patients with anorexia nervosa. J. Psychiat. Res. 19, (2/3), 247-251, 1985. 51 Khanna S. and Channabasavanna S. M. Birth order in obsessive-compulsive disorder. J. Psychiat. Res. 21, 349-354,

451

1987.

52. Raboch J. and Raboch J. Number of siblings and birth order of sexually dysfunctional males and sexual delinquents. J. Sex Marit. Ther. 12, (I), 73-76, 1986. 53. Bijur P. E., Golding J. and Kurzon M. Childhood accidents, family size and birth order. Sot. Sci. Med. 26, 839-843, 1988. 54. Zilber N., Kutai-Berman M., Kahana E. and Korczyn A. D. Multiple sclerosis and birth order. Acta neurol. stand. 78, 313-317,

1988.

1954.

65. Camitteri A. P. and Cremona V. The effect of parity on birth weight. J. Obstet. Gynec. Br. Common. W. 77, 145-147,

1970.

66. McDonald A. D. Early prenatal factors and prematurity. J. Obstet. Gynec: Br. Common. W. 69, 502-505, 1962. 67. Scott K. E. and Usher R. Fetal malnutrition: its incidence, causes and effects. Am. J. Obstet. Gynec. 94, 951-963, 1966. 68. Dowding V. M. New assessment of the effects of birth weight. Br. Med. J. 282, 683-686, 1981. 69. Thomson A. M., Billewicz W. Z. and Hytten F. E. The assessment of fetal growth. J. Obstet. Gynec. Br. Common. W. 74, I-IO, 1968. 70. Beitchman J. H., Peterson M. and Clegg M. Speech and language impairment and psychiatric disorder: the relevance of family demographic variables. Child. Psychiat. Hum. Dev. 18, 191-207, 1988. 71. Belmont L. and Marolla F. A. Birth order, family size, and intelligence. Science 182, 1096-I 101, 1973. 72. Breland H. M. Birth order, family configuration, and verbal achievement. Child Deu. 45, 1011-1019, 1974. 73. Butler N. R.. Peckham C. and Sheridan M. Sneech defects in children aged 7 years: a national study. Br. Med. J. 3, 253-257, 1973. 74. Kolvin I., Fundudis T. and Scanlon E. Early development type and prevalence. In Speech in Retarded and Deaf Children: Their Psychological Developmenr

(Edited by Fundudis T., Kolvin I. and Garside R.). Academic Press, London, 1979. 75. Tomblin J. B. Familial concentration of developmental language impairment. J. Speech Hear Disord. 54, 287-295, 1989. 76. Wadsworth J., Burnell I., Taylor B. and Butler N.

Family type and accidents in preschool

children.

J. Epidemiol. Community Hlth 37, 100-104, 1983.

77. Husband P. and Hinton P. E. Families of children with reported accidents. Arch. Dis. Child. 47,396-400, 1972. 78. Krall V. Personality characteristics of accident repeating children. J. Abnorm. Sot. Psychol. 48, 99-107, 1953.

452 79

80

81

82

83

84 85.

86. 87. 88. 89.

90. 91. 92.

93.

94. 95.

96. 97. 98. 99.

100.

101.

102.

BARBARA A. ELLIOTT Beautrais A. L., Fergusson, D. M. and Shannon F. T. Childhood accidents in New Zealand birth cohort. Aust. Pediat. J. 18, 238-242, 1982. Beautrais A. L., Fergusson, D. M. and Shannon F. T. Accidental child poisoning in the first three years of life. Aust. Pediat. J. 17, 1044109, 1981. Bijur P. E., Stewart-Brown S. and Butler N. Child behavior and accidental injury in 11,966 preschool children. Am. J. Dis. Child. 140, 487-492, 1986. Nixon J. and Pearn J. An investigation of sociodemographic factors surrounding childhood drowning accidents. Sot. Sci. Med. 12. 387-390. i978. Bharathi V. V. and Venkatramaiah S. ‘R. Birth order, family size, and anxiety. Child Psychol. Q. 9, (3), 11-17, 1976. Doss S. A study of maladjustment in relation to birth order. J. Psychol. Res. 24, (3), 156-159, 1980. Rosenow C. and Whyte A. H. The ordinal position of problem children. Am. J. Orthopsychiat. 1, 430-444, 1931. Schachter S. Birth order eminence and higher education. Am. Sot. Rev. t8, 7577767, 1963. Schooler C. Birth order and hospitalization for schizophrenia. J. Abnorm. Psychol. 69, 574-579, 1964. Sletto R. F. Sibling position and juvenile delinquency. Am. J. Sot. 39, 657-669, 1934. Touliatos J. and Lindholm B. W. Birth order, family size and children’s mental health. Psychol. Rep. 46, (3, pt. 2) 1097-1098, 1980. Morrison J. R. Early birth order in Briquet’s syndrome. Am. J. Psychol. 140, 159661598, 1983. Woerner P. I. and Guze S. B. A family and marital study of hysteria. Br. J. Psychol. 114, 161-168, 1968. Ziegler F. J., Imboden J. B. and Meyer E. Contemporary conversion reactions: a clinical study. Am. J. Psvchol. 116. 901-910, 1960. Stephens J. H. and Kamp M. On some aspects of hysteria: a clinical study. J. Nero. Ment. Dis. 134, 305-315, 1962. Price J. S. and Hare E. H. Birth order studies: some sources of bias. Br. J. Psychol. 115, 633-646, 1969. Hare E. H. and Price J. S. Birth order and family size: bias caused by changes in birth rate. Br. J. Psychol. 115, 647-657, 1969. Bytheway B. A Statistical trap associated with family size. J. Biosoc. Sci. 6, 67-72, 1974. Ljungberg L. Hysteria: birth order. Acta psychiaf. stand. Suppl. 32, 1055106, 1957. Roy A. Hysteria: a case note study. Can. J. Psychol. 24, 157-160, 1979. Tsuang M. T. Birth order and maternal age of psychiatric in-patients. Br. J. Psychol. 112, 1131-I 141, 1966. Wilson-Barnett J. and Trimble M. R. An investigation of hysteria using the Illness Behavior Questionnaire. Br. J. Psvchol. 146, 601-608, 1985. Smith G.-R., Monson R. A. and Ray D. C. Psychiatric consultation in somatization disorder-a randomized controlled study. N. Engl. J. Med. 314, 1407-1413, 1986. Greenwood M. and Yule G. U. On the determination of size of family and of the distribution of characters

103.

104. 105.

106 107. 108.

109.

110. 111.

112. 113.

114.

115.

116.

117. 118. 119.

120.

121.

122.

123.

124.

in order of birth. J. R. Stat. Sot. 77, 1799199, 1914. American Psychiatric Association, DSM-III. Diagnostic and Statistical Manual of Mental Disorders, 3rd edn. APA, Washington, DC, 1980. Ernst C. and Angst J. Birfh Order-Ils Influence on Persona&y. Springer, Berlin, 1983. Kayton L. and Boyrge G. F. Birth order and the obsessivecompulsive character. Arch. gen. Psychiat. 17, 751-754, 1967. Snowden J. Family size and birth order in obsessional neurosis. Acta nsvchiat. stand. 60. 121-128, 1979. Barry H. and Barry M. Birth order, family size and schizophrenia. Arch. gen. Psychiat. 17, 4355440, 1967. Coryell W. Obsessive compulsive disorder and primary unipolar depression: comparisons of background, family history, course and mortality. J. New. Ment. Dis. 169, 220-224, 1981. Erlenmeyer-Kimling L. The problem of birth order and schizophrenia: a negative conclusion. Br. J. Psych. 115, 659-678, 1969. Granville-Grossman K. L. Birth order and schizophrenia. Br. J. Psychol. 112, 1119-1126, 1966. Crisp A. H. Reported birth weights and growth rates in a group of patients with primary anorexia nervosa. J. Psychosom. Res. 14, 23-50, 1970. Crisp A. H. Anorexia Nervosa: Let Me Be. Academic Press, London, 1980. Crisp A. H., Hsu L. K. G., Harding B. and Hartshorn J. Clinical features of anorexia nervosa: a study of a consecutive series of 102 female patients. J. Psychosom. Res. 24, 179-191, 1980. Garfinkel P. E. and Garner D. M. Anorexa Nervosa: A Multidimensional Perspecfive. BrunnerjMazel, New York, 1982. Hall A. Family structure and relationship of 50 female anorexia patients. Aust. NZJ. Psychiat. 12, 263-268, 1978. Theander S. Anorexia nervosa-a psychiatric investigation of 92 female patients. Acta psychiat. stand. Suppl. 24, l-2, 1970. Pratt R. T. C. The familial occurrence of disseminated sclerosis. Ann. Eugenics, Lond. 16, 45-59, 1951. Cendrowski W. S. Birth order of multiple sclerosis subjects. Psychol. Neural., Base1 151, 161-167, 1966. Amerovich A.. LeCanuet F. and Marteau R. Birth order and risk of multiple sclerosis: are they associated and how? Acta neural. stand. 63, 136-138, 1981. Isager H., Anderson E. and Hyllested K. Risk of multiple sclerosis inversely associated with birth order position, Acta neurol. stand. 61, 3933396, 1980. Crippe J., Rudd A. and Ebers G. C. Birth order and multiple sclerosis. Acta neural. stand. 66, 342-346, 1983. Visscher B. R., Shen-Liu K., Sullivan C. B. et a/. Birth order and multiple sclerosis. Acta neurol. stand. 66, 209-215, 1982. Antonovsky A., Leibowitz U., Smith H. et al. Epidemiologic study of multiple sclerosis in Israel. Arch neurol. 13, 183-193, 1965. Gutensohn M. and Cole P. Epidemiology of Hodgkin’s disease. Semin Oncol. 7, 92-102, 1980.