PERSONALITY DEVELOPMENT IN TWINS

PERSONALITY DEVELOPMENT IN TWINS

Competence in the Newborn

and;Presch~~hPeriods OF MICHIGAN'

Iv :, V1 J 1~j 12 M, Ej)1 t. , CAl.. i::fSRARX

Donald]. Cohen) M.D.) Martin G. Allen) M.D.) William Pollin, M.D.) Gale Inoi], B.A.) Martha Werner) M.A.) and Eleanor Dibble) M.S. W. This study of twins aims at a traditional goal of longitudinal research: understanding the origins of those characteristic patterns of behavior and feeling which make each child a unique person. The analysis of a young child's personality always discloses an abundance of potentiallyimportant constitutional and environmental determinants of development, over which the twin method of comparison permits some control. With the longitudinal approach, we attempt to observe, in situ} the developmental forces which appear significant from our previous investigations of the lives of adult twins discordant for Dr. Cohen was Clinical Associate, DL Pollin is Chief, Mrs. Werner is Research Psychologist, and Mrs. Dibble is Research Social Worhel', Section on Twin and Sibling Studies, Adult Psychiatry Branch, National Institute of Mental Health, Bethesda, M'aryland. Dr. Allen is Assistant Professor of Psychiatry, Georgetown University Medical School, Washington, D.C. Miss Inof] is Psychologist, Child Research Branch, NIMH, Bethesda, Maryland. D1·. Cohen now is Associate Projessor of Psychiatry and pediat-rics, Yale Uniuersity School of Medicine and Child Study Center. Axel Hoffer, M.D., now Assistant Director, Massachusetts Mental Health Center, Boston, Massachusetts, collaborated in the initiation of the stUdy and served as the Research Psyc/:!iatrist during the first year. We are also appreciative of the collaboration of Charles Haluerson, Ph.D. (nursery school), Ann Lodge, Ph.D. (developmental evaluations), and Beale Ong, M.D. and Richmond Paine, M.D. (neurological evaluations) . . An earlier version of this paper was presented at the annual meeting of the AmericanPsychiatric Association, Washington, D.C., May J, 1971, under the title, "Personality Development in 10 Sets of Twins." , Reprints may be obtained from Dr. Pollin, National Institute of Mental Health, Section on Twin and Sibling Studies, Building 10, Room 4N206, Bethesda, Ma1yland 20014. '

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psychopathology (pollin et al., 1965, 1966; Pollin and Stabenau, 1968; Stabenau and Poll in, 1968; Allen et al., 1971) . In this report, we explicate some relationships between constitutional or biological adequacy during the first week of life and competence, as assessed by several independent techniques, during the preschool period (ages 3! to 4! years) . METHODOLOGY The methodology of this study involves several different strategies. The longitudinal approach of following children from gestation to school age allows for repeated observations and the study of developmental processes. The intra-twin comparison method provides one way of reducing socioeconomic, familial, and genetic variability and provides the observers (including the parents) with a built-in "standard of comparison." The use of different techniques of behavioral assessment permits greater security in findings which are consistent between techniques, especially if the observers are initially "blind" to each other's findings. Finally, the methodology involves an attempt to integrate clinical observations and more explicit procedures for personality assessment, and to define the variables which go into clinical assessment of infants and young children. This study has been the basis for formulating hypotheses and devising instruments, which are currently being further refined and tested with larger numbers of singletons, twins, and triplets. SAMPLE Twenty-two families referred by obstetricians and Mothers of Twins Clubs were seen for prenatal interviews. The cohort chosen for follow-up consisted of all 8 families with MZ (monozygotic or identical) twins and the first 2 families with DZ (dizygotic or fraternal) twins, which were included to allow for comparison between MZ and DZ development. This paper focuses on the 5 oldest pairs (ages 3! to 4! years currently) , followed from before birth and old enough for nursery school participation: Pairs I and 2 (DZ) and Pairs 3, 4, and 5 (MZ). In all 5 families in-the present study, both parents were in the

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627

home; they were white and middle or upper-middle socioeconomic class (white-collar workers and professionals) . METHOD AND PROCEDURE

Longitudinal Study Descriptions of the prenatal interviews and observations during the first year of life have been reported previously (Allen et al., 1971). Parents were interviewed during the pregnancy (about their life histories, feelings about the twin pregnancy, and expectations), during the lying-in period (about their immediate responses to the children) ,and every 2 or 3 months thereafter (about the children's development and family functioning) . A research psychiatrist observed the deliveries, assessed the children's conditions during the first week of life, and observed the children at 2- to 3-month intervals at the time of the home visits for parent interviews. Nurses were interviewed about the children and their notes reviewed. Neurological and pediatric examinations were performed during the first week. Another neurological examination was performed when the children were toddlers. The neurologist assessed general development, "soft" neurological signs, and minor differences between the children, and in addition examined for more clearly defined neurological signs and symptoms.

Newborn Constitution Constitutional competence during the first week of life was scored by a new technique, the First-Week Evaluation Scale, FES (Table 1). Each of 6 newborn variables is rated from 1 to 5, with a score of 1 indicating a serious problem and a score of 5 indicating normal functioning. The variables are health, physiological adaptation, calmness, vigor, attention; and neurological examination. The sum of ratings in these 6 areas is the FES. A high score indicates an excellent newborn constitution as demonstrated in the first week of life. The FRS covers a longer time and broader range of variables than the Apgar score, a measure of the child's physiological competence during only the first 2 to 5 minutes of life (Apgar, 1966) . The FES scores were determined by 2 psychiatrists independently reviewing the reports of each child's first week, which included infor-

628

Donald ]. Cohen etal. TABLE 1 FIRST-WEEK EVALUATION SCALE, FES

VARIABLE HEALTH

SCORE OPERAT/O'NAL DEFINITION 5 No physical problems, high Apgar 4 3 2 1

PHYSIOLOGICAL ADAPTATION

5

4 3 2 1

CALMNESS

5 4

3 2 1

VIGOR

5 4 3 2 1

ATTENTION

5 4

3 2

L

NEUROLOGICAL

5 4,' 3.' 2

Slight problem, but nothing serious Transient problem, e.g., jaundice Serious problem, e.g., needed oxygen or exchange transfusion Significant problem, e.g., prolonged respiratory disease Normal eating, sleeping, defecating, smooth breathing Some problems, but nothing serious Difficult to feed, occasional spitting up, diarrhea Real difficulties with eating, irregular breathing Frequent vomiting, very poor appetite Comfortable, easily consoled Enough crying to be a little difficult, but calmed O.K. Somewhat fussy, easily startled, e.g., by flashbulbs Unusual amount of crying and fussing Often inconsolable, spontaneous startles, jittev¥,irritable Good muscle tone, strong grasp, assertive kicks", Relatively good strength and muscle tone Moderately sluggish, didn't lift head up More sluggish and lacking in vigor than normal Floppy, weak, lethargic Followed objects, interested in sights and sounds Frequently looked at mother during feeding, occasional response to moving objects Occasional visual pursuit, but not much interest Never followed moving things with eyes, but rare glances at mother during or after feeding No following or apparent eye contact, no response to sounds Normal newborn Nonspecific variations attributable to low birth weight Bdrderline .normalv (increased or decreased tone) Nonspeeific , neurological-. abnormalities or asymmetries ' Discrete neurological-pathology

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629

mation from the obstetrician, neurologist, pediatrician, nurses, research psychiatrist, and family. There was good agreement between the 2 psychiatrists' scores for any child (maximum difference of 1 point on the scale for any child) , and there was complete agreement between the relative scores assigned to the children in a twinship. Where there was initial disagreement, the final score was decided after discussion between the psychiatrists. Reliability testing of the FES using a large sample of children is being planned.

Nursery School Between ages 2! to 3 years, the children attended a half-day standardized nursery school session.' The children came to an attractive, well-equipped nursery school room with their mothers, who were instructed to sit quietly. After a period of free play, a man and woman teacher separately interacted with the childreri. The mother then went with one teacher to an adjoining room; the children played freely and then interacted with the other teacher. The children's behavior was measured and classified throughout the morning by 2 independent observers seated behind a one-way mirror. In addition, at the end of the session, 2 other independent observers rated the children's general behavior. Teachers and observers had no previous knowledge of the children. Measures and ratings from different observers were averaged for each variable. Four types of data were collected: (1) general behavior ratings, e.g., play involvement and vigor; (2) amount of talking; (3) play activities; and (4) attention span. Because the ratings contain a basis of comparison to a standard (i.e., the observers' assessment of how this child's behavior compares with other children seen in the same nursery school setting) , the ratings are reported as "raw" data. The measures of talking, play, and attention were normalized in relation to the nursery school's pilot cohort in order to provide a meaningful basis of comparison to a standard." 1 Nursery school studies were conducted in the Section on Child Behavior (Charles Halverson, Ph.D., Chief) of the Child Research Branch, NIMH (Richard Q. Bell, Ph.D., Chief) . Description of this approach to understanding preschool behavior can be found in Bell et al. (1971). 2 Behavior ratings were on an ll-point scale. The behavior variables rated and interobserver reIiabiIities (corrected r by Spearman-Brown formula) were as follows:

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Rorschach Behavior The Rorschach was administered to each child at 3! years by an experienced tester with no previous knowledge of the children. The situation was used by the experimenter as a structured technique using a novel task with an adult. The child's general behavior during the testing, and not simply the content of his verbalizations, was rated by the examiner using a newly devised Situational-Task Rating Scale. This scale assesses social, emotional, and cognitive functioning, and verbal content only in the sense that it may reflect such variables as the child's flexibility (imagination, creativity) , language skills, and, in short, his style in meeting this novel task. In this way the extended use of the Rorschach is different from its classical clinical use as a projective tool with a child or adult population. Nineteen variables are rated from 1 to 5, with 1 indicating the worst performance and 5 indicating the best ." For example, moving flexibly from one percept to another on the same card and from card to card is rated 5, while rigid perseveration in response to several cards is rated 1. Entering play involvement (.87) , exploration (.90), vigor (.75) , originality (.85) , inability to delay (.88), frenetic behavior (.86) , fear of strangers (.96), fear of environment (.92), dependency (.86) , coping (.87) , animation-expression (.92), tractability (.91), and excitability (.89). Measures of talking, play style, and attention were derived in the following ways: (1) talking was measured by di viding' the number of 30-second intervals during which the child spoke at least once by the number of 3O-second intervals during which he was observed; (2) play style was determined by the proportion of time spent in various types of play, e.g., thematic or tactile; (3) attention span was determined by the lengths of episodes of pla y" and the number of shifts of attention. For data analysis, ratings on the ll-point scale were converted to proportions (e.g., a rating of 5 = 5/11 = .45). T alking, play style, and attention measures were converted into a more meaningful score by seeing how each child's score fit into the rank order of scores for the 30 pilot children from the nursery's longitudinal study. Thus, if a twin talked as much as the most talkative cohort child; his rank was given as 30; if the same as the least talkative, his rank was given as 1. For convenience in analysis, ranks were converted into proportions, and stated in decimals, e.g., a twin whose talking score was equivalent to the highest in the pilot cohort was given the final score of 30/30 = 1; the lowest score would be 1/30 = .03. s The Situational-Task Rating Scale, used by us in several experimental situations, appears to be sensitive and reliable in our hands; detailed reliability studies have not been completed. A copy of the form is available on request from the authors. The vari ables include ratings of the child's style in entering the situation, initial response to the task, boldness of speech, language skills, articulation, social competence, fearfulness, attitude to examiner, playfulness, motivation, intelligence, motor control, frustration tolerance, attention span, distractibility, concentration, flexibility, and tendency

to act out rather than verbalize concepts.

Personality Development in Twins

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the situation confidently is rated 5; crying and requiring strong encouragement is rated 1. The Total Rorschach Score is the sum of the scores on the 19 variables of the Situational-Task Rating Scale; the best possible score is 95. The score is a reflection of general development or the overall way in which the child copes with the novel, interpersonal situation.

Developmental Testing Development was formally assessed at ages 3t to 4t by another highly experienced psychologist with no previous knowledge of the children. The Stanford-Binet was administered and detailed behavioral observations were reported.

Child Psychiatric Evaluation A child psychiatrist, initially unfamiliar with the children's histories, assessed their social, emotional, and intellectual development at about 3 years of age, and again one year later. The children were engaged in play interviews, separately and together. The psychiatric assessment included the children's social relationships, play, speech, motor skills, attention, anxiety, and level of cognitive development. Family Studies When the children were 3t to 4t years old, both parents were interviewed on 2 to 4 occasions by a research social worker who had no information about IQ or nursery school behavior. The standardized interview included detailed items about the childhood of both parents, the background and course of the marriage, the family's social, emotional, and economic functioning, and the development of the twins. RESULTS AND ANALYSIS

Some observations of the families during the twins' first year of life have been reported previously (Allen et al., 1971) .

Newborn Constitution The First-Week Evaluation Scale (FES) scores ranged from 15 to 30. The Apgar scores at 2 minutes forall 10 children were either 9 or 10.

Donald]. Cohen et al.

632

For the purposes of analysis, the FES scores were divided into 3 groups of children: High (mean FES 29), Middle (mean 25) , and Low (mean 16) (see Table 2) . The High and Middle FES groups were close in FES and were separated only for analysis; they both contrasted with the Low FES group; it is on the contrasts between the best and least endowed children that we shall focus. The FES variable that best discriminated between High and Low FES groups was the most general one, "Health." The High FES group, in comparison with the Low, contained relatively more girls, more firstborn chil-

TABLE 2 FIRST-WEEK EVALUATION SCALE GROUP High FES

Middle FES

Low FES

CHILD 4A 1A SA

FES 30 29

3A

26

4B .2B 5B

25 25 25

1B 2A

17 16

3B

15

GROUP MEAN 29

28

25

16

TABLE 3 FES GROUPS AND DISTRIBUTIONS BOYS GIRLS FIRSTBORN TWIN (A) SECONDBORN TWIN (B) HEAVIER TWIN LIGHTER TWIN VERTEX DELIVERY BREECH DELIVERY

HIGH FES 1 2 3

MIDDLE FES 3 1 1

LOWFES 3

o

3

2

2 1 3

2 2 3

1 2 1 2

o

1

o 1

Personality Development in Twins

633

dren (Twin A) , more children who were the heavier of the set, and more children of vertex delivery (Table 3) . The MZ twins seemed more alike than the DZ: the mean FES difference between the 2 children for the MZ sets (sets 3, 4, and 5) was 6.3; for the DZ sets (sets I and 2), 10.3. Individual children's FES and group FES scores were used in analysis of preschool behavior assessments (reported below) . Individual FES scores were correlated with preschool variables by the Spearman 'rank-order coefficient, r, (Tate and Clelland, 1957) . FES groups were compared using group mean scores. In addition,FES groups were compared qualitatively (e.g., in relation to psychiatric and family assessments) .4

Nursery School The nursery behavior of High and Low FES children contrasted sharply (Table 4) . The typical High FES child talked more and coped better (e.g., succeeded in reaching his goals and carrying through intentions) ; he was involved with his play, which was original and included mature, structured home-situation themes; he was appropriately tractable (followed the teacher's suggestions) , but was also emotionally expressive and animated. In contrast, the Low FES child was markedly more fearful of strangers and dependent on his mother; his style of play was less well organized and included more gross play, frenetic activity, and relatively immature, but highly engaging tactile activities (e.g., play with dough and water); he was less able to inhibit his behavior to achieve an end. Middle FES children were intermediate between High and Low FES children on 11 variables. For the remaining variables, they resembled High FES children (painting, originality, vigor; fearlessness, tractability), but had a somewhat longer attention span, less excitability, and more animation. 4 Spearman rank-order coefficients, r , were calculated for the correlation between a d child's rank order for FES and his rank order for each nursery school variable, Rorschach variable, and IQ score. The I'd varies from +1.00 (perfect direct relationship between 2 variables) to -1.00 (perfect inverse relationship between 2 variables); the more I'd departs from 0, the stronger the relationship. To compare FES groups, group mean scores were calculated for each variable. Differences between group means reflected the magnitude of differences between the groups. Absence of overlap of individual scores between two FES groups was considered prima facie support for meaningful group differences.

Donald ]. Cohen et al.

634

TABLE 4 NURSERY SCHOOL VARIABLES AND CORRELATION WITH FES 1 HIGH FES LOW FES GROUP GROUP MEAN MEAN

DIFFERENCE IN MEANS

CORRELATION BETWEEN VARIABLES & FES

BEHAVIOR CHARACTERISTICS OF HIGH FES CHILDREN Talk Thematic play Coping Painting Originality Tractability Animation-expression Play involvement Exploring Vigor

.76 .68

.35 .30 .54 .61 .55 .60 .37 .82 .70 .59

.41 .38 .23 .22 .21 .14 .13 .12 .06 .06

.27 .43 .16 .25

.80 .83 .-.52

.13

.39 .49 .78 .27 .31 .60

.53 .40 .36 .32 .26 .18 .15 .14 .10 .06

.77 .83 .76 .74 .50 .94 .76 .65

.67** .58* .75*** .38 .30 .56* .26 .57* .04 .01

BEHAVIOR CHARACTERISTICS OF LOW FES CHILDREN Fear of strangers Tactile play Gross play Dependency Disorganized play Inability to delay Attention Frenetic Fear of environment Excitability

.31 .63 .13 .21 .54

.~7

-.47 -.23 -.41 -.51 -.33 -.28 -.13 -.26 -.23 -.18

1 Variables are separated into those demonstrated by High FES children and those demonstrated by Low FES children based on differences in the group means for High and Low FES groups and Spearman rank-order coefficients (when positive, directly related to FES; when negative, inversely related to FES). Within each list, the variables are ordered in terms of decreasing differences between the group means. Spearman rankorder coefficient, rd, represents correlation between FES and each nursery school variable on the basis of the results for all 10 children. *p<.10 **p<.05 ***p<.02

Personality Development in Twins

635

As reported in Table 4, the 5 nursery school variables which correlated significantly with FES (and the levels of statistical significance) were the following: coping (rd .75, p<.02), talking (rd .67, p<.05), play involvement (r, .57, p
Rorschach Behavior The Rorschach situation revealed major differences between the 3 groups of children. The mean values for the Total Scores of each group were the following: High FES 'gro\lp-87; Middle FES-78; and Low FES-61. The correlation between FES scores and the T 0tal Rorschach Scores was highly significant (rd .82, p<.01) . Overall, the HighFES children were more competent; their initial response to the examiner and task was more positive; their speech was more bold, linguistically mature, and well articulated. They were more playful, motivated, and well adjusted. The Low FES chil-

636

Donald ]. Cohen et al.

dren were more fearful and perseverated much more, indicating more anxiety. Their behavior was less well organized and controlled. The Middle FES children were intermediate on 13 variables, better than, the High FES on 3 variables, and worse than the Low FES on 2 variables.

Developmental Testing When the IQ scores for the 10 children were ranked, the High PES children were all in the top half of the cohort, the Low FESchild.'ren in the bottom (Table 5). The mean IQ scores were: High,FESchildren-133; Middle FES-116; and Low FES-IOr. The correlation between FES and IQ scores was at borderline significance (I1d .59; p<.IO) . The psychologist's verbal descriptions were remarkably consistent for FES groups. The High FES children were described as confident, assertive, imaginative, verbal, and social. The Low FES children were described as relatively passive, immature, stubborn, and fearful.

Child Psychiatric Evaluation The child psychiatrist's narrative descriptions of the interviews and summary statements about the children's personalities revealed clear personality differences between children in a twinship and between sets of twins. All of the High FES children were unusually well developed, mature, outgoing, and imaginative youngsters-who were independent and well organized, without being excessively constricted. Two out of 3 Low FES children were quise.iinmamre, I being very frightened and inhibited (2A) , and the ather rather silly and flighty (IB) . All High FES children had unusually well-developed language skills, without articulation disturbance; all Low FES children had speech immaturities, including stuttering, baby talk, and lisping. Two out of 4 Middle FES children (4B and 5B) had mild articulation difficulties. As will be discussed below, the influence of the family environment was most clearly seen in the development of the Middle FES children. One of the 4 Middle FES children (2B) was very similar to his brother (2A) in the LowFES group: immature, frightened.andlacking in vigor. Another Middle FES child (4B) was-unusually poised and verbal, resembling her sister (4A) in the High FES group. The

637

Personality Development in Twins

TABLE

5

STANFORD-BINET IQ AND TOTAL RORSCHACH SCORES

FES GROUP High

CHILD 4A lA

TOTAL RORSCHACH SCORE 87 94

IQ 127 132

5A

68

139

83

133

3A

85

120

4B

84

106

2B

83

89

5B

61

150

78

116

1B

62

111

2A

64

79

3B

59

113

61

101

MEAN Middle

MEAN Low

MEAN

FROM IQ EXAMINER'S COMMENTS "Quick, goal-directed." "Warm responsiveness, rapid-fire speech." "Better motor control, innerdirected, persistent, bright." "Confident, assertive, vocal." "Less friendly and talkative, more resistant." "Shy, whiny, immature socially & intellectually, better speech." "Dependent, friendly, exceptionally bright, poor articulation." "Distractable, sweet, dependent, restless." "Refusals, negativism, immature socially & intellectually." "Clinging, less positive relationship, poorly motivated."

. other 2 Middle FES children remained developmentally intermediate. Family Studies

The social worker's evaluations relied on the parents' anamnestic reports about their lives and the children's development. Her findings were consistent with, and gave broad shape to, the impressions about

Donald l- Cohen et

638

at.

family life and the children's development which were derived from the series of psychiatric interviews conducted from the time of the children's births. Because of this complementarity, information from both sources will be reported together. Clinical assessment of the families indicated major differences in parental mutuality, maturity, involvement with the children, and what we came to feel was the major thrust of the style of parenting (Table 6) . For example, one father discouraged lively play and running; another initiated roughhousing with his boys. In one household, the maternal grandmother, who lived with the family, was very actively involved in mothering' the children; in another family, the mother maintained an intense, artistically sensitive, and rather exclusive relationship with her sons. One household was always neat and structured, another frequently in lively chaos. As previously described (Allen et al., 1971), from the first months of the children's lives, families reported developmental differences between the twins. High FES children were reported by their parents to be developmentally more advanced and assertive; and Low FES children were described as relatively passive and irritable. The detailed psychiatric and casework evaluations gave a rich picTABLE 6 FAMILY STYLES FAMILY

FATHER Inv.'

+++ 2

+

3 4

++ +++

5

++++

MOTHER

Inv.! DX 2 Normal, hard-driving Obsessive/ schizoid Depressive Normal/ obsessive Narcissistic/ adolescent

+++ ++

++++ ++++ ++++

MAJOR FEATURE

DX2 Normal, en- Autonomy Vigor thusiastic Depressive Detachment Passivity Hysterical Normal/ hysterical Normal/ mildly depressive

Inv, = Involvement (+ + + + = Strong involvement; 2DX = General characterological diagnosis

1

FOSTERED RELATIONSHIP

Dependency Creativity Autonomy Maturity Autonomy

+ =

Intelligence

Weak involvement)

Personality Development in Twins

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tureof family and personal functioning. These studies indicated ways in which expectations and feelings of parents were responsive to the child's early constitutional competence. For example, the smaller twin was likely to be seen as more vulnerable to illness and, in general, more "sensitive." Parental attitudes were also importantly shaped by the parents' own life experiences and personality structures, and by the nature of the marriage. For example, a father with an organic illness and relatively low self-esteem felt close to the child who had an organic illness during the first month of life. In general, differential feelings and attitudes about the 2 children led parents to behave differentially toward them; and this differential reinforcement, in turn, progressively shaped the children's developing personalities. One example of a type of complex relationship between constitutional competence and parental behavior was observed in respect to parental linkage. In this group of families, regardless of which child the father felt closer to during the first months of life, by the time the children were toddlers, the fathers appeared to be more involved with the child who was developmentally more advanced.

Medical History The children had a large number of medical problems subsequent to the first week of life. The Low FES children suffered more frequent and severe illness, and required more hospitalization. The major medical problems encountered in the 5 pairs of twins were the following: in the High FES group, one boy (5A) had several febrile convulsions. In the Middle FES group, the brother (5B) of the High FES boy with convulsions also had one febrile convulsion and, in addition, needed a herniorraphy. In the Low FES group, one child (3B) had surgery for pyloric stenosis; another (IB) had strabismus and life-threatening pneumonia; and the third (2A) had persistent vomiting for the first 3 months, eczema, strabismus, and needed eyeglasses. DISCUSSION Although the sample is relatively small, the findings are convincing because of the longitudinal, in-depth observations; the consistency of results from different techniques of personality assessment (including mutually "blind" observations and different investigators); sta-

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tistical significance of the correlations; and marked differences in means, and absence of overlap between values, for High and Low FES groups. This longitudinal study of twins and other work with twins (Pollin et al., 1965, 1966; Pollin and Stabenau, 1968; Stabenau and Pollin, 1968; Allen et al., 1971; Gifford et al., 1962, 1966) lead to the generalization that the better endowed newborn develops into the more secure, trusting, and developmentally advanced child during the first year of life and the more assertive, actively coping, and competent toddler. Our observations support the special importance of early differences in arousal and attention patterns in shaping early development (Wolff, 1966) . The children's differing abilities to attend to the environment and to modulate their responses to internal and external stimulation were noted quite early by parents. In general, the adaptive value of an infant's being attentive and calm is consistent with several lines of clinical and experimental evidence about stimulus barriers and attention (Bergman and Escalona, 1949; Kessen et al., 1970) . For example, Bell et al. (1971) found that coping as a toddler is associated with two characteristics in infancy: (a) a long latency period before starting to cry when a nipple on which the infant is sucking is withdrawn; and (b) a high threshold to tactile stimulation. It has also been shown that there are marked differences at times of extreme stress (e.g., circumcision) in cardiac acceleration, and in infants' abilities to suck and inhibit distress (Cohen, 1970), as well as to make use of a pacifier to inhibit hunger crying (Cohen, 1967). Wolff and White (1965) have demonstrat.ed that the optimal time for eliciting visual following in infants is during the period after : feeding when the child is attentive and calm. Such moments of comfort when the child is able to scan visually his mother's face are probably of major significance in the early development of mutual attachments; it is the better endowed, High FES child who is more likely to have frequent, mutually satisfying periods of this type during the first weeks of life. Thus, family responses interact with constitutional endowment. The attentive and comfortable infant is more easily consoled; he needs and receives less care-giving from his mother, but probably is involved, for this reason, in relatively more mutually satisfying en-

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gagement. Our observations support the impression that the better endowed infant does not tend to arouse the same degree of guilt, anger, and subsequent ambivalence that is aroused by the fussier child. The capacity to be easily comforted and remain attentive is associated with the ability to modulate responses to both internal and external stimulation. While the better endowed infant is more attentive to the world, he is also less frightened by sudden changes. Interestingly, in our series, this child is less likely to be ill or hospitalized during the first year of life. All in all, the better endowed infant develops a greater trust in his body and in the world, and a less ambivalent relationship with his family (Erikson, 1959). Normal attachments unfold without excessive dependency, and he is less likely to experience upset with strangers or separation. The father gravitates to the child who tolerates his rough play and who is more assertive, e.g., who can grab desired objects held by his sibling. The fortunate child becomes more exploratory and autonomous during the second year, secure in his abilities and relationships. His assertiveness is reinforced by his parents; because of his trust and reinforced self-esteem, he may be more capable of open expression of negativism for a period of time, an indication of his expanding autonomy. In a myriad of transactions, children and family reciprocally reinforce perception and self-perception of competence (or incompetence), assertiveness (or passivity) , and dominance (or submission) . One danger in this dichotomization is that both children's real needs and competencies may, at times, be overlooked. This schema is simplified and requires an understanding of specific environmental factors. For example, we have observed a twin show remarkable gains following successful mastery of a stress which was accompanied by increased parental closeness. At times, mastery or overcoming stress and coping with psychological sensitivities in early childhood may be associated with later developmental assets, such as the capacity for creativity and warm social relations (Zetzel, 1970). This would be consistent with the areas in which the Middle FES children excelled in nursery school, e.g., originality, animation and expression of feelings, and low level of fear. Another important domain to consider in relation to the children's growth from infantile dependency to autonomy relates to the character structure of the parents and to the conclusions from E. Kris's longitudinal study of "the

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mutual influence of equipmental and environmental factors on the developmental processes of identification" (Ritvo and Solnit, 1958, P: 65) . For example, in family #2 of this paper, the father is relatively obsessive and the mother inhibited; their sons. .DZ, are very similar in their immaturity, lack of spontaneity, and inhibition of imagination as toddlers, although their newborn constitutions were different (see Escalona, 1963) . Some studies of twin development have led to the suggestion that the characteristics of assertiveness and sociability may be split between the 2 children (Gifford et al., 1962, 1966). For the twins we have studied, this does not appear to be the case. In this cohort, overall competence, coping, assertiveness, and language and social skills characterized the well-endowed child; the relatively less well-endowed child was characterized by his fearfulness and dependency. The impression that the less well-endowed twin becomes more socially advanced Illay result from observations of his close tie to his mother, which we have also seen . However, while he may talk more with her, the less well-endowed child usually talks less well and less often in other social situations, where he is likely to be more inhibited than his sibling. In general, we have been impressed that cognitive, social, and emotional maturity are closely linked; in a set of twins, one child is likely to be advanced in all spheres. This hypothesis, of course, will need much further testing and elaboration, but is consistent with our research on adult twins, which has demonstrated that the less wellendowed child is mu ch more vulnerable to later psychopathology (pollin et aI., 1965, 1966; Poll in and Stabenau, 1968; Stabenau and Poll in, 1968; Mosher et al., 1971) . SUMMARY The longitudinal study of twins by the intra-twin comparison method is an approach to studying development in which each child has a control who shares many genetic, socioeconomic, and environmental variables and through which an investigator can focus on a more limited set of potential determinants of development. This study emphasizes that even if there is genetic identity, twins may be quite different constitu tionally and behaviorally from birth. The present study analyzes some relations between early constitutional differences and

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behavior at ages 3! to 4!. We have tried to understand ways in which familial responses to constitutional differences relate to personality development in early childhood. The hypotheses from this study will be tested as the other 5 pairs of twins in this longitudinal study become older, and in our ongoing, large-sample study of personality development during the preschool years. For this cohort, early constitutional differences were strongly associated with specific behavioral patterns of preschool age children. The group of newborns characterized by good health, physiological adaptation, vigor, good attention, calmness, and normal neurological examinations developed into the better coping, more competent, trusting, and linguistically more mature preschoolers; the group characterized by problems during the first week of life developed into more fearful and frenetic children. These associations between constitution and preschool development may be shaped in part by the ways in which the children's constitutional differences and subsequent development relate to the life experiences and feelings of the parents.

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