Behavioral contrasts in twinships: stability and patterns of differences in childhood.

Semistructured interviews with 305 mothers of same-sex twins were conducted periodically over the course of the twins' first 6 years. During each of the interviews, the mothers were asked to contrast the twins on a variety of specific behaviors. From the body of reports, 5 ages and 16 behaviors were selected for analysis as most illustrative of the main developmental trends for intrapair differences and similarities. For each of the behaviors, intrapair contrasts were moderately stable across age spans as long as 18-36 months. A set of behaviors related to negative aspects of temperament and a set of behaviors related to sociability were obtained at 6 months and were replicated at later ages. Concordance rates for the identical twin pairs were uniformly higher than for the fraternal pairs, suggesting that genotypic similarity was associated with intrapair similarity for aspects of emotionality and sociability.

Relations between twins' similarity of appearance and behavioral similarity: testing an assumption.

Questionnaires rating twins' physical similarity and similarity of dress were obtained from the parents of 121 identical and 70 same-sex fraternal twin pairs. Within-pair difference scores on several behavioral measures (two intelligence tests, two perceptual tests, one reading test, one test of speech articulation, and one personality inventory) were correlated with the twins' scores for physical similarity and similarity of dress. The correlations revealed no systematic relation between the similarity of appearance and the similarity of behaviors for either the identical twin pairs or the same-sex fraternal twin pairs. The assumption that twins' behaviors are more alike because they are more similar in appearance does not seem warranted.

Twins with academic learning problems: antecedent characteristics.

Developmental histories of 46 children experiencing academic difficulties, all of whom had participated since birth in a longitudinal study of twins, were compared with a matched control group of twins. Genetic influences on the liability for academic difficulties are suggested, since identical pairs were more likely than fraternal pairs to be concordant both for academic difficulties and for preschool behavioral characteristics specific to the index group.

Twins: within-pair similarity on Bayley's Infant Behavior Record.

Summary scores for behaviors rated on Bayley's Infant Behavior Record were obtained by summing sequential ratings for same-sex twins participating in a longitudinal study. The first year scores were based on visits made at 3, 6, 9, and 12 months, by 55 identical and 27 fraternal pairs; the second year scores were based on visits at 18, 24, and 30 months, by 47 identical and 27 fraternal pairs. During the first year, within-pair correlations for almost all behaviors were stronger for identical than for fraternal pairs; in the second year, within-pair correlations for identical twins were similar to those of fraternal twins except for the more cognitive behaviors. Composite scores were calculated for two behavioral clusters--one composed of behaviors related to Primary Cognition, and the other composed of behaviors related to Extraversion. Within-pair correlations were higher for identical than for fraternal twins in both the first and second years, indicating stronger and more consistent genetic influences on the more cognitive behaviors.

Persons, situations, and time: a genetic view of behavioral change in children.

Twin children participating in a longitudinal study were observed at 9, 12, 18, 24, and 30 months of age in two settings: unstructured free play and relatively structured test taking. Behaviors relating to adaptability to the two settings were rated and the scores analyzed for evidence of continuity both across settings and across ages. Age-to-age correlations for the adaptability scores for each setting yielded a simplex pattern of low-moderate order. Correlations of individuals' scores across settings were also of low-moderate order. Within-pair correlations indicated that identical twins were more similar that same-sex fraternal twins in both settings, but correlations were consistently stronger in the playroom setting. Analysis of variance was performed to test for within-pair concordance in the direction and degree of change. Identical pairs remained significantly more similar in adaptability from one setting to another even when change in adaptability occurred. Similar results were found for change across ages; however, these differences were significant only in the playroom setting. Results suggest that situation variables contribute to the low stabilities frequently reported for personality dimensions but that the direction and degree of behavioral change are genetically conditioned.

Cognitive and personality factors associated with the double trapezium illusion.

76 pairs of 9- to 11-yr.-old twins (41 female, 35 male) who were discordant on the magnitude of error for the double trapezium illusion were given the Children's Personality Questionnaire and the Wechsler Intelligence Scale for Children (WISC). The Wilcoxon matched-pair signed-ranks test was used to determine whether the twins with relatively smaller magnitudes of error than their co-twins also manifested systematic differences on the personality factor scores and the WISC subtest scores. Among the female pairs, twins with smaller magnitudes of the illusion were relatively more stable (Factor C), more shrewd (Factor N), obtained higher scores on Similarities and Digit Span, and had longer reaction times before responding to the figures. Among the male pairs, similar trends were found but the differences were not as pronounced. The results are discussed in terms of Piaget's theory concerning children's deployment of attention and Kagan's reflective-impulsive dimension in children's cognitive judgments.

Sex and genetic differences in hair color changes during early childhood.

Hair color was assessed routinely from three months to six years for children participating in a longitudinal study of twins: 169 female twin pairs, 161 male pairs, and 60 opposite-sex pairs. Age trends, established by sampling only one number of every pair, showed marked changes in hair color for both sexes, but there was a consistent excess of light-haired males and dark-haired females. Within-pair concordance rates were calculated for same-sex pairs whose zygosity had been determined independently through bloodtyping. A high rate of concordance was found for MZ twins at every age in spite of the general change in hair color, indicating a strong genetic influence in the timing of color changes. The results are discussed in terms of accelerated maturation of females, and the need for genetic models of the inheritance of hair color which are age- and sex-specific.