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.

Retardation and twin concordance in infant mental development: a reassessment

A reappraisal is made of Nichols and Broman's conclusion that there is no evidence that genetic influences are important in infant mental development if severely retarded twins are excluded from the sample. The reappraisal touches on certain idiosyncrasies of their sample, the test scores reported, and their definition of severe retardation. Data are presented for a sample of White twins who have been followed longitudinally since birth. For Bayley Mental Scale scores at 9 months of age, the monozygotic within-pair correlation was 0.85 and the dizygotic within-pair correlation was 0.62. With 13 retarded twins excluded, the correlations became 0.81 and 0.64, respectively, with the monozygotic correlation still being significantly larger (p less than 0.01). The present results indicate that genetic factors play a role in infant mental development.

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.

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.

Hereditary components of response to the double trapezium illusion

The double trapezium illusion was studied in 68 pairs of same-sex twins ranging in age from 9 to 11 yr. Serological tests indicated that there were 39 identical and 29 fraternal twin pairs. Intrapair differences in the magnitude of the illusion were significantly less for identical pairs, indicating an hereditary component in the illusion.

Articulation proficiency in twins and singletons from families of twins

Two hundred sixty-three twins and 94 singletons from families of twins were administered the 1960 version of the Templin-Darley Screening Test of Articulation for three to eight year olds. Mean scores for the twins fell significantly below the standard test norms. This study provides additional evidence that position of birth in large families, as well as SES level, can affect development in articulation for twins and singletons alike. The within-pair similarity of articulation was significantly greater for male identical twins than for male fraternal twins. No such difference was found between the female identical and fraternal twins.

Personality factors associated with the Ponzo illusion: study using the co-twin method

29 pairs of 9 to 11-yr.-old MZ twins (11 male, 18 female) who were discordant on the magnitude of their susceptibility to the Ponzo illusion were given the Children's Personality Questionnaire (CPQ). The Wilcoxon matched-pair signed-ranks test was used to determine whether the twins with relatively larger magnitudes of the Ponzo illusion than their co-twins also manifested systematic differences on CPQ factor scores. Among males, twins with larger magnitudes of the illusion were relatively more calm and stable (Factor C), conscientious (Factor G), critical, restrained and evaluative (Factor J); among females the twins with larger magnitudes of the illusion were more calm and stable (Factor C), more shrewd (Factor N), and less apprehensive and anxious (Factor O) than their co-twins. Results are discussed in terms of Piaget's theory of children's deployment of attention and Kagan's reflective-impulsive dimension in children's cognitive judgments.

Activity, motor coordination and attention: individual differences in twins

Differences in temperament, attention span, and activity level as reported by mothers of 56 pairs of 12-mo.-old twins were related to tested differences in cognitive abilities at age 4 yr. The more active twins were found to show temper more, have shorter attention span in play, and walk earlier but to have poorer manual dexterity than their co-twins. The twins with longer attention showed less temper and walked later but had better manual dexterity than their co-twins. Twins who were less active or had longer attention span at 12 mo. had significantly higher WPPSI Performance IQs than their co-twins when tested at age 4 years. Verbal IQ differences were in the same direction but not significant.