As part of a long-range study on mental functioning in old age, 53 twins (mean age 84 years) were given the Kent-Rosanoff Word Association test, a test which measures the cognitive ability to give discrete verbal association to common stimulus words. In general, the results obtained with the aging twins were similar to those previously reported for young adults tested in the early 20th century, nearly 90% of the responses being found in the 1910 Kent-Rosanoff norm tables. Our results are consistent with the trend reported in the literature toward diminished primary responses with advancing age. Unlike reports in the literature, however, the present study detected no sex differences, possibly due to the small sample size (18 men and 35 women). Moreover, Kent-Rosanoff performance was not significantly related to organic brain syndrome, as measured by psychiatric evaluation, Graham-Kendall Memory-for-Designs test, and Stroop-Color-Word test, possibly because most of the twins were diagnosed as having only a “mild” or “moderate” degree of impairment. The intraclass correlations were higher for MZ than DZ twin partners, the differences reaching statistical significance for two scoring categories, primary and unusual responses. Even though the sample was small, only six DZ and 12 MZ pairs, this finding suggests a possible hereditary component in the verbal associative behavior of octogenarians.

The classical twin study method was used to assess the relative contributions of genetic and environmental components to individual variation in several aspects of cognitive functioning. Tests of logico-mathematical concept formation, as well as vocabulary, nonverbal reasoning, and visual memory, were administered to 137 MZ and 72 DZ, same-sex white twin pairs. These children were individually tested on the Piagetian Mathematical Concepts Battery (PMCB), Peabody Picture Vocabulary Test (PPVT), Raven Coloured Progressive Matrices (PM), and a Visual Memory (VM) test. The Attitudes Toward Education (ATE) questionnaire and the Moos [63] Family Environment Scale (FES) were used to collect additional data from the parents. Twins were 4 to 8 years old, with a mean age of 71 months, and most were from middle- and upper-middle-class families. Zygosity was determined from dermatoglyphic information and responses to a questionnaire asking mothers about twin similarities and confusion between the twins by others. These data were analyzed by a simple pair concordance procedure and by a discriminant function analysis. In addition, blood typing was done on 32 pairs for whom zygosity was not possible to determine by these methods.

Previously reported patterns of intercorrelations among the 10 subscales of the FES, as well as the subscale structure, were verified by factor analysis. A factor analysis of the ATE yielded three factors: Basic Academic Education, Parental Participation, and General Utility of Education. These factors correlated significantly (P < 0.01) with various environmental indices (including father's occupation and education, Achievement Orientation, Expressiveness, etc). A factor analysis of the PMCB tasks gave some support for the existence of Piaget's underlying concepts of conservation of number, seriation, and classification.

No sex differences were found for any of the specific cognitive abilities or any of the environmental variables. Correlations with age were substantial: 0.75 for PMCB, 0.70 for PPVT, 0.59 for PM, and 0.43 for VM. Because of the high correlations with age, the effect of age on these variables was partialed out in all further analyses. PMCB correlated most highly with PM (r = 0.41), and with PPVT (r = 0.36). Nonverbal reasoning and vocabulary were relatively independent of each other (r = 0.23). Correlations between visual memory and all other tests were low.

MZ and DZ intraclass correlations for height and weight were similar to values reported in other studies. After correcting for test reliability, significant genetic variance (P < 0.01) was found for both PMCB and PPVT, and was suggested for VM. Genetic variance for PM was not significant (P > 0.05). Correction for reliability could not be employed in this case because an accurate estimate of the PM test–retest reliability is not available. There was no significant effect of age on the magnitude of the MZ or DZ intraclass correlations.

A stepwise multiple regression on the environmental variables was performed for each cognitive test. The environmental variables considered were number of siblings, parental education and occupation, the 10 FES subscales, and the three ATE factors. Age was entered first in the regression equation for each test, and it accounted for 18% to 57% of the total variance in cognitive performance. Parental education accounted for 3% of the total variance in both PMCB and PPVT performance. This was considered as an environmental influence, but the possible confounding with a genetic element in parental IQ was discussed. Achievement Orientation exhibited a significant negative relationship (R2 = 0.02) with PM performance. Cohesion in the Family was positively related to PPVT performance (R2 = 0.02). In addition, Intellectual–Cultural Orientation predicted VM performance (R2 = 0.02). Overall, those environmental variables found to have a small effect suggest the value of a warm, stimulating, supportive (but not “pushy”) family environment for normal cognitive development in young children.

Examination of the genetic and environmental results indicated that 49% of the variance in age-corrected PMCB performance was accounted for by the genetic variance (estimated from twin comparisons) and parental education. Similarly, variables identified in this investigation accounted for 60% of the variance in age-corrected PPVT performance, 29% of the age-corrected PM performance, and 32% of age-corrected VM performance.

In conclusion, this was the first large twin study to find both genetic and environmental influences on the development of Piagetian logico-mathematical concepts and other specific cognitive abilities. The results illustrate the feasibility of investigating cognitive development in a theoretical framework such as Piaget's.

Previous twin studies provide evidence for genetic contributions to individual differences in reading achievement, but the nature of those genetic effects is uncertain. Reading is a complex behavior composed of many lower-level skills including attention, memory, learning ability, and the integration of auditory and visual information; genetic influences may exert their effects through these lower-level skills. This study evaluated the relationship between auditory-visual integration (AVI) and reading achievement and assessed genetic variance in AVI and reading achievement using the conventional twin model (comparison of identical twins with fraternal twins).

The final sample consisted of 109 first- and second-grade volunteer twin pairs. Of the 89 same-sexed pairs, 57 pairs were classified as monozygotic, 31 pairs as dizygotic, and one remained unclassified; zygosity determination included examination of the intrapair similarity of genetic markers, dermatoglyphics, and/or physical appearance. The childrens' test battery included reading achievement tests (Woodcock Reading Mastery Tests) and the AVI task as well as estimates of general intellectual ability (Vocabulary and Block Design tests from the Wechsler Intelligence Scale for Children-Revised), auditory memory (Auditory Sequential Memory subtest of the Illinois Test of Psycholinguistic Abilities), comparison of auditory patterns, and comparison of visual-spatial patterns. Although the presence of heritable variation in reading achievement has been reported previously [22, 23, 30, 34, 50, 57], it does not account for all of the reliable variance; accordingly, two family questionnaires (Attitudes Toward Education and Moos' Family Environment Scale) and one dealing with first-grade method(s) of reading instruction were included to assess possible environmental factors.

The most unique aspects of this study were the young age of the children studied and the twin analyses of the auditory-visual integration test battery. Evidence for heritable variation in reading achievement was found in this young sample consistent with the results of studies using older children. The main thrust of this project was to investigate possible “lower-level” sources of this heritable variation, particularly auditory-visual integration. As a first step, it was necessary to establish the presence of a significant relationship between reading achievement and auditory-visual integration, independent of general intellectual ability. The partial correlations between the reading achievement measures and AVI (WISC-R test scores partialled out) ranged from 0.10 to 0.49 and were generally significant. Factor analysis of the children's test battery supported the idea that the integration aspect of the AVI task (auditory-visual and/or spatial-temporal) was the aspect of this skill related to reading achievement rather than auditory memory or visual-spatial ability, which also contribute to success on the AVI task. Twin analyses of the AVI task revealed evidence of the presence of heritable variation. However, the task relates to other skills exhibiting genetic variance, and it could not be determined whether aspects of the AVI task independently related to reading achievement were the sources of the heritable variation observed.

Although this study emphasized genetic relationships, environmental contributions to variation in reading achievement were also considered in a preliminary fashion. Family environmental data suggested that exposure to and emphasis on intellectual and cultural activities promote reading achievement.

In conclusion, a positive and significant relationship between reading achievement and auditory-visual integration, independent of general intellectual ability, was established for a first- and second-grade sample. One environmental variable, intellectual-cultural orientation of the family, was also related to reading achievement. Evidence indicative of the presence of genetic variance was presented for reading achievement, as previously reported in older twin samples, and for auditory-visual integration.