Specific cognitive abilities in 5- to 12-year-old twins

Eleven tests of specific cognitive abilities were administered to 108 pairs of young twins (average age of 7.6 years). Internal consistencies are high for all measures except Raven's Coloured Progressive Matrices and Delayed Picture Memory. Two-month, test-retest reliabilities are also reported. The twin sample is representative in terms of both means and variances when compared to normative data from standardization samples, and twin correlations for height and weight are similar to those obtained in six other twin studies. Because all measures were highly correlated with age (average correlation with age was 0.64), scores were aged adjusted. Previous twin studies of specific cognitive abilities in adolescents and adults found genetic variance for nearly all tests. In contrast, our study of young twins yielded significant genetic influence for only 1 of the 11 measures, PIAT Reading Recognition, and suggested the possibility of genetic influence on 2 others (vocabulary and WISC-R mazes). Environmental influences seem to dominate, particularly for nonverbal measures, as children begin their education. In accord with other studies, we found that between-family environmental factors have an important influence on the development of nearly all of the measures of specific cognitive abilities. However, we found that our tests of perceptual speed and memory were substantially influenced by within-family environmental factors independent of error.