Three recent studies have used twin data to explore the possibility of differential contributions of heritability and environmentality to individual differences in cognitive ability as a function of ability level (Detterman, D. K., et al., Behav. Genet. 20:369-384; 1990; Bailey, M. J. and Revelle, W., Behav. Genet. 21:397-404, 1991; Cherny, S. S., et al., Behav. Genet. 22:153-162, 1992). All arrived at different conclusions: higher heritability at the low end, higher heritability at the high end, and no differential influence, respectively. The current report involves a sample of 148 identical and 135 fraternal twin pairs from the Western Twin Project who were tested on a battery of intelligence and achievement tests to further explore the issue. The results suggest no significant differences in heritability at either the high or the low end, although a trend toward higher heritability for children of higher ability is evident. Individual differences for a composite ability/achievement score showed significantly greater influence of shared family environment at the low end than the rest of the distribution. In general, results for cognitive ability and academic achievement were highly similar.