A twin study of gender-influenced individual differences in general cognitive ability

In order to test the hypothesis that the etiology of individual differences in general cognitive ability differs in males and females, full-scale IQ data from a sample of 426 twin pairs [211 monozygotic (MZ) twin pairs, 128 same-sex dizygotic (DZ) twin pairs, and 87 opposite-sex DZ twin pairs], with a mean age of 11.85 years, were fitted to a structural equation model of sex-limitation. Resulting estimates of heritability did not differ significantly in males and females (0.69 and 0.41, respectively), and corresponding estimates of the variance due to shared-environmental influences (0.10 and 0.32) were not significantly different from zero. Moreover, the estimate of the between-sex genetic correlation was one, with a 95% confidence interval of 0.61-1.00. Thus, individual differences in general cognitive ability appear to be due substantially to common genetic influences in males and females.