A TWIN STUDY OF GENDER-INFLUENCED INDIVIDUAL-DIFFERENCES IN GENERAL COGNITIVE-ABILITY

In order to test the hypothesis that the etiology of individual differ ences in general cognitive ability differs in males and females, full- scale IQ data from a sample of 426 twin pairs [211 monozygotic (MZ) tw in 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 struc tural equation model of sex-limitation. Resulting estimates of heritab ility did not differ significantly in males and females (0.69 and 0.41 , respectively), and corresponding estimates of the variance due to sh ared-environmental influences (0.10 and 0.32) were not significantly d ifferent 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.