Valerie S. Knopik1 & John C. DeFries1
Although the average scores of males and females are highly similar on various tests of general cognitive ability, variances tend to be somewhat higher in males (L. V. Hedges and A. Nowell, 1995, Science 269, 41-45). Thus, the etiology of individual differences in general cognitive ability may differ in males and females. In the present study, this hypothesis was assessed by fitting a structural equation model of sex-limitation to full-scale IQ data from monozygotic (MZ), same-sex dizygotic (SSDZ), and opposite-sex dizygotic (OSDZ) twin pairs tested as control subjects for the Colorado Learning Disabilities Research Center. A total of 426 twin pairs (92 MZ males, 119 MZ females, 68 SSDZ males, 60 SSDZ females, and 87 OSDZ pairs) were used in the analysis. When the full model was fitted to the data, 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 1.00 (0.61-1.00, 95% confidence interval). Thus, individual differences in general cognitive ability appear to be due substantially to common genetic influences in males and females.
Address: Valerie S. Knopik, Institute for Behavioral Genetics, Campus Box 447, University of Colorado, Boulder, CO 80309, Telephone: 303/492-7362, FAX: 303/492-8063, E-Mail: Valerie.Knopik@Colorado.EDU
1Institute for Behavioral Genetics, University of Colorado, Boulder, Colorado 80309. 2Supported in part by NICHD grant HD-27802, HD-11681, and NIMH grant MH-16880.