Sex differences in early verbal and non-verbal cognitive development

The present study of over 3000 2-year-old twin pairs used a sex-limitation model to examine genetic and environmental origins of sex differences in verbal and non-verbal cognitive ability. Girls scored significantly higher on both measures (p < 0.0001), although gender only accounted for approximately 3% of the variance in verbal ability and 1% of the variance in non-verbal cognitive ability. For the verbal measure boys showed greater heritability than girls. Also the twin-pair correlation is significantly lower for opposite-sex twins than for non-identical same-sex twins. This indicates that individual differences in verbal ability include some sex-specific factors. Non-verbal cognitive ability did not differ in aetiology for boys and girls. We conclude that genetic and environmental influences differ for girls and boys for early verbal but not non-verbal development.