Genetics and intelligence: Recent data

During the last three years, cognitive data have been reported for 4,639 pairings using the family design, 2,540 pairings using the adoption design, and 2,164 pairs of twins. Together, these extensive data point to less heritable influence on IQ than is indicated by the widely cited older data. The difference between the newer and older data could be due to environmental or genetic secular changes in the population or to methodological differences. Contrary to usual assumptions, the newer data also suggest that more of the environmental influences for parents and offspring and for older siblings operate within families (making family members different from one another) than between families (making family members similar to one another).