I predict that the next generation of psychologists will wonder what all the nature–nurture fuss was about. It will seem obvious that genetic differences as well as environmental differences contribute to individual differences in cognitive abilities and disabilities. This prediction relies on the hope that psychology will continue to be a science, because in science, data reign supreme, and the data are clear: Genetics is important. Converging evidence from family, twin, and adoption studies provides a better case for significant and substantial genetic contributions to cognitive abilities, especially general cognitive ability (intelligence), than for any other domain of behavior or medicine (Plomin, Owen, & McGuffin, 1994b). Few scientists seriously dispute any longer the conclusion that cognitive abilities show significant genetic influence. (See, chapters 4 and 5, this volume, for discussions of this issue.)

The magnitude of genetic influence is still not universally appreciated, however. For general cognitive ability, the world's literature suggests that about half of the total variance in IQ scores can be accounted for by genetic variance (Chipuer, Rovine, & Plomin, 1990; Loehlin, 1989). It should be noted that the total variance includes error variance. Correcting for unreliability of measurement, heritability would be higher. Also, the world's literature includes disproportionate numbers of young children. As discussed below, new evidence indicates that adults show greater heritability. Estimating the magnitude of the genetic effect is more difficult than determining its statistical significance, but regardless of the precise estimate of heritability (the genetic effect size), the point is that genetic influence on IQ test scores is not only significant; it is also very substantial.