Children increasingly resemble their parents in cognitive abilities from infancy through adolescence. Results obtained from a 20-year longitudinal adoption study of 245 adopted children and their biological and adoptive parents, as well as 245 matched nonadoptive (control) parents and offspring, show that this increasing resemblance is due to genetic factors. Adopted children resemble their adoptive parents slightly in early childhood but not at all in middle childhood or adolescence. In contrast, during childhood and adolescence, adopted children become more like their biological parents, and to the same degree as children and parents in control families. Although these results were strongest for general cognitive ability and verbal ability, similar results were found for other specific cognitive abilities--spatial ability, speed of processing, and recognition memory. These findings indicate that, within this population, genes that stably affect cognitive abilities in adulthood do not all come into play until adolescence and that environmental factors that contribute to cognitive development are not correlated with parents' cognitive ability.