In her 1958 Presidential Address to APA, Anne Anastasi urged psychologists to move beyond the question of studying “how much” variance was accounted for by genetics and environment and instead to address the question of “how” genotypes were translated into phenotypes. Despite the passage of nearly 40 years, Anastasi's challenge has not been taken up. Today, we know much more than we once did about the role of genetics in intelligence, the distribution of alleles at neurologically relevant sites, and the molecular chemistry of brain functioning, but we still know as little as ever about the mechanisms through which genetics get translated into phenotypic intellectual expression.

In this chapter, we endeavor to say something about the process that translates genes into phenotypes and, in so doing, say something about the environmentally loaded nature of heritability and the genetically loaded nature of the environment. While acknowledging the important role that genetics plays in intellectual development, we shall nevertheless take issue with the traditional “percent of variance accounted for” model of behavior genetics. In its place, we will suggest that traditional heritability measures do not inform us about the manner in which genes exert their influence, nor even about the organism's genetic potential. Rather, they tell us what proportion of individual differences in already actualized genetic potential has been brought to fruition by the prevailing environment. Left unknown, and unknowable from the traditional model, is (1) the amount of the organism's unactualized genetic potential, and (2) the absolute level of intellectual competence around which individual variation occurs.