Genetic aspects of intelligence.

Abstract

What do we really mean when we say we want to understand the genetics of IQ (or nose length)? There are three levels at which we might be operating. Most basically, we might want to know how the primary products of all the structural genes and their controlling elements enter into the synthetic and metabolic processes that underlie central nervous activity and the way in which that activity is modulated by external inputs. It is in this sense that we understand the genetics of protein coat formation in A phage. But no serious person would suggest that genetic analysis of intelligence in man is anywhere near such a level of understanding. We may even doubt that when the tools and concepts for such an analysis become available, anyone will bother using them for an analysis of intelligence. Second, and much closer to what is possible in higher organisms, we may be content to carry out a phenogenetic analysis at a gross phenomenological level. That is, we may simply wish to get our hands on as many different genotypes as possible, allow these genotypes to develop in a wide range of environments, varying in as many different environmental dimensions as seem relevant for the species, and plot the norms of reaction for the various genotypes. Indeed, this study of norms of reaction is the proper object of research-if we are interested in knowing how various historical changes in human social organi­ zation and educational practice will affect human behavior. This is the only correct sense in which we can study the nature-nuture problem, the problem of the interacting genetic and environmental causes of phenotype (31). It is in this sense that we analyze the genetics of larval viability in Drosophila (8). But even this level of investigation is denied us for human traits, most especially behavioral traits, because we simply cannot replicate human genotypes over and over and follow their development in different environments. Indeed, we do not even know what we mean by environment in this case since it presumably includes the overwhelming complexity of social milieu and is itself an autocor­ related developmental process.