The intention of this paper is to review the evidence support­ ing the major thesis that a knowledge of genetic architecture within a species gives clues to the evolution of behavior. To this end, a study of some of the origins of this idea, both within genetics and psychology, will be embarked upon, together with a review of the experimental evidence supportive of it. This review will concentrate on behavioral phenotypes, though not to the exclusion of other, usually morphological, character on which the original enunciation of the proposition was based. Essentially, the rationale is disarmingly simple. The study of the gene action governing a behavioral or other characteristic, by revealing the genetic architecture of the organism or species, indicates the forces of natural selection which have moulded the genetic architecture in the way that it is observed today. Thus natural selection leaves its imprint on the genome and it is argued that a sophisticated analysis of that genome in turn allows an inferential statement about the nature of those forces. It will be at once apparent that the substructure for this type of argument is that of Darwinian evolutionary theory, which is so widely and so pervasively accepted in contemporary biology that it seems hardly necessary to argue its case.