On the organization of individual differences in sexual behavior

SYNOPSIS. Research on the sexual differentiation of mating behavior has impeded progress in our understanding of the proximate bases of individual differences in two ways. First, by viewing variation as categorical rather than continuous, and second, by not placing sufficient emphasis on the fact that males and females differ genetically in many vertebrates, environmental contributions to individual differences cannot be distinguished from genetic contributions. Just as there are two levels of organization of sex, so, too, are there two levels of organization of the sexual phenotype. Primary organization is the process of sexual differentiation that follows gonadal determination and is manifest as the morphological, physiological and behavioral aspects of the sexual phenotype. There is a second, and subsequent, level of organization, however, that is directly related to primary organization and is the basis of individual variation in sexually dimorphic behaviors. This level can be termed secondary organization. Because individual variation is the substance of evolutionary change, understanding its organization will require both new paradigms and alternative animal model systems that allow separation of the effects of genes and hormones from environmental and experiential stimuli. The plasticity of the sexual phenotype and how each individual emerges from its own unique circumstances integrates these different levels of organization. As might be predicted, recent research suggests that the relationship between primary and secondary organization and the development of individual differences in sociosexual behaviors involves more than just sex steroid hormones.