This paper proposes that mammals exhibit three primary emotion categories for mating and reproduction: (1) the sex drive, or lust, characterized by the craving for sexual gratification; (2) attraction, characterized by increased energy and focused attention on one or more potential mates, accompanied in humans by feelings of exhilaration, “intrusive thinking” about a mate, and the craving for emotional union with this mate or potential mate; and (3) attachment, characterized by the maintenance of close social contact in mammals, accompanied in humans by feelings of calm, comfort, and emotional union with a mate. Each emotion category is associated with a discrete constellation of neural correlates, and each evolved to direct a specific aspect of reproduction. The sex drive is associated primarily with the estrogens and androgens; it evolved to motivate individuals to seek sexual union. The attraction system is associated primarily with the catecholamines; it evolved to facilitate mate choice, enabling individuals to focus their mating effort on preferred partners. The attachment system is associated primarily with the peptides, vasopressin, and oxytocin; it evolved to motivate individuals to engage in positive social behaviors and assume species-specific parental duties.

During the evolution of the genus Homo, these emotion systems became increasingly independent of one another, a phenomenon that contributes to human mating flexibility and the wide range of contemporary human mating and reproductive strategies.