The sexual behavioral system evolved to motivate reproductive acts. The primary strategy for achieving this goal is to approach a potentially fertile partner, convince him or her to have sex, and engage in genital intercourse. However, human offspring are vulnerable throughout an exceptionally prolonged development period. Hence, in ancestral environments, sexual partners needed to stay together long enough to jointly care for their offspring during the period of maximum vulnerability, thereby increasing the offspring’s chances of survival and future reproductive success.
Over the course of human evolution, selection pressures have produced mechanisms that keep sexual partners attached to each other for an extended period, motivate them to remain in a committed relationship and engage in co-parenting behaviors following an offspring’s birth.1 Several characteristics of human sexuality suggest that the sexual system has been “exploited” by evolutionary processes to serve such a function.2 Humans, for example, tend to have sex in private and to sleep together afterwards. Humans also frequently have sex in the “missionary position”, which, by contrast to the typical sex positions of most mammals (e.g., canines), allows partners to maintain face-to-face contact during sexual intercourse. These and other similar behavioral tendencies foster extended close contact between sexual partners and make them feel more intimate with each other, thereby promoting enduring attachment bonds between them.