Human sexual behavior differs from that of most other mammals, including most other primates, in that we have no detectable ovulation and sexual consciousness. Sexual attraction, norms, and attitudes are strongly tied to cultural concepts of personality, standards of beauty, and individual psychologies.

It is possible that this difference may have evolved early in hominid history as a mechanism to strengthen and increase the direct involvement of males in the care and nurturing of offspring. In essence, males were added to the standard primate family unit that had consisted of the young and females. Sexual interest would enhance motivations to form personal, emotional, and economic bonds between parents. The evolution of bipedalism was a catalyst to this change, and an integral part of this bonding process, improving mobility and the acquisition and sharing of resources among family members.

The antecedents of our sexual behavior are reflected by examining the social behavior of one of our close relatives, the bonobos. Humans are a sexually dimorphic species. Many of our dimorphic features may be understood in the context of the different roles likely played by males and females among the early hominids. Some features make sense as clear, visible signs of male or female identity. Still others, shared by both sexes, can be seen as evidence for the importance of personal sexual attraction having replaced the automatic, innate reproductive signals of most primates.

Sex refers to differences between male and female biological characteristics. These differences are translated by cultural systems into the identities and roles of men and women. These categories are referred to as gender, and they differ widely from culture to culture and from time to time.

Cultural universals are behavioral patterns found in all societies. Two of these universals may be explained by our identity as the sexual primate: marriage and incest taboo. Both universals may be explained as cultural manifestations of biologically based themes: the cooperative bond that was the adaptive focus of the evolution of our sexual behavior.