Many animal behaviors differ between the sexes and are therefore referred to as sexually dimorphic (dimorphic means having two forms). Most of these sexually dimorphic behaviors are part of the reproductive repertoire. A good example is apparent in songbirds. In many species, the male produces complex song, whereas the female does not. The production of song in male birds arises from the activity of specific brain nuclei whose growth and connectivity depend on the presence of testosterone during a critical period of development (see Box B in Chapter 24). In rodents, many sexually dimorphic behaviors are also associated with reproduction. Examples are priming of the genitalia for sexual intercourse, and a stereotypical position assumed while having sex (lordosis for females, mounting for males). Just as courting and behaviors associated with the sex act can be dimorphic, other reproductive behaviors such as building nests, caring for the young, foraging for food, nursing, and so on can take two different forms in females and males. In humans, the different behaviors of males and females can be far more subtle, including one's sense of sexual identity, the choice of a sexual partner, and behaviors that are not related directly to sexual or reproductive function, such as spatial thinking and use of language.

In both human and animal examples, behavioral differences are based on the details of the underlying neural circuitry. Accordingly, neurobiologists have long looked for differences between the brains of females and males that might explain sexually dimorphic behaviors and, as described in subsequent sections, have found many examples. These differences in the nervous system, like the behavioral differences they give rise to, are also referred to as sexually dimorphic. Bear in mind, however, that while brain differences in animals like rodents often have two distinct forms, in human females and males these neural differences probably vary along a continuum.