Findings in the field of neuroendocrinology, which explores the functional relationships between hormones and the brain, have indicated that the release of testicular (androgens) and ovarian (estrogens) hormones during critical periods of brain development exert a profound effect on the genesis and survival of neurons in specific brain areas, resulting in sex differences in reproductive behaviors. Sex differences are not restricted to the reproductive sphere, however. They are found in a broad range of nonreproductive behaviors such as aggression, locomotor activity, play behavior, and learning abilities. While the debate on whether sex differences in cognition are determined by biological or sociocultural factors is long lasting, increasing experimental evidence indicates that sex differences could be determined by genetic or hormonal factors. Support for this conclusion is of four kinds. First, sex differences have been reported in various learning abilities that depend on neural...