Behaving like the other sex has been observed in a number of species of mammals, although such behaviour is generally more common in the female than the male. This study discusses why such capacity is so common in sexually differentiated animals. The contributors gather together information on the generation of heterotypical sexual behaviour and on certain forms of aggression. They provide a review of the current state of knowledge from both animal experimentation and human clinical studies, looking at the role of physiological mechanisms and experiences in such controversial topics as the genesis of homosexuality.