Human sexuality seems normal to us, but it is bizarre by the standards of other animals. Diamond arues that our strange sex lives were as crucial to our rise to human status as our large brains. Among the aspects of human sexuality discussed are female menopause, our having sex in private, our often having sex for fum rather than procreation and the expansion of women's breasts even before use in lactation, and the battle of the sexes over parental care. These features might seem too natural to require explaination, but on reflection they prove surprisingly difficult to account for.