It is perhaps rare for readers of British tabloid newspapers to ponder the same questions as evolutionary biologists, but that may have been the case last week. The tabloids enjoyed themselves at the expense of women suffering from a rare and often debilitating condition: persistent sexual arousal syndrome. (The Boston Globe also carried the story, though it didn't aim to titillate its readers.)

The syndrome can lead to spontaneous, almost continuous orgasms, leaving the affected woman unable to concentrate on even mundane tasks. Some readers may have wondered what causes the syndrome (unlike the tabloid editors, who didn't seem to care), but no one really knows. As a consequence, women affected can be offered no instant cure.

From wondering about what causes the syndrome, more thoughtful readers might have been prompted -- understandably -- to question why males are necessary, and indeed why sex between males and females exists at all. It is this question (the two ask essentially the same thing) that is a favorite of evolutionary biologists. This is not just because evolutionary biologists enjoy thinking about sex, but because it is one of the great mysteries of science.