"Why is Sex Fun?"

Physiology Professor Jared Diamond, an evolutionary biologist, was well into a study of birds when he got the idea for his book, "Why Is Sex Fun?: The Evolution of Human Sexuality." (Basic Books)

"We're used to thinking of birds as having unusual sex lives," he said. "In fact, birds are normal. We're the ones with the weird sex lives."

So Diamond set out to explain why human sexuality developed as oddly as it did. He explains why humans insist on privacy for sex, why men don't breast-feed their babies, why women undergo menopause and why the human penis is "so unnecessarily large." The book's title question turns out to be "the most difficult question in human sexuality," Diamond said. He argues that the purpose of fun sex changed as humans evolved.

"It started as a means by which women could distribute their favors, so when they became pregnant, each male would think, 'It might be mine,' and not kill the child when it was born," he said. "Then it ends up being the reason the husband had to stay home, because if he wandered off, it might be the day his wife was fertile and in bed with some other man."

And as to the question of phallus size? "You'll have to read the book for that," he said. Here's an excerpt from Amazon.com.

- Carroll Lachnit