Sex on the Brain presents a convincing case that we're products of both our biology and our culture - and that the two perform an intricate dance whose steps are, to some extent, ones we can choose.

Pulitzer Prize-winning science writer Deborah Blum has synthesized research so new - from the fields of evolutionary biology, anthropology, animal behavior (especially primatology), neuroscience, psychology, and other disciplines - that scientists are just beginning to publish it. She provides the best picture yet of the biological underpinnings of the differences between the sexes.

Examples of what she has discovered include: Men's testosterone levels drop when men are in happy marriages, so dramatically that some scientists speculate that women may use monogamy to control male behavior. On the other hand, new studies show that women in high-stress professions display a rise in testosterone levels (and possibly competitive behaviors). A study done with sweaty T-shirts suggests that a man's scent carries information about his immune system.

And women find that scent sexiest when the male immune system is different from their own - which just happens to make for healthy, diverse genetic matches. The exceptions are women using birth control pills, who consistently choose men with the "wrong" immune systems. Some scientists think that many men are attracted to blond women because of a male predisposition to choose youthful mates.

Fair hair is generally considered a biological indicator of youth, especially among certain races - since more children than adults are blond.