The new science of sex.

Two centuries ago, protofeminist Mary Wollstonecraft wrote a treatise entitled "A Vindication of the Rights of Woman" in which she theorized that men and women are essentially the same. The roles they play, she suggested, are merely social constructs. The buzz phrase since then has been that "the mind has no sex."

But there is growing scientific evidence that the mind does have a sex, and that other unexpected components of the body have a sex as well. An article in the New York Times Magazine by former New Republic editor Andrew Sullivan helped start the sex-difference debate up again early in 2000. After experiencing upsurges in various feelings since he began injecting testosterone in order to combat the effects of his disease (Sullivan is HIV positive), he started to look into the scientific research on testosterone's consequences. Although the different effects of male and female hormones have been known for some time, the research that Sullivan uncovered indicates a much more fundamental role for testosterone than was previously acknowledged. There are significant differences between men and women in their brains and genes as well.

There are two strands to this data: animal research and human research. Newborn female rats develop penises and the willingness to use them after injection with testosterone. While male zebra finches sing, females don't unless they are injected with the hormone. In species where females are dominant, meanwhile, such as certain hyenas, the females naturally have higher levels of testosterone than the males. Among animals, it seems to be testosterone that is associated with "male" behavior.

Much the same is true in humans. One study Sullivan cited showed that men (and women) with high testosterone levels "experienced more arousal and tension than those low in testosterone.... They spent more time thinking, especially about concrete problems in the immediate present. They wanted to get things done and felt frustrated when they could not." Human studies show that our testosterone levels rise in response to confrontation and sexual situations. Athletes' testosterone rises in competition, and it remains high in the event of victory, but lowers in defeat. The same is true, interestingly, of the fans following the sport.

All this holds true for both men and women. The crucial difference is that men have 250 to 1,000 nanograms of testosterone per deciliter of blood plasma, while women have 15 to 70. Testosterone is crucial in making men men--literally. It is an infusion of testosterone around six weeks after conception that makes an embryo male (the default sex for humanity is female), and it is a further rush at puberty that lowers male voices, produces body hair and builds muscles. Testosterone is clearly associated with aggression and risk-taking.

We know, however, that testosterone levels can be influenced by the social environment. An Emory University study found that an alpha-male monkey had, as expected, high testosterone levels, but that placing him in an environment with hostile females lowered his testosterone levels to those of submissive males. His initially high testosterone levels did not protect him or maintain his dominance. So while testosterone is important, it does not seem to be the final determining factor in what makes men and women different.

What about genetics, then? Males possess a Y chromosome, which women do not. The role of genetics in sex is much deeper than that, though. It is now generally accepted, for instance, that it is the father's genes that build the placenta. This is one aspect of a mysterious process known as "imprinting," whereby the genes of placental mammals seem to remember ...