On measure after measure, Baumeister found, women were more sexually adaptable than men. Lesbians, for instance, are more likely to sleep with men than gay men are with women. Reports indicate that women's attitudes to sex change more readily than men's do. For instance, in one study, researchers compared the attitudes toward sex of people who came of age before and after the sexual revolution of the 1960s; they found that women's attitudes changed more than men's.

The sexual patterns of couples also indicate that women are sexually adaptable. The female libido fluctuates throughout the month, based on ovulation and the menstrual cycle. But couples do not appear to have sex more or less frequently based on what time of the month it is. Rather, couples have sex in weekly and daily patterns--in the evenings and/or on weekends. A 1991 survey looked at how the gap between how frequently men and women desire sex and how often they actually have sex; the gap is bigger for women, 82 percent of whom had sex when they did not desire it, compared to 60 percent of men.

What could explain this flexibility? Baumeister proposed that "Women might be more willing to adapt their sexuality to local norms and contexts and different situations, because they aren't quite so driven by strong urges and cravings as men are."

When Baumeister set out to compare the male and the female sex drive about a decade ago, the four leading psychology textbooks of the time either did not address the fact that the male and female sex drive were different, or they suggested that they were the same. When he presented his hypothesis--that the male sex drive is stronger than the female drive--to peers in his field, they were skeptical. They believed, as Baumeister puts it, that "the idea that men have a stronger sex drive than women was probably some obsolete, wrong, and possibly offensive stereotype."

So Baumeister and two (female) colleagues set to work reviewing hundreds of studies about human sexuality and found consistently that women are less motivated by sex than men are.

For men, they found, the goal of sex is sex itself. One 1996 study found that seven in ten men--compared to four in ten women--said the goal of sexual desire was simply having sex. In the same study, 35 percent of women said that love and intimacy were important goals of sex compared to 13 percent of men. Men also think about sex more, according to studies. When men and women monitor their sexual urges over a seven-day period, men report having twice as many sexual urges as women do.

Bergner and others might chalk these findings up to society's sexual double standard: Men are allowed to be more sexual than women and, therefore, they are more forthcoming about their sexual urges. But this doesn't seem to be the case.

Men feel guiltier about sex. They feel guiltier about masturbating than women do (13 percent versus 10 percent) and they feel guiltier about thinking about sex than women do. For instance, men report having more unwanted and uncontrollable thoughts about sex. In one survey, men responded more affirmatively to the following statements than women did: "I think about sex more than I would like" and "I must fight to keep my sexual thoughts and behavior under control."