Many women interviewed were having sex purely because they wanted the experience.
It turns out that woman have sex for all of these reasons and more, and that their choices are not arbitrary; there may be evolutionary explanations at work.
Psychologists Cindy Meston and David Buss, both professors at the University of Texas at Austin, decided that the topic of "why women have sex" deserved a book of its own. They've woven scientific research together with a slew of women's voices in their new collaborative work, "Why Women Have Sex," published September 29 by Times Books.
"We do bring in men occasionally by way of contrast, but we wanted to focus exclusively on women so that the complexity of women's sexual psychology was not given the short shrift, so to speak," said Buss, a leading evolutionary psychologist.
The authors conducted a study from June 2006 to April 2009 that asked women whether they had ever had sex for one of 237 reasons, all of which had emerged in a previous study. About 1,000 women contributed their perspectives.
It turns out that women's reasons for having sex range from love to pure pleasure to a sense of duty to curiosity to curing a headache. Some women just want to please their partners, and others want an ego boost.
Research findings
Purposely made partner jealous?31 percent women vs. 17 percent men have tried to evoke jealousy in a partner.
Had sex out of sense of duty?
84 percent wives vs. 64 percent husbands usually or always comply when a spouse wants sex but they don't.
Partner choice for casual sex?
63 percent of women prefer to have casual sex with a friend vs. 37 percent who prefer sex with a stranger.
Steal someone else's mate?
38 percent of women say they've "poached" someone for a short fling.
Buss said he found it surprising how dramatically and variably sexual experience seemed to influence women's feelings of self-esteem.
"Some sexual experiences that women in our study reported just had devastating effects and long-lasting negative effects on their feelings of self-worth," he said. "But then for others, their sexual experiences provided the soaring height of euphoria and made them feel alive and vibrant."
Meston said some 20-somethings defied the gender stereotypes that women should be more chaste than men and not sleep around as much.
"Many of the women were having sex purely because they wanted the experience, they wanted the adventure, they wanted to see what it was like to be with men of different ethnicities," she said. "Some women said they wanted more notches on the belt. They simply wanted to get rid of their virginity."
Some women have sex to make money, and not just in the conventional manner of prostitution. A woman from California who goes by "Natalie Dylan" garnered national attention this year with her campaign to sell her virginity and said in January that her top bid of $3.8 million came from a 39-year-old Australian.
There are more factors that influence a woman's sex drive than a man's, the authors said, and the factors that make men attractive to women -- personality, sense of humor, self-confidence, status -- are less important considerations for men when they are choosing women.
There is also evidence that sexual arousal is more complicated for women than for men, the authors report.
A study from Meston's lab showed a strong correlation between how erect a man's penis is and how aroused he says he is. By contrast, the link is much weaker between a woman's physical arousal (as measured inside her vagina) and the arousal she says she feels, the researchers found. This is why drugs to treat erectile dysfunction such as Viagra don't work as well in women, the authors said.
That makes sense from an evolutionary perspective, even though men and women may not consciously think about their choices that way, the authors said. If the goal of a man is to spread his genes, he would need to look for signs of fertility in a woman, which are historically associated with physical cues, Buss said.
"The adaptive problem that women have had to solve is not simply picking a man who is fertile but a man who perhaps will invest in her, a man who won't inflict costs on her, a man who might have good genes that could be conveyed to her children," he said.
In this context, women must also be more selective, because wrong choices can lead much higher costs than for men: pregnancy and child-rearing.
In studies, women have consistently shown preferences for men with symmetrical bodies, a subtle mark of genetic fitness and status, the book said. In fact, simply by smelling T-shirts that men had worn for two nights, women judged the odors of symmetrical men to be the most attractive, and the asymmetrical men's odors the least attractive, in one study.
Still, symmetry isn't everything, Meston and Buss said. They pointed to singer-songwriter Lyle Lovett as someone with other positive attributes, such as musical talent and personality, who has clearly done well with women despite asymmetrical features.
"Women are evaluating men on multiple attributes," Buss said.
Kissing also turns out to be more important for women than for men in some respects: In one study, 53 percent of men said they would have sex without kissing, but only 15 percent of women said they would even consider sex without smooching first, the book said. For women, kissing is "an emotional litmus test," the authors wrote.
The medicinal value of sex also comes into play for some women, the book said. Sex can help a woman relax and sleep better, and it can ease the pain of menstrual cramps and headaches -- and some survey participants cited these as reasons they've had sex.
A study from Rutgers University found that, during orgasm, women were able to tolerate 75 percent more pain. Though Meston has not studied the phenomenon in men, she said she would expect sex to have the same effects of reducing headaches and other pain.
The authors collected stories from 1,006 women from 46 states, eight Canadian provinces, three European countries and Australia, New Zealand, Israel and China. Participants came from a variety of ethnic and religious -- as well as non-religious -- backgrounds and socioeconomic statuses. About 80 percent of the women said they were in a relationship at the time, and 93 percent said they were predominantly or exclusively heterosexual.
The book also explores how women's perception of sex may change over time, according to whom they're with and whether they are married.
A 26-year-old heterosexual woman wrote, "When I was single, I had sex for my own personal pleasure. Now that I am married, I have sex to please my husband. My own pleasure doesn't seem as important as his. I believe he feels the same way."