A fierce debate about whether jealousy, lust and sexual attraction are hardwired in the brain or are the products of culture and upbringing has recently been ignited by the growing influence of a school of psychology that sees the hidden hand of evolution in everyday life.
Fresh sparks flew last month when a study of more than 16,000 people from every inhabited continent found that men everywhere -- whether single, married or gay -- want more sexual partners than women do.
"This study provides the largest and most comprehensive test yet conducted on whether the sexes differ in the desire for sexual variety," wrote lead researcher David P. Schmitt, an evolutionary psychologist at Bradley University in Peoria, Ill. "The results are strong and conclusive -- the sexes differ, and these differences appear to be universal."
The idea that male promiscuity is hardwired -- and therefore "normal" -- drew swift and furious criticism. Scholars who assert the primacy of culture in shaping human behavior charged Schmitt with choosing his facts, making his conclusions less about science than "wishful thinking."
The debate won't be settled soon, if ever. For the real arguments are about social mores, gender roles and sexual politics. The real question isn't about evolution, but society's view of appropriate behavior for men and women.
Ohio State University psychologist Terri Fisher said she knows the new study will be misused. Each year, when she teaches her college students about the research into sexual variety, the young men smile and nod and the young women look appalled.
"I bet a lot of males might leave class and talk to their girlfriends and say, 'You know what I learned in class? It's natural I don't want to commit to you and that I feel attracted to other women -- it's because I am a man,' " Fisher said.
The basic idea of evolutionary psychology is that human behavior -- like human physical features -- is the product of evolution. Unlike with bones and tissue, however, there is no fossil record of behavior, so psychologists draw inferences from current behavior as to why people developed certain ways of acting.
There is little controversy that evolution played some role in sculpting behavior. Neuroscientists have studied emotions such as fear and found that many species freeze when panicked, meaning that this is probably an evolved behavior. But when evolutionary psychologists use the same argument about complex behaviors such as sexual attraction, the debate becomes heated.
For if men and women naturally have different desires for sexual variety, this easily becomes a justification of male promiscuity. Sociologists and social psychologists assert that differences in sexual proclivity arise because of a double standard in male-dominated societies, where female sexuality is tightly controlled: Thus, a man with multiple partners is a "stud" while a woman with multiple partners is a "slut."
Using genetics to bolster such beliefs, these critics say, gives gender inequality the imprimatur of biology.
"Arguments about evolved dispositions have the implication of defending the status quo," said Alice Eagly, a professor of psychology at Northwestern University. "They have implications for power and status."
Schmitt's study, which was published in the July issue of the Journal of Personality and Social Psychology, involved 16,288 volunteers from 50 countries in the Americas, Europe, Africa and Asia, as well as Australia. Asked how many partners they desired over the next month, men on average said 1.87, while women said 0.78. Men said that over the next 10 years they wanted 5.95 partners, while women said they wanted 2.17.
More than a quarter of heterosexual men wanted more than one partner in the next month, as did 29.1 percent of gay men and 30.1 percent of bisexual men, the study said. Just 4.4 percent of heterosexual women, 5.5 percent of lesbians and 15.6 percent of bisexual women sought more than one partner.
Men were also more willing to enter into sexual relationships with partners they had known for short periods of time, said Schmitt in an interview.
"It is the first systematic, massive, scientific study of these sex differences," said David M. Buss, an evolutionary psychologist at the University of Texas in Austin who wrote "The Evolution of Desire." Calling the Schmitt paper definitive, Buss said, "The evidence he presents is irrefutable."
Schmitt thinks the roots of the differences his study found lie in ancient hunter-gatherer societies. Men who sought sexual variety had a greater chance of passing on their genes -- and their promiscuous proclivities. Women who kept their mates improved the chances of raising children and were more likely to pass on their genes -- and their monogamous proclivities.
Many evolutionary psychologists say these divergent sexual strategies also explain two corollary findings of modern studies. One says men seem more disturbed by sexual infidelity and women seem more disturbed by emotional infidelity. The other says heterosexual men seek women who are young and beautiful because these are viewed as signs of fertility, while heterosexual women seek men who are rich because that helps in raising children.
Schmitt and Buss said that the findings help account for the fact that men are more interested in pornography, more likely to flirt with strangers and more likely to stray as spouses.
But social psychologists and even some evolutionary psychologists aren't convinced. They say Schmitt's study is impressive, but his findings are far from universal. And they challenge every one of Schmitt's and Buss's assumptions and conclusions.
Because of society's double standard, Fisher said, women are hesitant to report their true sexual desires. In one study, she asked men and women to report whether they masturbated, watched soft-core pornography or hard-core pornography. Each "yes" got a point. She found, on average, that men scored 2.32 and women 0.89.
But she also found that women's scores changed depending on how confident they were of remaining anonymous. In the study, both men and women had been told to hand their questionnaires to a researcher. But when women were told to deposit their answers in a locked box supervised by a researcher, their average score jumped to 1.53. And when the women took the test alone in a locked room and then deposited their answers in a locked box -- ensuring privacy and anonymity -- their score shot up further, to 2.04. The men's answers did not change significantly, indicating they were less concerned about their opinions being discovered.
In Schmitt's international study, students answering the survey sat together in classrooms, filled out the questionnaires and deposited them in a locked box. Some were asked to mail in their responses.
Fisher, who works at OSU's Mansfield campus, also found that when anonymity was guaranteed, women reported having sex for the first time at a younger age. Men guaranteed anonymity raised the age when they first had sex.
"No parent in any culture ever tells a daughter, 'By all means, go have sex,' " said Pamela Regan, an evolutionary psychologist at California State University in Los Angeles who disagrees with Schmitt and Buss. By contrast, she said, "many expect their sons to 'be men,' which implies sexual experience."
Eagly and Regan argue that men's and women's sexual choices and desires grow more similar in societies with greater gender equality -- a contention supported by Schmitt's own data.
Regan added that other evolutionary theories are just as plausible as the male promiscuity argument: Men in hunter-gatherer societies who stuck with a single mate and helped raise children might have been more genetically successful -- because passing on genes means not just having children but ensuring they survive long enough to reproduce in turn.
Other research has contradicted the finding that heterosexual men mainly seek young, beautiful women, while heterosexual women are most drawn to rich men. Last month, Stephen T. Emlen, an evolutionary biologist at Cornell University, reported in a study that people basically want partners with qualities they attribute to themselves. Contrary to stereotypes about wealthy Mr. Rights and beautiful Ms. Wonderfuls, he said, attractive people tend to value attractiveness, wealthy people value mates with money, and ambitious types and family-oriented souls tend to gravitate to others like themselves.
The desire for similar mates, Emlen found, was five to six times more powerful than the desire for beautiful or wealthy partners. Emlen's study was published in the Proceedings of the National Academy of Sciences.
And David DeSteno, a psychologist at Northeastern University in Boston, took aim at the other corollary of the evolutionary psychologists' theory: that men fear sexual infidelity while women fear emotional infidelity. Under test conditions designed to elicit gut responses, both men and women reported that sexual infidelity would bother them more, said DeSteno, whose work was published last year in the Journal of Personality and Social Psychology.
If there's one thing almost everyone agrees on, it is that genes do not decide what people ultimately do.
Indeed, the interests of individuals often conflict with the interests of their genes -- and the strongest evidence for this is that in most industrialized societies, birth rates are falling. People are choosing not to have children, or to adopt, or to enter into gay relationships -- all antithetical to the idea of passing on genes.
"I have heard people say, 'I can't help it, I am a man -- I have to spill my seed,' " said Regan. "That's using science to justify your bad behavior.