For more than three decades evolutionary psychologists have advanced a simple theory of human sexuality: because men invest less reproductive effort in sperm than women do in eggs, men's and women's brains have been shaped differently by evolution. As a result, men are eager for sex whereas women are relatively choosy. But a steady stream of recent evidence suggests this paradigm could be in need of a makeover.



"The science is now getting to a point where there is good data to question some of the assumptions of evolutionary psychology," says social psychologist Wendy Wood of the University of Southern California (U.S.C.).



The eager males–choosy females paradigm doesn't imply that men and women literally make conscious decisions about how much effort they should put into short- and long-term mating relative to their costs of reproduction—minutes versus months. Instead the idea is that during human history, men and women who happened to have the right biochemical makeup to be easy and choosy, respectively, would leave more offspring than their counterparts.



In 1993 psychologists David Buss and David Schmitt, then at the University of Michigan at Ann Arbor, used that idea to generate a series of predictions about men's and women's sexual behavior. As part of their study, Buss and Schmitt surveyed college students about their desire for short- and long-term mates (that is, one-night stands versus marriage partners), their ideal number of mates, how long they would have to know someone before being willing to have sex, and what standards a one-night stand would have to meet. In all categories the men opted for more sex than the women.



Although the study has been cited some 1,200 times, according to Google Scholar, there were "huge gaps from what I'm used to as a scientist," says Lynn Carol Miller of U.S.C. Miller says that in order to evaluate the relative proportion of mating effort devoted to short- and long-term mating in the two sexes, the proper method is to use a scale such as time or money, which has the same interval between units, not the seven-point rating scale that Buss and Schmitt used.



In a study to be published in the journal Sex Roles: A Journal of Research, Miller and her colleagues carried out their own version of Buss and Schmitt's work, asking how much time and money college students spent in a typical week pursuing short-, intermediate- or long-term relationships. The proportion of mating effort dedicated to short-term mating was the same for men and women. Similarly, both men and women showed an equivalent tendency to lower their standards for sex partners, and men did not report feeling constrained to have far fewer sexual partners than they truly desired.



"I'd certainly accepted the idea that men pursue purely sexual relationships with greater fervor than women do," says Paul Eastwick of the Texas A&M University in College Station. "This is the first time I've seen data that makes me think, 'Hmm, I wonder if that sex difference isn't so robust.'" Miller says the results are to be expected if paternal investment boosted the survival rate of offspring during our species' 200,000-year history. If both sexes invest in their offspring's survival, she says, they should both show similar mating adaptations.



As a corollary to male eagerness for sex, men are also supposed to be bothered more by sexual infidelity than emotional infidelity, because men have a vested interest in making sure their offspring are their own and not another man's. Surveys have indeed found that in the U.S. and several other industrialized countries more men than women express greater concern with sexual infidelity than with emotional infidelity (falling in love with someone else). But another recent study suggests jealousy patterns could have something to do with glitches in people's ability to form secure relationships.



Psychologists Kenneth Levy and Kristen Kelly of The Pennsylvania State University surveyed 416 undergraduates to see which type of jealousy bothered them more. They also assessed the students' so-called attachment styles. Previous studies had found that more men than women have what's called a "dismissing avoidant" style in relationships, meaning they tend to deny their emotions and their need for the other person.



When Levy and Kelly broke down their jealousy results by attachment style, they found that men and women who had secure attachment styles were both more likely to view emotional infidelity as more upsetting than a sexual affair. Men with the dismissing style were more bothered by sexual infidelity, but women who manifested this style were also, although the effect was more pronounced in the males.



Levy says attachment styles are largely determined by early experiences with caregivers—usually mom and dad. To explain why more men than women exhibit the dismissive style, he says, "we would have to hypothesize that men are more likely to be raised in such a way that would promote dismissive attachment."



Beyond simply poking holes in the standard evolutionary psychology narrative, researchers have another paradigm ready to put in its place: U.S.C.'s Wood and Alice Eagly of Northwestern University propose that men and women adapt their outlooks to fit their society's division of labor between the sexes, which results from physical differences in size, strength and mobility (during pregnancy).



In a 2009 study Eagly, along with Eastwick and another colleague asked college students of both sexes to imagine themselves as either a future homemaker or provider. Students who imagined being homemakers rated their anticipated spouse's provider qualities as more important than that spouse's homemaker qualities. The finding fits with data indicating that women and men who earn more are more likely to get married, suggesting they make more attractive partners.



"In more equal actual roles, men and women have more similar mate preferences," Eagly says. "In very different marital roles that confine women to a domestic role, men and women choose differently."



The evidence, however, does not move Buss, now at the University of Texas at Austin. He calls Eagly and Wood's theory "bizarre" for positing that "natural selection has shaped sex differences in male and female bodies, but not in male and female brains and the psychological adaptations those brains contain."



In Wood's view the traditional evolutionary psychology paradigm was attractive because it explained the pattern of sex differences people saw around them in a way that made those differences seem natural. It assumed that men and women have always interacted in the way they do now. "We would say that men and women have evolved to act in a lot of different ways," Wood says. "We're the ultimate flexible species."