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Sunday, October 17, 2010

More on human sexual strategies and preferences

Thanks to Claudio T for the link!
From J.R. Minkel's blog: A Fistful of Science
David Buss defends evolved sex differences (exclusive!)

This week Scientific American ran an article of mine, “Student Surveys Contradict Claims of Evolved Sex Differences.” Here’s the gist:

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.

A highly cited 1993 paper on evolved sex differences (linked to below) served as the story’s jumping off point and foil. Evolutionary psychologist David Buss of the University of Texas at Austin, a co-author of that paper, kindly responded to a query of mine while I was writing the story, laying out his objections to the evidence I cited in the article. I knew I wasn’t going to have room to do justice to his views, so I asked him if I could post his comments to this blog. He did me one better: he wrote a direct response to my article, which I’m reprinting below in its entirety. Naturally, this will make more sense after you’ve read my article. [I've posted it below - MA]

Evolved Sex Differences: Not Gone, Not Forgotten, and Not Explained by Alternative Hypotheses

It is both astonishing and disturbing that evolutionary psychology as a field, and the specific hypotheses that have been advanced under its broad umbrella, continue to be so badly mischaracterized by other “scientists” as well as by science journalists writing for popular media (see, for example, Dr. Kurzban’s recent account of one vivid example directly related to the current article). That these errors and scholarly lapses continue to occur, despite numerous attempts to correct common misunderstandings about them, suggests poor scholarship, non-scientific motivations of the ideological or religious kind, or both.

Given the large number of lapses contained in the current article and in thecomments of the authors it quotes, I’ll restrict my comments to a few of the more egregious errors and point interested readers to the relevant scientific sources so they canjudge for themselves.

The first problem is that sexual strategies theory, initially advanced in 1993, is erroneously depicted. Contrary to the cartoonish depiction of “eager males-choosy females,” our theory proposed that BOTH men AND women have evolved short-term AND long-term sexual strategies. Subsequent publications from my own lab and the labs of other scientists (e.g., Gangestad, S.W., & Thornhill, R. (2008). Human oestrus. Proceedings of the Royal Society B, 275, 991-1000) have tested specific evolutionary hypotheses about the benefits to women of short-term mating strategies. Although the final word is not in, currently viable hypotheses for women’s short-term sexual strategies include increased access to resources, securing good genes, and trading up to a better mating partner. Readers in the science of female sexuality should check out my recent book, co-authored with Dr. Cindy Meston, entitled Why Women Have Sex.

Just as some women pursue a short-term sexual strategy some of the time, many men pursue a long-term mating strategy marked by love, commitment, and heavy parental investment in children. Humans have a complex “menu” of mating strategies, selectively deployed depending on predictable contexts such as population density, matevalue, and sex ratio. Given that these elements have been central to sexual strategiestheory since its inception 17 years ago, responsible scientists and science journalists should put a stop to the cartoonish depiction of sexual strategies theory.

Now let’s address the issue of whether men and women differ in their psychology of short-term mating. Among the 9 key hypotheses and 22 empirical predictions contained in the original version of sexual strategies theory is indeed the hypothesis that men will attach a higher motivational priority to short-term mating, depending on contextual factors such as personal mate value, sex ratio in the mating pool, risk, and cost. The logic follows from Trivers’s theory of parental investment and sexual selection, which has been abundantly supported in the non-human and human scientific literatures. Indeed, the scientific evidence supporting this prediction for humans is overwhelming. Contra the data cited in the current article, readers should consult the massive cross-cultural studies that have been conducted both by evolutionary psychologists such as David Schmitt and by scientists who have no commitments to evolutionary psychology, such as Dr. Richard Lippa. The Lippa study, for example, tested more than 200,000 individuals across 53 different nations and found robust sex differences precisely as predicted by sexual strategies theory, as well as findings flatly contradicting the Eagly-Wood “alternative” theory [that women and men are simply responding to their society's division of labor]. I urge readers who really doubt the existence of sex differences on variables such as desire for sexual variety to consult these massive cross-cultural studies. Indeed, these psychological sex differences are among the largest psychological sex differences ever discovered, as meta-analyses by Dr. Janet Hyde of the University of Wisconsin have documented.

Although some researchers try to market their data as contradicting hypotheses about evolved sex differences, viewed in the broader context of the massive data sets such as those of Janet Hyde, Richard Lippa, and David Schmitt, dispassionate readers will come to the conclusion that massive weight of the scientific evidence supports thefundamental tenets of sexual strategies theory.

David M. Buss, Ph.D. Professor of Psychology Head, IDEP Area

For those who want to know how Eagly and Wood’s alternative paradigm might account for the cross-cultural data collected by Schmitt and Lippa, here’s your starting point. Happy reading!

Original Article in Scientific American by J.R. Minkel
Student Surveys Contradict Claims of Evolved Sex Differences

New data is undermining the evidence that has long been proposed to support the eager males—choosy females paradigm
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."

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