Did men evolve to go for Jessica Rabbit?
By Meredith Reiches, Heather Shattuck-Heidorn, Marion Boulicault, Nicole Noll, Brianna Weir, & Sarah Richardson
Filmmaker Robert Zemeckis described his voluptuous cartoon creation, Jessica Rabbit, as the “ultimate male fantasy,” expressing the common belief that slender women with tiny waists and curvy hips are the ideal of all men. Two recent articles by William Lassek and Steve Gaulin in the journal Evolutionary Psychology (here, and here) challenge the idea that this preference is universal and the result of evolution. They review evidence for the claim that slim women with small waists enjoy better health and higher fertility than women with higher waist-hip ratios and body mass indexes (BMI). They find that it is not supported. The theory that slim, small-waisted women are healthier and more fertile—and that men have been under selection over the course of human evolution to find them attractive—has had a considerable foothold in both scientific and popular culture for over two decades. Let’s take a look at these claims and Lassek and Gaulin’s critique of them.
Evolutionary psychology and the waist-hip ratio hypothesis
It’s important first to understand the starting assumptions of evolutionary psychology, the subdiscipline of psychology that produced the tiny-waist-equals-health-and-fertility idea. Evolutionary psychology approaches the human mind as the result of thousands of years of natural selection. Beneath the trappings of culture, researchers believe, humans across time and space—from undergrads in Psych 101 classrooms to cattle herders on the East African savanna—share a fundamental nature. This nature, if you dig deep enough, expresses itself through similar—if unconscious—preferences and behaviors.
The slim women, small waists concept is part of a larger claim in evolutionary psychology that sex differences in men’s and women’s minds arose because males and females make distinct contributions to reproduction. Men, physically able to donate sperm and end their parental contributions there, are adapted to maximize the number of fecundable—that is, able to get pregnant—women they inseminate. Women, meanwhile, saddled with nine months of pregnancy, not to mention months or years of calorically-expensive nursing, evolved to tempt men, with the embodied promise of more sex and babies, into sticking around to help raise the kids. Men with a keen eye for potential fertility in women—and the women whose kids they help to raise—have the best shot at passing on their genes.
The waist-hip ratio hypothesis[1] is a central empirical premise of evolutionary psychology's picture of male and female evolutionary strategies. According to this hypothesis, evolution explains why men prefer female partners who have small waists, both overall and relative to their hips (and usually low BMIs, too). These preferences, purportedly in evidence on the pages of men’s magazines and in beauty pageants, make sense because they increase men’s evolutionary fitness: low waist-hip ratio and low BMI in a woman signal good health and high potential fertility.
The idea that heterosexual men are hard-wired to find slim, hourglass-shaped women attractive circulates in popular discourse—see exhibits A, B, and C—in ways that lead people to assume that the waist-hip ratio hypothesis is solid science. People have incorporated it into their conventional wisdom about the world. Indeed, the hypothesis does heavy lifting for the validation of evolutionary psychology, serving as proof of the concept that evolved sex differences shape bodies and behaviors. This makes a recent refutation of the hypothesis, by leading scientists in evolutionary psychology itself, especially important.
Weighing and measuring the evidence
For starters, the authors find that men’s preferences for low waist-hip ratios are not universal. They vary by the type of society in which men live. In well-nourished populations—that is, populations with access to abundant food, labor-saving technology, and Western-style health care—men do prefer images of slender women with slim waists. However, by contrast, men living in several subsistence and forager groups prefer images of women with higher BMIs[1]. This is an important distinction. People in subsistence and foraging populations get the majority of their food through small-scale farming, gardening, hunting, and gathering. They are more likely to face periodic food shortages, to expend significant energy obtaining food, and to lack access to the technological and medical resources of industrialization. In these ways, though not in many others, they live in conditions believed to more closely resemble those that humans encountered during our evolution. Their preferences should, in theory, bear some relationship to preferences that helped humans survive in similar environments. The fact that men in subsistence and foraging populations don’t show a preference for women with low BMIs suggests that that preference wasn’t historically important to men’s ability to pass on their genes.
If men have preferred and had children with low waist-hip ratio, low BMI women since the dawn of time, then selection should lead to the preferred traits becoming more common: the average woman ought to have a waist-hip ratio and BMI near men’s ideal. This isn’t the case, and this is Lassek and Gaulin's second main finding. Lassek and Gaulin found that only a tiny percentage of women in well-nourished populations like the United States have waist-hip ratios as low as the ones men prefer when shown images in a testing setting[2]. That is, men’s preferences in places like Pittsburgh and Santa Barbara bear little relation to body proportions of the population of available women, even young ones..
Fertility data don’t support an evolutionary explanation for the waist-hip ratio hypothesis, either. In industrialized societies, women with low waist-hip ratios and BMIs tend to be younger than 20. That’s at least five years younger than they’ll be when they have the most frequent ovulatory menstrual cycles, e.g. cycles when it would be possible for them to get pregnant[3]. Furthermore, Lassek and Gaulin review nearly twenty papers that examine whether waist size relates to fertility. None of these studies, they report, finds that women with small waists are more likely to get pregnant than women with average waist sizes[4]. In fact, the reverse is true: women with the most coveted BMIs and waist-hip ratios are less fecund than their somewhat heavier, higher BMI peers[5]. As they strikingly write, “If men had evolved to prefer women with high fertility, they should prefer women with BMIs between 20 and 29 who are between 25 and 29 years old”[6]. For context, according to the National Institutes of Health and the Centers for Disease Control and Prevention, a BMI of 25 or higher is “overweight.” Women with the highest fertility have BMIs that are neither at the lowest nor the highest end for their populations[7].
And that doesn’t only apply to women in their peak childbearing years. In a large, nationally representative sample in the United States—the National Health and Nutrition Examination Survey—the authors found a positive relationship between lowest BMI after 18 years of age and lifetime fertility, controlling for marital status and oral contraceptive use, among other variables. That is, women with higher BMIs in young adulthood had more children over the course of their lives. Taking into account age, marital status, education, and race, all of which influence the number of children a women has, the authors found that women whose lowest adult BMIs fell between 25 and 29 had the most children[8].
Low waist-hip ratio and low BMI in women also don’t predict better health outcomes for women or babies. Women of all ages with low BMIs are more likely to give birth to a low-birth-weight baby—that is, a baby that weighs less than 2,500 g and that is therefore at higher risk for health problems and infant mortality[9]. The authors report in their paper on BMI, waist-hip ratio, and health that, in subsistence populations of the kind described above, women with higher BMIs have better chances of survival[10]. This is important because, in the absence of high tech medical care, heritable traits that help a person to survive—in this case, higher BMI—would be the ones that get passed down most often to children, suggesting that high rather than low BMI, under certain conditions, is at a premium.
The lowest mortality rates in well-nourished populations, meanwhile, don’t appear at the low end of the BMI scale but rather at or near the population average[11]. Having a run-of-the-mill BMI, not the BMI of a supermodel, means your survival chances are better. As far as self-reported health goes, low BMI and waist-hip ratio women don’t have it any better than average-sized women, either. In fact, women with very low BMIs—under 20—reported poorer health, including more disability and days in the hospital[12].
What’s at stake?
These papers offer a great example of what it looks like when scientists drill down and examine the state of empirical evidence for widely held claims central to their field. The source of the article and venue of publication are particularly encouraging. Steve Gaulin is a professor of anthropology at UC Santa Barbara in a department that includes other established evolutionary psychology researchers, John Tooby and Leda Cosmides (who is affiliated with anthropology but based in the Psychology and Brain Sciences Department). These thinkers have been foundational to evolutionary psychology, forging it out of sociobiology in the 1980s. Gaulin’s prior work examines other evolutionary psychological hypotheses such as the relationship between men’s voice timbre and testosterone and correlations between women’s facial symmetry and attractiveness. William Lassek, a medical doctor who works as an adjunct epidemiologist at the University of Pittsburgh and holds a research affiliation at UC Santa Barbara, blogs at Psychology Today. He published a 2011 book with Steve Gaulin about the evolutionary underpinnings of women’s fat cravings and metabolism.
It is notable, however, that the realization that waist-hip ratio is a poor indicator of either fertility or health—and that it does not seem to predict men’s partner choices—comes so late to many evolutionary psychologists. Researchers in human biology, anthropology, and biomedicine have long known that the lowest BMIs and smallest waists aren’t associated with high fertility. In addition, that men's preferences for low waist-hip ratios and BMIs on surveys would not indicate their actual behavior would be no surprise to mainstream psychologists, who recognize that self-report surveys cannot straightforwardly predict a person's preferences in the real world (though some researchers in evolutionary psychology report this, too). Behavioral measures—including how people in fact choose partners—carry much more weight. That is, psychologists would not necessarily assume that a man’s choice of whom he finds attractive on a survey would predict his behavior in choosing a mate. The assumption that the survey choice is a strong gauge of behavior, though, is crucial to the waist-hip ratio hypothesis.
If we reject explanations for men’s preferences for low waist-hip ratios and BMIs based on the idea that these phenotypes in women predict better health or fertility, where does this leave evolutionary explanations for the survey results that come up again and again in industrialized populations? As often happens in evolutionary psychology, the target wriggles out from under contradictory evidence and keeps moving. Lassek and Gaulin conclude by suggesting that waist-hip ratio and BMI communicate not health or fecundability per se but rather age as a proxy for future fertility: men in well-nourished populations, the story goes, prefer low BMI women because they are likely to be young. Men in subsistence populations prefer higher BMI women because, when women don’t get much to eat, their BMIs decrease with age, so higher BMI women in their communities are also likely to be young [13]. Young women have more years of childbearing ahead of them. Men’s waist-hip ratio and BMI preferences, the authors insist, can still be explained as evolutionary adaptations offering long-term reproductive payouts.
No one doubts that hetero-inclined men find women’s secondary sexual characteristics like breasts and hips attractive, or that evolution has something to do with why: it makes sense that men who desire women would cue on signals that a woman is sexually mature and not already pregnant. Likewise, human preferences for signs of good health in a mate—clear skin, a full set of teeth, etc.—may well have been targets of selection.
But why do men in Melbourne, Milan, and Miami choose pictures of women with waist-hip ratios and BMIs well below those of the young, fecundable women around them? We know from research in psychology, public health, and gender studies that sociocultural variables like media exposure, the beauty ideals that surround people in early life, and sexist beliefs are associated with body types preferences in men and women and with changes in those preferences across time and space. Cultural context seems to play a significant role in shaping what people find attractive. While there may be an evolutionary explanation for why men’s preferences respond to culture—Gangestad, Haselton, and Buss proposed one in 2006—evolution may not be the most relevant factor in explaining variation (here, and here), or even consistency, in those preferences.
The Tinder, “swipe right” paradigm of attraction favored by evolutionary psychologists ignores the cultural environments and social contexts in which people forge their preferences and choose their partners. To understand why many men do a double take when Jessica Rabbit sashays by, we might do better looking to history than to prehistory.
Tweetable version
Two recent papers challenge the idea that men evolved to prefer slim women with small waists because they are healthier and more fertile.
How to cite this blog
Reiches, Meredith, Shattuck-Heidorn, Heather, Boulicault, Marion, Noll, Nicole, Weir, Brianna, and Richardson, Sarah. “Did men evolve to go for Jessica Rabbit? Two new papers take the measure of the waist-hip ratio hypothesis, an evolutionary explanation for men’s mate preferences,” GenderSci Blog, November 14, 2018, https://projects.iq.harvard.edu/gendersci/blog/did-men-evolve-go-jessica-rabbit-two-new-papers-take-measure-waist-hip-ratio
Statement about intellectual labor
Reiches drafted the blog posted and integrated lab member comments and edits during the revision process. Shattuck-Heidorn generated an initial reaction statement situating the Lassek and Gaulin papers in disciplinary context. Richardson proposed an initial outline for the post. All authors provided substantive contributions to the ideas expressed in this post and participated in its preparation and final revision.
References:
[1]Lassek, W. D., & Gaulin, S. J. (2018a). Do the Low WHRs and BMIs Judged Most Attractive Indicate Higher Fertility?. Evolutionary Psychology, 16(4), 1474704918800063, 2.
[2]Lassek and Gaulin (2018a), 2.
[3]Lassek and Gaulin (2018a), 2.
[4]Lassek and Gaulin (2018a), 3.
[5]Lassek and Gaulin (2018a), 4.
[6]Lassek and Gaulin (2018a), 9.
[7]Lassek and Gaulin (2018a), 4.
[8]Lassek and Gaulin (2018a), 7.
[9]Lassek, W. D., & Gaulin, S. J. (2018). Do the Low WHRs and BMIs Judged Most Attractive Indicate Better Health?. Evolutionary Psychology, 16(4), 1474704918803998, 8.
[10]Lassek and Gaulin (2018b), 3.
[11]Lassek and Gaulin (2018b), 4.
[12]Lassek and Gaulin (2018b), 7.
[13]Lassek and Gaulin (2018a), 9.
[1]While the waist-hip ratio hypothesis is the one that captures the public imagination, Lassek and Gaulin, the authors of the studies under discussion, note that the prime mover is actually waist size, i.e., how small a woman’s waist is compared to her height