A brief rant about waist-to-hip ratio

This is a bit off topic, but it's a good illustration of William James' notion of the psychologist's fallacy and it addresses a pet peeve of mine.

Evolutionary psychology is becoming more and more popular and the media is one of its biggest fans.One thing that annoys me is how quickly and uncritically people latch on to these stories and use them to justify the status quo. One of the most popular stories is that men prefer women with small waists and big hips. This is measured using the Waist to Hip Ratio (WHR). The WHR is the circumference of your waist divided by the circumference of your hips. The links below will tell you that men are irresistibly drawn to women with WHRs of .70. This number is apparently imbued with evolutionary significance because prepubescent girls have WHRs close to 1 (their waists are the same size as their hips), while post-pubescent girls have WHR less than 1 (waists smaller than hips); and also because low WHRs are associated with a good hormonal balance. One thing that makes this idea attractive is that it conforms to our modern, western experience - many women who are considered to be extremely attractive have low WHRs and it's difficult to generate examples of women who are famous for their beauty, but who have high WHRs. This evolutionary angle legitimizes our society's standard of attractiveness. We assume that everyone else basically shares our own preferences (the psychologist's fallacy), so, rather than this result simply telling us something about modern, western mens' judgments of attractiveness, there is the irrisitable pull to generalise this preference to ALL men.

To give a sense of the way the popular media handle this topic, here are a couple recent stories about the WHR: 1 (this one includes exercise tips to help women appear to have a more ideal WHR ratio) 2, and 3 (this one also claims that "men's perfect lovers come with a waist-to-hip ratio of .70", implying, I suppose that WHR ratio influences how good you are in bed??). Science reporting is rarely subtle and these articles are no exception. They talk about "males", "females", "mate preference", and "evolutionary" indicators of fertility. This language suggests to the average reader that these results are universal. That they reflect the preferences of people in general. But, does the research behind the headlines support this universality?

Evolutionary psychology, like most other branches of psychology, tends to lack cultural population validity. This problem is well-summarised in Henrich et al. (2010; see also here for interesting commentary on the original paper). The gist is that we can never take culture for granted in psychological research; there is no realm in which it is safe to assume that an effect is universal. The burden of any serious evolutionary psychology research program must be to establish the generality of their results across cultures. It doesn't matter how cool the evolutionary angle is - oh, look, this co-varies with fertility!!. It doesn't matter how obvious the effect seems to us. If male preference for women with low WHRs doesn't obtain across cultures then it's not universal. This isn't to say that there couldn't still be an evolutionary component to our preferences. It would be remarkable if there were not. But, genetic contributions to behaviour are complicated. So, failure to establish the generality of a preference for low WHR doesn't necessarily imply that men aren't sensitive to information that conveys fertility in potential partners. But, it does mean that there is not a universal reliance on this one particular type of information. It is quite likely that a whole lot of cues interact in a complex system of perceived attractiveness, to the extent that it doesn't make much sense to isolate one variable. So, anyway...

What IS the evidence for a low WHR ratio preference across cultures? Well, it's actually quite muddled. Westman and Marlowe (1999) provide a pretty good intro to the evidence for the WHR preference, so I'd recommend their paper for a quick overview. They point out that the  majority of studies on WHR rely on American undergraduates, although there is also evidence for a similar preference in Hispanic, British (although see below), and American-Indonesians. Some researchers (e.g., Singh, 1993) suggest that this preference is universal across cultures (p. 305). But, rather than jump straight into a statement of universality, Singh says something a bit more measured. He claims "the fact that WHR conveys such significant information about the mate value of a woman suggests that men in all societies should favor women with a lower WHR over women with a higher WHR for mate selection or at least find such women sexually attractive." That last bit is interesting. It merely suggests that men shouldn't find women with low WHR unattractive. This is a very different argument than the oft repeated universal preference for low WHR.

Unfortunately, Singh's perfectly reasonable prediction has morphed into a presumption of universal preference for low WHR. This means that we hear little about evidence that contradicts this assumption. But, as it happens, there is quite a bit of evidence against this claim. Westman & Marlowe (1999) tested the effect of weight and WHR on perceived attractiveness, health, and suitability as a wife in the Hadza of Tansania. The men in that society showed no preference for women with low (.7) or high (.9) WHR, but they did show a distinct preference for heavier (cf. thin) women. Yu and Shepard (1998) also failed to find an effect of WHR on attractiveness among the Matsigenka. Swami et al (2007) looked at WHR preferences among males in Spain, Portugal, and the UK.In all three countries BMI, not WHR, accounted for the most variance in perceived attractiveness. WHR influenced attractiveness judgments for Spanish and Portugese, but not British men. However, even in the Spanish and Portugese samples WHR accounted for only about 18-19% of the variance, while BMI accounted for over 70% of the variance in perceived attractiveness. This paper also has a great summary of methodological issues with prior WHR studies (e.g., the use of two dimensional line drawing, failing to control for BMI). Cornelissen et al (2009) looked at patterns of British male gaze fixation during attractiveness judgments of pictures of women. Men tended to look at the upper abdomen and face, not the hip or pelvic area. The pattern of gaze fixations matched they way men evaluated the same pictures when estimating body fat, and did not match the way men evaluated WHR. Reading these papers suggests a lively debate in the literature about the universality of low WHR preference. I am not an expert in this area, and these examples don't even scratch the surface, but they do indicate lack of consensus on the generality of the low WHR preference.

So, what does WHR even mean, evolutionarily speaking? Most people seem to argue that low WHR indicates a good balance of estrogen to other hormones, which is important for fertility. Fertility, undoubtedly, is essential to evolutionary fitness but 1) WHR isn't going to be the only cue to fertility and 2) there are other important characteristics that may account for more variance in reproductive success in some situations (e.g., if the vast majority of women in a certain age range are fertile). Cashdan (2008) looked at actual average WHRs in a variety of cultures, mostly non-Western. She found that the average WHR was > .80 (remember, .70 is supposedly the magic number). Cashdan pointed out that androgens and cortisol both increase abdominal fat in women (increasing WHR). But, higher levels of these hormones are also associated with increased strength and stamina, which come in handy in less than optimal circumstances. She says: "Waist-to-hip ratio may indeed be a useful signal to men, then, but whether men prefer a WHR associated with lower or higher androgen/estrogen ratios (or value them equally) should depend on the degree to which they want their mates to be strong, tough, economically successful, and politically competitive" (p. 1104). This suggests that it's possible to construct a perfectly reasonable evolutionary account for why men might prefer a high, rather than low, WHR (i.e., given a stressful environment where strength and stamina matter). The variables that dominate in a particular situation will likely depend on a number of specific environmental and cultural conditions. In other words, it's complicated.

This story, unlike the one about low WHR preference, doesn't seem to reflect our (modern, western) experience, so it's less likely to catch the popular imagination. We don't tend to think of male attraction based on female heartiness, but we also live in a particularly rich culture where we don't spend a lot of time physically searching for / killing food or building shelters. So, here's the psychologist's fallacy again. Evolution is complicated and the features that confer fitness are necessarily dependent on context. This means that it's not too difficult to think of a number of plausible evolutionary explanations for a particular phenomena. The preferred explanations are most likely going to be the ones that fit with our current experience, but this doesn't make them better explanations.