I Like Vertebral Wedging And I Cannot Lie

Men prefer big butts, right? Everybody knows that. Or do they? A recent paper in Evolution and Human Behavior seeks to answer this most important question. The hypothesis is that the male preference for particular female anatomical shape derives from selection pressure to prefer females whose spines were better adapted to carry the weight of pregnancy after the evolution of bipedalism. But is this good science or just an evolutionary psych just-so story?

According to Lewis, et al. our female hominin ancestors faced selection pressure to have a certain lumbar curvature to properly distribute weight during pregnancy as they adapted to bipedal life:

The combination of bipedalism and increased abdominal mass during pregnancy uniquely posed ancestral hominin females with the adaptive challenge of a forward-shifted center of mass (COM) during pregnancy. If this COM were not moved back over the hips, ancestral women would have been subjected to a nearly 800% increase in hip torque during pregnancy (Whitcome, Shapiro, & Lieberman, 2007).

Yikes. Sounds bad. This would have apparently left females (and their offspring) at a nutritional disadvantage because it would reduce the ability to forage because of back pain and fatigue. Luckily, evolution came to to the rescue:

The female hominin spine—but not the male hominin spine—possesses evolved morphology to deal with this adaptive challenge: women possess wedging in the third-to-last lumbar vertebra (Whitcome et al., 2007). This wedging helps pregnant women shift their COM back over their hips, thereby reducing hip torque by over 90% (Whitcome et al., 2007). 

So far so good, sort of. The challenges of bipedal pregnancy (ladies, you know what I'm talking about, right?) very plausibly might have resulted in a change in the distribution of female spinal shapes to cluster around an optimum over time. This vertebral wedging can thus reasonably hypothesized as an adaptation to conditions early in the evolution of bipedalism.

But it's still basically a speculation, and one for which there would be potential empirical remedy. But this article contains no empirical support. Where are the field studies? I get that it's not possible to ride a rocket ship back to the Pleistocene to measure spines, but there are (unfortunately) still cultures in which pregnant women do manual labor for a living. Does the variance in spinal curvature for these women make a substantial difference in back pain and fatigue? Are those with 35° or 55° lumbar curvatures able to work less than those lucky enough have a 45° angle? There's just not a lot of real evidence presented here.

The authors then want to take this "finding" and build on it to claim that male sexual preference evolved to favor females whose spines are the best shape:

The fitness benefits experienced by ancestral women with a beneficial intermediate degree of lumbar wedging between hypolordosis and hyperlordosis would have created the background selective conditions for the evolution of a male mate preference for such women. Men who preferred and selected these women as mates would have gained several key fitness benefits, including having a mate who was less vulnerable to spinal injuries, better at foraging during pregnancy, and better able to sustain multiple pregnancies without debilitating injury. On this basis, we advance the hypothesis that selection fashioned psychological adaptations in men to detect cues to lumbar vertebral wedging in women and regulate mating attraction accordingly.

But first thing's first. The researchers needed establish that male preference for the supposed optimum lumbar curvature actually exists (lumbar curvature is used as a proxy for vertebral wedging since, as the authors helpfully point out, "[a]ncestral men could not have directly observed potential mates' vertebrae").

A study was conducted that showed a sample of men a series of photographs of women in profile and asked them to rate the attractiveness of the females on a ten point scale. The photos were manipulated using Photoshop to change the visible curvature of the lower back but nothing else.

Here's the schematic of the range of angles shown to subjects (my captions throughout):

The one in the middle is the pretty one

The results:

Science!

Fairly strong curve, there, seemingly showing a strong preference for the curvature shown in the middle in the first figure. The researchers used values found naturally occurring within the population (14–69°) . The paper does not, however, tell us the normal distribution of these naturally occurring values. This 55° of variance seems  high for a trait that was supposedly under such strong selection pressure if the distribution doesn't produce a tall, skinny bell curve. If the actual distribution clusters around 45° that could support the idea that selection was as strong as the hypothesis requires. Of course, if the actual distribution clusters too closely to 45° then the simplest explanation might be that males pick that trait as most attractive because it's most frequent (rather than because of its adaptive advantage).

But setting that aside, I'm sure that the researchers engaged in a detailed historical study of prior conceptions of beauty, or at the very least gathered a cross-cultural, diverse group of male subjects to use in the study in order to avoid having culturally conditioned preferences creating noise in the data. Any trait that evolved along with bipedalism should be more or less culturally universal, and one way to substantiate that evolutionary history would be to show its universality.

The subject sample:

One hundred two men (M_age = 19.00 years, SD_age = 2.41, age range: 17–34 years) were recruited from the psychology subject pool at The University of Texas at Austin. Participants received course credit for participation.

Oof. Median age 19, and given course credit for participation. Basically what we have is a sample of college freshmen taking PSYCH 101 at the University of Texas. Not exactly showing much here other than the silhouette in the middle up there looks most like the girls wandering around campus in Austin. With something as culturally laden as attractiveness, one would think some better steps to avoid this kind of sampling bias might have occurred.

Now that they've demonstrated beyond all doubt that the male preference for a certain angle back there is totally a real thing, the researchers then want to distinguish between two distinct traits that can lead to the appearance of lumbar curvature: big butts (or, "buttock mass" if you want to be all sciency about it) and vertebral wedging. Here the preference of two hundred college freshmen in a second study was very clear:

These results show that men's preference for lumbar curvature is unequivocally not a by-product of a preference for buttock mass. Men (1) discriminate between women whose external lumbar curvature reflects vertebral wedging and those whose (identical) curvature reflects buttock mass, (2) prefer women with greater vertebral wedging over women with greater buttock mass at sub-optimal levels of lumbar curvature—when the former's underlying spinal structures are closer to the optimum, and (3) exhibit the expected reversal of this preference at certain angles of lumbar curvature above optimum.

So it's not the big butt after all.

Except that this suffers the same issue as the first study: subject sample. There's absolutely nothing in this study either that gives us any reason to believe that the men serving as subjects in the study are expressing anything other than cultural norms. Due to the problem with the sample, this study can't even tell us that there is wide agreement about the "most attractive" shape among men living across the globe today, let alone that the preference is ingrained in us through natural selection and has persisted across millennia.

This study will get a lot of press, though, since it'll go nicely alongside a picture of Kim Kardashian.