In the comments section to a 2blowhards post on booty shakin', blogger Alias Clio puts forth an argument from incredulity regarding several hypotheses I proposed: 1) that male preferences for different parts of the female body have, over time, correlated with personality traits; 2) that natural selection has had a role in causing some men to prefer one body part over another; and 3) that the correlation could be caused by some simple mechanism. She also repeats an evolutionary just-so story about why human females developed large breasts -- that is has something to do with face-to-face sex -- and that too is worth taking a hard look at (the story, that is).

As to 1), the available data do paint a somewhat clear picture that assmen, boobmen, and legmen are not the same on average for personality traits. What Clio doubts is that the correlations here and now can be projected back into the past or into other parts of the world. That's true enough, but it's true of anything psychological, and only for technical reasons: when we discover which gene variants in males are implicated in preferring T over A, we can dig up or unfreeze ancient humans, sequence their DNA, and see if the males were boobmen or assmen. That's how we found out that some Neanderthals were probably red-headed, despite the fossil evidence not telling us anything about their hair color.

Though everyone knows it's a dubious move, the best we can do to see what preferences may have been like in sub-Saharan Africa 100,000 years ago is to investigate present-day hunter-gatherers in Africa. The Hadza are a well studied hunter-gatherer group who live in Tanzania, and a study by Marlowe et al. (2005) (free PDF) shows that Hadza males prefer females who have a low waist-to-hip ratio in profile (i.e. due to protruding buttocks), rather than from the front (i.e. an hourglass or wasp-waist shape), while Western males prefer the converse. The authors did not collect personality data on the Hadza males, and did not test to see whether a male preferred boobs or buttocks, but in principle this would not be difficult to do, and we could see whether a similar pattern showed up among African hunter-gatherers.

That brings us to 2), whether or not natural selection had a role in the emergence of boobmen. Clearly they are a new morph within homo sapiens. They are too high in frequency to be the result of de novo mutations here and now, and they did not all migrate from some pre-historic Martian colony of homo sapiens. That leaves genetic drift or natural selection. Genetic drift can cause allele frequencies to go up or down over time, but it cannot produce design. Mate preferences are too specific and coordinated during development to admit a believable drift explanation: natural selection appears to have fashioned them.

But toward what end? We don't need to know, really. With the completion of the HapMap project, we are learning of tons of cases of natural selection in human beings, and we largely have no clue what it was up to. The numbers don't lie. Still, let's indulge in a little conjecture just to show that the idea isn't so perplexing in the case of boobmen.

In many areas of life, there is no one best solution, and we face a trade-off. If I develop conspicuous ornaments, that may make me more attractive to females, but it may also give me away to predators more easily, or provoke the envy of duller looking males, who might ostracize me (no small matter in a social species). Duller looking males might avoid predators and envy-based ostracism, and may be able to work better in groups because of this, but they won't be as attractive physically to females. The result is that some fraction of males will be dull and the rest conspicuous. We would need tools from game theory and differential equation modeling to spell out what parameters are involved, and what the exact frequency of each would be at equilibrium. But the point is that neither is universally favored, so both will co-exist.

So it could be with boobmen and assmen. I don't think these preferences per se were the target of selection, but again that they correlate with other personality traits that have been under selection. For instance, everyone says that compared to boobmen, assmen are more likely to have polygynous tendencies, to prefer short-term relationships, and to emphasize female qualities most relevant to the short-term (such as her most sexual body part, the derriere). We don't know if that's true, but it would be surprising if everyone had the same specific delusion. Since both short-term and long-term strategies have pros and cons, both could co-exist.

If being a boobman is linked to a more monogamous orientation, we are asking how natural selection could have driven up the frequency of monogamous males in societies where boobmen are common, such as Northern Europe. Maybe agriculture there requires the father to stick around and provide for his kids, whereas in parts of sub-Saharan Africa where farming has lower energy requirements, females can farm on their own and not worry about whether the father will stay with her. I don't claim that this is the only way it could have happened; this example is just to illustrate how simple the process can be.

Turning to 3), the mechanism does not have to be known in order to talk about the adaptive value of the trait (see Niko Tinbergen's Four Why's for clarification). We know that lighter eyes were selected for in Europeans, but we could know this fact even if we didn't know what biochemical pathways are involved in eye color. Still, let's indulge in a little more conjecture just to show how non-mysterious the mechanism can be. It may be as simple as testosterone level, with assmen having higher T than boobmen.

This is an incredibly easy hypothesis to study empirically, though from Googling it looks like no one has done so. To repeat a finding from the boobman, assman, and legman study, though, the assmen and those who prefer both large breasts and large buttocks have more ambitious personalities and are Type-A businessmen. We also know that in various species, such as the dark-eyed Junco, higher testosterone makes a male more polygynous and less likely to stick around to help raise the kids. Whatever the mechanism turns out to be, investigating the matter is not so perplexing that we don't even know where to start looking.

Last, let's examine some very popular but utterly ridiculous hypotheses for why human females evolved large breasts, summarized here. First of all, it is not true that human females have large breasts -- some have small, some medium, and some large. Look at the picture of the chimp in that summary -- you see human females with breasts that small (or large) all the time. This is not hairsplitting: it suggests that breast size reflects some trade-off.

For example, the trade-off could be in fat deposition: if you have a fixed amount of fat and want to be conspicuous, you had better put the bulk of it in one place or the other. Only gifted (or cursed) females have so much to go around that they can have large breasts and large asses. Those who put it in their chests are probably pursuing a long-term mating strategy, and those who put it in their behinds are probably pursuing a short-term strategy, on the assumption that female supply has evolved to meet male demand.

The evolution of breasts has nothing to do with mimicry of the buttocks -- can you think of any other way that a man might view buttocks-resembling things on a woman if he wanted to? Moreover, do assmen respond at all similarly to boobs as to the buttocks? This hypothesis predicts that they should be roughly interchangeable, but I don't even notice who has big or small boobs unless someone points it out to me, and I have no way of judging what "good boobs" look like, according to boobmen. It also has nothing to do with our species' face-to-face sexual position -- again, can you think of any other way a man might look at buttocks-resembling things while having sex? And as misleading as the name may be, doggy-style is not a trait that humans have lost, like a coat of body fur.

Neither does it have to do with our bipedal posture: it's true that this posture would have obscured any rump swellings (as chimps have), but the fleshy buttocks have still been in plain view ever since -- and typically, more viewable from afar than the breasts, as they take up more volume. Five-hundred years from now, the scientific consensus will be that invoking bipedal posture as a driver of some clearly unrelated change was the 20th century equivalent of ancient Greek theories about trepidation of the spheres.

Since such hypotheses are so easily debunked, why have they persisted for as long as they have? Napoleon said that you should never attribute to malice that which can be explained by incompetence, and here the incompetence surely stems from the majority of researchers and commentators in this area being boobmen, legmen, or women. (Women will grudgingly admit that boobs and legs can be things of beauty, but recoil in disgust upon pondering the booty.) We all have a tendency to extrapolate from the personal to the universal, but when I find out that someone thinks or behaves differently than I do, I ask what forces could cause them to think or behave in such deviant ways. That's how you get a clearer picture of how the world works, but it relies on there being a diversity of views within the field. It's about time some assmen joined the ranks of sex researchers to set the field straight.

Labels: babes and hunks, Human Evolution