Maybe beauty is just “cognitive cheesecake”?

41czavSUnNL._SY344_BO1,204,203,200_Beauty matters a lot in our world. The entertainment and fashion industries are based on beauty. Obviously some aspect of beauty is socially constructed and contextual. Beauty standards can change. There was a time when many aspects of European physical appearance, from light hair and eyes, down to the lack of an epicanthic fold, were excluded from idealized canons in East Asia. Obviously that is not the case today, and one can give a very plausible explanation through recourse to recent history as to why those norms shifted. Similarly, there is even a possibility that something as central to evolutionary psychology as preference for a particular waist-to-hip ratio may vary as a function of material conditions. I is clear from the social historical scholarship that the ideal characteristics of a female mate are strongly conditioned on the resources of the male; lower status males put greater emphasis on the direct economic benefits which their partner may bring because they are more on the margin of survival. For much of history lower status males meant most males. That is, peasants.

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And yet cross-culturally there does seem to be a certain set of preferences which one might argue are “cultural universals.” People from “small-scale” societies are still able to consistently rank photographs of people from WEIRD societies in facial attractiveness which correlation with results from participants in developed nations. This indicates that there is a strong innate basis. An element of taste deep in our bones, even if we may inflect it on the margins, or increase or decrease its weight in our calculations of what makes an optimal mate. There may be societies where Lena Dunham’s “thick” physique may be preferred to Bar Refaeli‘s svelte profile, but I am skeptical that there would be societies where the former’s facial features would strike individuals as preferable to those of the latter (one might have to correct for Refaeli’s species-atypical hair and eye color, but European norms are pretty widespread outside of small-scale societies now, so that shouldn’t be a major issue). So the question then becomes: is this adaptive?

Evolutionary psychologists have a panoply of ready explanations. They are often grounded in correlations, and then adaptationist logic. For example, women with lower waist to hip ratios (0.7 being the target) have more estrogen, and are more likely to be nubile, and so are more fertile, all things equal. Since being more fertile is going to be a target of selection, a lower waist to hip ratio is going to be a target of selection, because implicitly there is a genetic correlation between estrogen and waist to hip ratio. The problem is that genetic correlations have to be proved, not assumed. Correlations are not necessarily transitive. Just because A has a positive correlation with B and B has a positive correlation with C, does not entail (necessarily) that A has a positive correlation with C.

With that in mind, a new paper looks at facial attractiveness, averageness of facial features, and heritability of both these traits. They use a twin design, with an N of ~1,800. And, they relate it to a comprehensible causal mechanism: mutational load resulting in increased developmentally instability. Basically, the more mutations you have, the more likely you have to exhibit facial asymmetry, and therefore facial averageness is a good proxy for genetic quality. It is well known that average faces tend to be rated better looking than non-average faces. This is part of an argument that Geoffrey Miller put forth in The Mating Mind, a very fertile work. There is an elegance to it. Unfortunately follow up work over the past ten years is suggesting that this simple model is either wrong, or, everything is a whole lot more complicated.

First, the paper, Facial averageness and genetic quality: Testing heritability, genetic correlation with attractiveness, and the paternal age effect. The abstract gives away the game:

Popular theory suggests that facial averageness is preferred in a partner for genetic benefits to offspring. However, whether facial averageness is associated with genetic quality is yet to be established. Here, we computed an objective measure of facial averageness for a large sample (N = 1,823) of identical and nonidentical twins and their siblings to test two predictions from the theory that facial averageness reflects genetic quality. First, we use biometrical modelling to estimate the heritability of facial averageness, which is necessary if it reflects genetic quality. We also test for a genetic association between facial averageness and facial attractiveness. Second, we assess whether paternal age at conception (a proxy of mutation load) is associated with facial averageness and facial attractiveness. Our findings are mixed with respect to our hypotheses. While we found that facial averageness does have a genetic component, and a significant phenotypic correlation exists between facial averageness and attractiveness, we did not find a genetic correlation between facial averageness and attractiveness (therefore, we cannot say that the genes that affect facial averageness also affect facial attractiveness) and paternal age at conception was not negatively associated with facial averageness. These findings support some of the previously untested assumptions of the ‘genetic benefits’ account of facial averageness, but cast doubt on others.

I’m going to reproduce some of the results from Table 4 below.

Averageness Attractiveness    
Heritability Non-heritable Heritability Non-heritable Genetic correl Env correl
Female 0.21 0.79 0.6 0.38 0.11 0.21
Male 0.22 0.78 0.62 0.39 0.11 0.08
Overall 0.21 0.79 0.6 0.4 0.11 0.16

What you see is very modest heritability for averageness, and a decent one for attractiveness. But, there’s no statistically significant evidence that the genetic correlation is there (the confidence intervals are huge around 0.11, from 0 to 0.35). Though they state the environmental correlation passes statistical muster (so common environmental variables might be producing attractiveness and facial averageness). Please note that a heritability of 0.6 does no mean a correlation of 0.6. The heritability of height is 0.8 to 0.9, but correlation of the trait across siblings is ~0.5. Heritability is the proportion of variation of the trait explained by variation in genes, in the population.

If you just look at heritabilities, averageness seems to have been under stronger selection than attractiveness all things equal. Usually strong directional selection removes the heritable variation on a trait. The high heritability gives us a clue that there are a lot of ugly people around still, and some of that is just the way they are born. In contrast, there are fewer people with lop-sided faces. These are subjects from a Western society, so I bet the results are going to be different in a high pathogen load environment (my expectation is that heritability will decrease, but perhaps it will actually increase because as genetic factors which allow for one to be robust to disease will become more important in explaining variation in the trait).

Finally, in the near future there will be high coverage genomic sequences from many people. If you hit the same marker more than 30 times you can conclude with decent confidence if it’s a de novo mutation unique to the individual. You can actually check how well mutational load tracks with averageness and attractiveness (each human has <100 de novo mutations, so there’s a lot of inter-sibling variance presumably). At this point I’m moderately skeptical of a lot of the selectionist models, whereas five years ago I’d have thought there would be something there, and it would be easy to discover. And it may be that beauty, like many aspects of culture, is not about adaptation and function in any direct sense, but simply a cognitive side effect. Like what Steve Pinker has stated about music. I don’t really believe that, but we can’t dismiss that position out of hand anymore.

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