Predicting The Future With Faces

“Your future will be horrible, but at least it will be short. So there’s that”

The future is always uncertain, at least as far as human (and non-human) knowledge is concerned. This is one reason why some people have difficulty saving or investing money for the future: if you give up rewards today for the promise of rewards tomorrow, that might end up being a bad idea if tomorrow doesn’t come for you (or a different tomorrow than the one you envisioned does). Better to spend that money immediately when it can more reliably bring rewards. The same logic extends to other domains of life, including the social. If you’re going to invest time and energy into a friendship or sexual relationship, you will always run the risk of that investment being misplaced. Friends or partners who betray you or don’t reciprocate your efforts are not usually the ones you want to be investing in the first place. You’d much rather invest that effort into the people who will give you better return.

Consider a specific problem, to help make this clear: human males face a problem when it comes to long-term sexual relationships, which is that female reproductive potential is limited. Not only can women only manage one pregnancy at a time, but they also enter into menopause later in life, reducing their subsequent reproductive output to zero. One solution to this problem is to only seek short-term encountered but, if you happen to be a man looking for a long-term relationship, you’d be doing something adaptive by selecting a mate with the greatest number of years of reproductive potential ahead of her. This could mean selecting a partner who is younger (and thus has the greatest number of likely fertile years ahead of her) and/or selecting one who is liable to enter menopause later.

Solving the first problem – age – is easy enough due to the presence of visual cues associated with development. Women who are too young and do not possess these cues are not viewed as attractive mates (as they are not currently fertile), become more attractive as they mature and enter their fertile years, and then become less attractive over time as fertility (both present and future) declines. Solving the second problem – future years of reproductive potential, or figuring out the age at which a woman will enter menopause – is trickier. It’s not like men have some kind of magic crystal ball they can look into to predict a woman’s future expected age at menopause to maximize their reproductive output. However, women do have faces and, as it turns out, those might actually be the next best tool for the job.

Fred knew it wouldn’t be long before he hit menopause

A recent study by Bovet et al (2017) sought to test whether men might be able to predict a woman’s age at menopause in advance of that event by only seeing her face. One obvious complicating factor with such research is that if you want to assess the extent to which attractiveness around, say, age 25 predicts menopause in the same sample of women, you’re going to have to wait a few decades for them to hit menopause. Thankfully, a work-around exists in that menopause – like most other traits – is partially heritable. Children resemble their partners in many regards, and age of menopause is one of them. This allowed the researchers to use a woman’s mother’s age of menopause as a reasonable proxy for when the daughter would be expected to reach menopause, saving them a lot of waiting. 

Once the participating women’s mother’s age of menopause was assessed, the rest of the study involved taking pictures of the women’s faces (N = 68; average age = 28.4) without any makeup and with as neutral as an expression as possible. These faces were then presented in pairs to male raters (N = 156) who selected which of the two was more attractive (completing that task a total of 30 times each). The likelihood of being selected was regressed against the difference between the mother’s age of menopause for each pair, controlling for facial femininity, age, voice pitch, waist-to-hip ratio, and a value representing the difference between a woman’s actual and perceived age (to ensure that women who looked younger/older than they actually were didn’t throw things off).

A number of expected results showed up, with more feminine faces (ß = 0.4) and women with more feminine vocal pitch (ß = 0.2) being preferred (despite the latter trait not being assessed by the raters). Women who looked older were also less likely to be selected (ß = -0.56) Contrary to predictions, women with more masculine WHRs were preferred (ß = 0.13), even though these were not visible in the photos, suggesting WHR may cue different traits than facial ones. The main effect of interest, however, concerned the menopausal variable. These results showed that as the difference between the pair of women’s mother’s age of menopause increased (i.e., one woman expected to go through menopause later than the other), so too did the probability of the later-menopausal woman getting selected (ß = 0.24). Crucially, there was no correlation between a woman’s expected age of menopause and any of the more-immediate fertility cues, like age, WHR, facial or vocal femininity. Women’s faces seemed to be capturing something unique about expected age at menopause that made them more attractive.

Trading off hot daughters for hot flashes

Now precisely what features were being assessed as more attractive and the nature of their connection to age of menopause is unknown. It is possible – perhaps even likely – that men were assessing some feature like symmetry that primarily signals developmental stability and health, but that variable just so happen to correlate with age at menopause as well (e.g., healthier women go through menopause later as they can more effectively bear the costs of childbearing into later years). Whatever systems were predicting age at menopause might not specifically be designed to do so. While it is possible that some features of a woman’s face uniquely cues people into expected age at menopause more directly without primarily cuing some other trait, that remains to be demonstrated. Nevertheless, the results are an interesting first step in that direction worth thinking about.

References: Bovet, J., Barkat-Defradas, M., Durand, V., Faurie, C., & Raymond, M. (2017). Women’s attractiveness is linked to expected age at menopause. Journal of Evolutionary Biology, doi: 10.1111/jeb.13214

What Can Chimps Teach Us About Strength?

You better not be aping me…

There was a recent happening in the primatology literature that caught my eye. Three researchers were studying patterns of mating in captive chimpanzees. They were interested in finding out what physical cues female chimps tended to prefer in a mate. This might come as no surprise to you – it certainly didn’t to me – but female chimps seemed to prefer physically strong males. Stronger males were universally preferred by the females, garnering more attention and ultimately more sexual partners. Moreover, strength was not only the single best predictor of attractiveness, but there was no upper-limit on this effect: the stronger the male, the more he was preferred by the females. This finding makes perfect sense in its proper evolutionary context, given chimps’ penchant for getting into physical conflicts. Strength is a key variable for males in dominating others, whether this is in the context of conflicts over resources, social status, or even inter-group attacks. Males who were better able to win these contests were not only likely to do well for themselves in life, but their offspring would likely be the kind of males who would do likewise. That makes them attractive mating prospects, at least if having children likely to survive and mate is adaptive, which it seems to be.

What interested me so much was not this finding – I think it’s painfully obvious – but rather the reaction of some other academics to it. These opposing reactions claimed that the primatologists were too quick to place their results in that evolutionary context. Specifically, it was claimed that these preferences might not be universal, and that a cultural explanation makes more sense (as if the two are competing types of explanations). This cultural explanation, I’m told, goes something like, “chimpanzee females are simply most attracted to male bodies that are the most difficult to obtain because that’s how chimps in this time and place do things,” and “if this research was conducted 100 years ago, you’d have observed a totally different pattern of results.”

Now why the difficulty in achieving a body is supposed to be the key variable isn’t outlined, as far as I can tell. Presumably it too should have some kind of evolutionary explanation which would make a different set of predictions, but none are outlined. This point seems scarcely realized by the critics. Moreover, the idea that these findings would not obtain 100 years ago is tossed out with absolutely no supporting evidence and little hope of being tested. It seems unlikely that physical strength yielding adaptive benefits is some kind of evolutionary novelty, or that males did not differ in that regard as little as a hundred years ago despite plenty of contemporary variance.

One more thing: the study I’m talking about didn’t take place on chimps. It was a pattern observed in humans. The underlying logic and reactions, however, are pretty much spot on.  

Not unlike this man’s posing game

It’s long been understood that strong men are more attractive than weak ones, all else being equal. The present research by Sell et al (2017) was an attempt to (a) quantify approximately how much of a man’s bodily attractiveness is driven by his physical strength, (b) the nature of this relationship (whether it is more of a straight line or an inverted “U” shape, where very strong men are less attractive, and (c) whether some women find weaker men more attractive than stronger ones. There was also a section about quantifying the effects of height and weight.

To answer those questions, pictures of semi-to-shirtless men were photographed from the front and side, and their heads were blocked out so only their bodies remained. These pictures were then assessed by different groups for either strength or attractiveness (actual strength measures were collected by the researchers). The quick run down of the results are that perceived strength did track actual strength, and perceptions of strength accounted for about 60-70% of the variance in bodily attractiveness (which is a lot). As men got stronger, they got more attractive, and this trend was linear (meaning that, within the sample, there was no such thing as “too strong” after which men got less attractive). This pattern was also universal: there was not a single women (out of 160) who rated the weaker men as more attractive than the stronger ones. Accounting for strength, height accounted for a bit more of the attractiveness, and weight was negatively related to attractiveness. Women liked strong men; not fat ones.

While it’s nice to put something of a number on just how much strength matters in determining male bodily attractiveness (most of it), these findings are all mundane to anyone with eyes. I suspect they cut across multiple species, and I don’t think you’re going to find just about any species where females prefer to mate with physically weaker males. The explanation for these preferences for strength – the evolutionary framework into which they fit – should apply well to just about any of the species in that list. While I initially made up the fact that this study was about chimps, I’d say you’re likely to find a similar set of results if you did conduct such work.

Also, the winner – not the loser – of this contest will go on to mate

Enter the strange comments I mentioned initially:

“It’s my opinion that the authors are too quick to ascribe a causal role to evolution,” said Lisa Wade…“We know what kind of bodies are valorized and idealized,” Wade said. “It tends to be the bodies that are the most difficult to obtain.”

Try reading that criticism of the study and imagine it was applied to any other sexually-reproducing species on the planet. What adaptive benefits is “difficulty in obtaining” supposed to bring and what kind of predictions does that idea make? It would be difficult, for instance, to achieve a very thin body; the type usually seen in anorexic people. It’s hard for people to ignore their desires to eat certain foods in certain quantities, especially to the point you begin to physically waste away. Despite that difficulty in achieving the starved look, such bodies are not idealized as attractive. “Difficult to obtain” does not necessary translate into anything adaptively useful. 

And, more to the point, even if a preference for difficult-to-obtain bodies per se existed, where would Lisa suggest it came from? Surely, it didn’t fall from the sky. The explanation for a preference for difficult bodies would, at some point, have to reference some kind of evolutionary history. It’s not even close to sufficient to explain a preference by saying, “culture, not evolution, did it,” as if the capacity for developing a culture itself – and any given instantiation of it -  exists free from evolution. Despite her claims to the contrary, it is a theoretical benefit to thinking about evolutionary function when developing theories of psychological form; not a methodological problem. The only problem I see is that she seems to prefer worse, less-complete explanations to better ones. But, to use her own words, this is “…nothing unique to [her]. Much of this type of [criticism] has the same methodological problems

If your explanation for a particular type of psychological happening in humans doesn’t work for just about any other species, there’s a very good chance it is incomplete when it comes to explaining the behavior at the very least. For instance, I don’t think anyone would seriously suggest that chimp females entering into their reproductive years “might not have much of an experience with what attractiveness means,” if they favored physically strong males. I’d say it’s fairly common such explanations aren’t even pointing in the right direction a lot of the time, and are more likely to mislead researchers and students than help inform them. 

References: Sell, A., Lukazsweki, A., & Townsley, M. (2017). Cues of upper body strength account for most of the variance in men’s bodily attractiveness. Proc. R. Soc. B 284