Sexual selection in humans: beards, chins and maps

Many researchers believe that certain human traits act as a signpost, advertising the fact that the individual has good genes, or is healthy, or just generally great. As such, finding these traits attractive will result in mating with healthy individuals with good genes. Then evolution kicks in, and the advantages of being attracted to these traits (and possessing them) spread throughout our population until it’s a ubiquitous characteristic of humanity. This is sexual selection.

For example, two researchers from the University of New South Wales recently conducted a study into how attractive men and women find beards. Dixon and Brook’s (2013) results indicate that the more beard a guy has, the more masculine and healthy they’re considered by both men and women alike. The sexual selection implication of this is that a guy with full beard has the genes and health needed to grow it, thus advertising just how good beardy is as a mate.

Except that women don’t find full beards most attractive. Sure they rate them as most masculine and most healthy, but heavy stubble, rather than full beard is rated as the peak of attractiveness. The researchers hypothesise that this is because picking a mate who is too masculine isn’t necessarily a good thing. They may be more likely to get into fights, take risks and wind up widowing the wife and leaving her to raise his beardy babies alone. Going for a stubble-sporting mate may be a safer bet.

In research, it's important to define "beard" with pictorial examples

In research, it’s important to define “beard” with pictorial examples

But before all you hip, happening bachelors start growing just the right amount of stubble to seem sexy; it’s worth noting that there’s a flaw in the evolutionary reasoning behind this research. Cast your mind back to how evolution actually works:

  1. Everyone is slightly different.
  2. Those differences lead to different rates of survival and reproduction
  3. In subsequent generations, more/less people have those differences depending on if they increased/decreased rates of survival and reproduction

Spotted the problem yet? Evolution works through the differential survival/reproduction of different individuals. It’s really hard to make a case for evolution (in the form of sexual selection) influencing how attractive beards are without showing that having that super-attractive heavy stubble actually increases your chance of having babies. And these researchers didn’t do that.

Now, waiting for the next generation to pop out takes time so most researchers try to think of shortcuts. Typically this is “look for universal traits.” After all, point 3 indicates that a beneficial, evolved trait will spread throughout the population. So if being attracted to beards is beneficial and thus has evolved, it should be something true for all women. However, given 80% of the women interviewed by the beard study were European, I hardly think they’ve identified a universal consensus on what is attractive. 

Indeed, there are many examples of researchers identifying a trend in a limited sample size; but having it disappear when they ask a greater variety of women (or men). For example, a guy develops a broad chin when they have a lot of testosterone during development (Enlow, 1982); making it a sign of well….testosterone! Having testosterone is generally a good thing for men, so if a woman is attracted to a broad chin they will be sure to bag a mate who will ensure her children have the genes needed for good testosterone.

And sure enough, studies indicated that broad chins were attractive (Rhodes et al., 2003). But then Thayer and Dobson (2013) analysed the skeletons of men from around the world and found major differences between their chins. No single type of chin – broad or not – was universal. This is the opposite of what we’d expect if evolution was spreading traits throughout the population; indicating that having a broad chin is not being (sexually) selected for.

The outlines of chins from around the world!

The outlines of chins from around the world!

It’s not just studies of the influence of sexual selection that suffer from this problem. Attempts to identify the impact of evolution on other behaviours and traits typically also use the “look for universals” approach; but make conclusions before having examined a large enough sample.

For example, men have better spatial memory than women. Not quite enough to justify tired stereotypes about women and maps, but the difference is still significant. This is a universal characteristic, and was long thought to be the result of evolution. Back when we were hunter-gatherers, men had to hunt over large territories; requiring them to develop better spatial memory to remember where everything was. Conversely women gathered, which didn’t involve travelling over quite so large an area.

But then more widespread study examined dozens of animal species and found spatial reasoning was not correlated with territory size. Males always had better reasoning, regardless of whether or not they had a larger territory than the females. Eventually the researchers realised extra testosterone associated with having balls was causing the effect and were able to make a female rate better at remembering where things were by injecting her with the stuff  (testosterone, not testicles) (Clint et al., 2012). 

Dimorphism = difference between genders. As you can see, there is no real relationship between different territory sizes and different spatial ability

Dimorphism = difference between genders. As you can see, there is no real relationship between different territory sizes and different spatial ability

So even when research does identify a universal trait it still isn’t necessarily an example of psychological evolution. It could be the result of chance, or testosterone, or something else. As such the only real way to investigate the influence of evolution on our mind is to look for evidence of steps 1 & 2 of evolution. To look for traits which actually give a guy (or a gal) an increased chance of producing children.

Sure, looking for universal traits is a good way for coming up with ideas about what may have been influenced by evolution; but that’s just the first step on a long road to show evolution is involved. So the next time you hear an evolutionary psychology story about why things are the way they are (like how rape evolved to give lesser-quality men a chance to reproduce) be skeptical. Ask for evidence of evolution actually taking place, not just “universal” traits. Otherwise their story is no different from fairy tales about why the leopard got his spots.

And thou shalt not use evolution to justify bollocks.


Clint, E. K., Sober, E., Garland Jr, T., & Rhodes, J. S. (2012). Male superiority in spatial navigation: adaptation or side effect?. The Quarterly review of biology,87(4), 289-313.

Dixson, B. J., & Brooks, R. C. (2013). The role of facial hair in women’s perceptions of men’s attractiveness, health, masculinity and parenting abilities.Evolution and Human Behavior.

Enlow DH (1982) Handbook of Facial Growth. Philadelphia: .W.B. Saunders

Rhodes G, Chan J, Zebrowitz L, Simmons LW (2003) Does sexual dimorphism in human faces signal health? Proceedings: Biological Sciences 270: S93–S95

Thayer ZM, Dobson SD (2013) Geographic Variation in Chin Shape Challenges the Universal Facial Attractiveness Hypothesis. PLoS ONE 8(4): e60681

29 thoughts on “Sexual selection in humans: beards, chins and maps

  1. My gut reaction was how much social norms influences the attractiveness or otherwise of the beard. We might very well get different survey results from countries where most men have beards (Polynesian islands for example) and shaven men might be considered unappealing because they are physically child-like. Or look at the opposite where beards are rarer and compare the results.

    And then I found this from the Media Institute of Cancer Research, always known for their carefully researched articles: how women don’t like beards

    • I share a house with 5 girls and there is such a variety in opinion about beards, subjectively this makes my gut feeling skeptical of any universal statements on the matter, unless they’re backed up with a comprehensive survey. Which this study wasn’t

      Gut feelings aside, the confounding factor of social norms is why it’s really important to get as universal a sample as possible; and why I’m very skeptical of their results given the vast majority were European.

      • Absolutely. I can’t believe they didn’t even consider social factors in this – it should have been obvious to any first year undergraduate that limiting a European sample would skew these results.

      • In fairness their sample did include people from every continent (save Antarctica); it’s just that all the others accounted for such a low percentage. Just having a token representation from say, Africa, doesn’t make your results universal!

  2. This might well be another example of the publish or perish policy in science these days. The complete study taking into account a much wider variety of women might have taken a lot more time and the result might well have been not so clear. So guess what, they looked at one group and deduce the world from it. I am not saying this is fraud but it somewhat borders with dishonesty. This kind of attitude was pointed out as being widespread in certain scientific fields, in particular in psychology and biology, in a series of reports analysing the reasons of the Diederik Stapel scandal.

    • Whilst extrapolting these results to the world may be unjustified, beyond that I don’t think there’s anything awfully wrong with these studies. EvoPsych is a good way to do preliminary research and generate hypotheses about what might be the case. If they had said “we found this result about beards, it suggests sexual selection. This is interesting, more work needed” I wouldn’t have said a thing bad about it.

      The same is true for most EvoPsych work. It tends to get roundly critised by scientists, but I think the only problem is people overstepping their bounds and saying more than the evidence shows. It’s not a bad way of doing things, it’s jsut that it’s only really good for preliminary work.

      • Well the reason why they tend to overstate their results is that there is a selective pressure for this. The selection is performed by referees who favor : 1) clear cut results that in fact rarely exists in psychology; and 2) amazing consequences to these results, again a normally pretty rare occurence. It’s quite telling that Stapel himself started his fraud after one of his article came back, refused, precisely because of the two points above. From then on, he decided to get clear cut spectacular results, telling more or less what was expected, which he produced by first fudging the data and then faking it completely. It’s a slippery slope which could be avoided if referees favores honesty over panache !

      • I hope that the rise of the e-journal, with it’s lower costs, will stop researchers being pressured to produce impressive results that ensure a return on investment.

        Such e-journals have allowed for the rise of student-based journals; where young people can publish their less than explosive results. Hopefully the same will eventualyl happen for academia, where the pressure to revolutionise the world with every paper you write is removed in favour of honsety.

      • Your comment about e-journals reducing over-selling of results seems unlikely. The easier it is to publish, the more noise there is, and the louder you have to yell to be noticed :(.

      • I don’t think that’s what’s happening. There are a select number of high quality e-journals, and they’re more willing to publish results even if they aren’t revolutionary. For example, PLoS accepts ~80% of submitted, whilst classic print journals like Nature only accept ~8%. PLoS isn’t being drowned into obscurity because it – and other e-journals – has a more liberal acceptance policy.

  3. Beard and hair lengths change rapidly, in mere flashes of time. In one generation they seem “the natural,” but the next finds them affected, and sees shaven and cropped as natural. Of course this is part of a much larger pattern by which humans strive to justify their dress, food, behavior toward children and animals, as natural. We even have origin and founding stories to show how our occupation of a particular landscape is “natural.” Swept up in these preferences, we select mates who exemplify the most appropriate attitude toward these ways of being natural, which must overlap with seeming fit.

    • One thing the study did note is that men and women appear to find different lengths of facial hair attractive. As such, it may be that the variation in facial hair you describe is a product of other factors dictating what is fashionable, such as impressing men. So whilst these additional factors are something that should be investigated, I don’t think they alone can rule out the idea that sexual selection is involved in facial hair attractivenss

  4. I think it’s more likely that beards are for impressing other men rather than women.

    In prehistoric societies women were treated like property by the men and didn’t have much say in who they married so the best way for a man to secure wives wasn’t to impress the women themselves but to impress the male relatives who controlled them. I think the purpose of displays of masculinity, aggression and hunting prowess are less about impressing women, who generally find them a bit silly, but impressing other men in tribe and gaining status that will give a man access to women.

  5. The cultural variability is a huge problem for researchers trying to infer universal behavior (or anything, really) from evolutionary game theory.

    The part about spatial memory is pretty interesting, I didn’t know that before. I was still stuck in the hunters-vs-gatherers mindset. I remember a similar evo-psych argument being made for vision: women have better peripheral vision because they needed to watch out for predators while gathering and men have better foveation because they needed to focus on the prey they were hunting. Do you know if that argument holds up, or is it also explained by a side-effect of something, or is the evidence inconclusive?

    • The spatial memory-gatherer link always seemed a bit suspicious in my eyes, given that a popular hypothesis for why hominins became encephalised is to improve the ability to remember where fruit and other plant resources are. It seemed like gathering drives large brains, except when it doesn’t.

      I have no idea about the vision thing, but given the number of anti-evopsych studies coming out in recent months I suspect it may be in for a challenging soon. If not there’s probably a paper or two in there for someone. Critisicing EvoPsych is the new cool science thing, apparently.

  6. Cool post! I so agree with you that looking for the universality of a trait is not enough to conclude with confidence that it was selected for by evolution.

  7. Apart from the obvious fact that these presumed “evolutionary psychology” adaptions are clearly influenced by cultural norms I think the whole model is kinda stuffed. The human genome is only so big, like 20,000 protein encoding genes total. The vast majority of these genes are concerned with getting the basic systems up and running, cell growth and management, the immune system and physiological functioning. I’m making a vaguely informed guess that the number that specifically relate to brain fabrication is well under a thousand. There’s a fair bit of known stuff for these guys to do, creating multiple types of neurons, neurotransmitters, regulators, brain structures, and so on.

    A car might has 5000 parts and I don’t believe that you could write 1000 lines of computer code that would tell a robot how to build the parts and assemble them into a car. Admittedly, biological systems are somewhat self-assembling and the detail of gene action is not well understood but there isn’t going to be a lot of instruction space available for facial hair choice as proposed – not too long, and not too short, just right – unless optimising facial hair length in partners was a major driver of brain evolution. It seems to me that there’s enough evolutionary psychology traits and preferences that have been propose by someone somewhere to use up the entire encoding capacity of those thousand genes several times over. It’s a magic black box mentality, isn’t it?

    AFAIKS, this hard coded preference model just can’t work on gene numbers. There are other equally massive problems, including, but not limited to: (1) the genetically coded brain structure that responds to partner beard length has not been found and is in fact ludicrous, (2) the idea is more or less ignores everything that is known about cognitive psychology, and (3) (for heaven’s sake) the unshaven look is a fashion – a couple of decades ago it didn’t pull quality chicks, quite the reverse. The sexual selection novelty mechanism is a way more likely explanation of current beard preferences.

  8. Interesting read.
    How attractive would a bloke suddenly become if he had a beard AND a Ferrari?
    Or of course, one could simply shave only one half of one’s face.

  9. Pingback: Which male traits are being sexually selected for? | EvoAnth

  10. Pingback: Why you shouldn’t trust 96% of Evolutionary Psychology | EvoAnth

  11. First, I thought you did a nice job dissecting the paper and pointing out how we should not take things at face value. Second, I agree with folks that stressed that we really like origin stories but these are hard to determine. However, I think your emphasis that a trait must be universal in order to show that it has an evolutionary origin is flawed. I study populations along an elevational gradient that is only 2,000 meters and find distinct genetic differences in the populations as they adapt to local conditions. If you look at the poison dart frogs in the Bocas del toro region in panama, you can see that each island has a different looking population but that they are all considered to be the same species. In sexual selection it is more than reasonable to expect that the cues that suggest male quality could differ and even that if there are many, that females may evolve the preference for one over the other in different places. Drift can also lead isolated populations of drosophila to speciate in Hawaii as they get separated from each other and female preferences carry. Finally, is also a misnomer to suggest that a trait should always reach fixation if it is adaptive. There are things such as trade-offs and selection may actually favor variation. In Pygmy grasshoppers, their coloration is quite variable and it has been shown that predators have a hard time developing a search image when they are variable. Enjoyed your blog post.

    • You raise some interesting points that present some very real limitations to the evolutionary psychology method. However, I’d wager most EP researchers would respond by arguing that humans simply haven’t been apart and/or in different environments long enough to develop such differences. They frequently propose that there has been an “adaptive lag” in our species.

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