Sexual selection is a variant of natural selection in which one gender prefers certain traits be present in their mate. Thus individuals with those attractive traits will have a high reproductive success, spreading their genes (and the trait) through the population.
This can also cause the attractive trait to become greatly exaggerated, so as to exploit the other gender’s preference for it. This is the process which resulted in the large, elaborate tails of peacocks.
Given the influence sexual selection can have on a population, researchers started to wonder if there were any traits in humans that were the product of mate-choice preferences. So they started to look at some seemingly “non-functional” aspects of humanity and seeing if they made someone more attractive.
One of the first traits they identified was male chest hair which seems to have no functional purpose (belly button cleaning aside) yet was strongly correlated with being perceived as attractive. This provides strong evidence that male chest hair is a sexually selected trait.
How attractive women find two body types with and without chest hair
Other traits that seemed to be sexually selected for were size and waist-hip ratio. Women, it would seem, look for men who are taller than they are (but only by ~9%). Importantly this was relative, not absolute, size. Height was not inherently attractive but being taller than they were was. A small woman might find a below average man attractive. Men look for women who are shorter than they are (again relatively) and with a 0.7 waist-hip ratio (i.e. the waist is 70% the width of the hip).
How attractive various waist-to-hip ratios are viewed by men and women.
Now, why might sexual selection be acting on these traits? Three main explanations have been put forwards as to why these traits are attractive.
- They indicate the potential mate is in good health/fertile. Waist-hip ratio is a product of fat deposition, so a sufficiently high waist-hip ratio would mean they are not malnourished. Alternatively if it were too high then they would be overweight and thus less healthy. One would expect for there to be a preferred medium and this does seem to be the case.
- They indicate the potential mate has good genes. Waist-hip ratio is, for some bizarre reason, associated with lower rates of some cancers.
- They exploit a sensory bias in the chooser. Maturity in men associated with wealth, making older men more attractive. Chest hair makes men look older. Perhaps this is an attempt to exploit a woman’s preference for mature men.
Because of this, not only is picking someone with these traits beneficial but the desire to pick someone with those traits is itself beneficial. If I find an indicator of fertility attractive I will be more likely to pick a fertile mate. I win.
In this manner natural selection will cause both the desire for the trait and the trait itself to spread throughout the population. At this point it stops mattering if the trait indicates a healthy individual etc., it will be selected for just because people find it attractive!
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Oh Darwin you hound-dog you.
At that point you might think we can sit back. After all, we have some traits (and explanations for them) that match predictions made by sexual selection. Case closed?
If I were a newspaper, or someone else more biased towards shock value than truth, that is certainly where I’d end it. But unfortunately the tale gets a little bit more complex and uncertain at this point and it all boils down to the two main ways of looking at how human evolution has influenced human behaviour.
The first – and more famous – field is evolutionary psychology. For it to conclude that a trait is the result of adaptation (in this case adaptation driven by sexual selection) it must meet a series of criteria that an adapted behaviour would meet, i.e. universal in a population, gives an advantage etc.
The other is human behavioural ecology. For it to decide a trait is the result of adaptation, it must be shown to confer some reproductive advantage. For example, you would have to demonstrate people with chest hair had a higher rate of reproduction.
Both approaches have their advantages and disadvantages.
By demanding evidence of a higher rate of reproduction, human behavioural ecology offers a more conclusive answer to whether a trait is adaptive. However, it can only be applied to organisms whose rate of reproduction you can study. Wanna see if something was beneficial in the past, but no longer is? HBE isn’t really for you.
Evolutionary psychology, with its list of criteria, is more generally applicable. For example, if something was previously adaptive, but no longer is, you would expect it to be universal. However since it doesn’t normally show if the trait does offer a genuine reproductive advantage it makes these conclusions with less certainty.
As you’ve probably noticed, a lot of work into human sexual selection has been done so in a manner more reminiscent of evolutionary psychology. As such there is still a rather large potential for these results to be wrong. So, whilst it looks just like chest hair is being selected for in men and waist-hip ratio is being sexually selected for in humans generally, be sure to take it with a grain of salt.
| Dixson AF, Halliwell G, East R, Wignarajah P, & Anderson MJ (2003). Masculine somatotype and hirsuteness as determinants of sexual attractiveness to women. Archives of sexual behavior, 32 (1), 29-39 PMID: 12597270 |
| Sybil A Streeter, Donald H McBurney (2002). Waist–hip ratio and attractiveness: New evidence and a critique of “a critical test” Evolution and Human Behavior, 24 (2), 88-98 : 10.1016/S1090-5138(02)00121-6 |
| Pawłowski B (2003). Variable preferences for sexual dimorphism in height as a strategy for increasing the pool of potential partners in humans. Proceedings. Biological sciences / The Royal Society, 270 (1516), 709-12 PMID: 12713744 |
Interesting; such a complex subject that it’s difficult to get your head around all of it and not think of a hundred questions.
When writing I did have so many interesting tangents to go off on, it got to a point where I just had to say “no, stop writing about that. Forget about how interesting it is if you want to be finished before midnight”
“They exploit a sensory bias in the chooser” + “the desire to pick someone with those traits is itself beneficial” does it. You don’t really need all of the speculation about why the trait might be adaptive in the first place. Once it starts being selected for sexually, it doesn’t necessarily have to be linked with it’s adaptive usefulness in some other context, it is self-reinforcing.
You don’t need to understand why it is adaptive in the first place, but none the less I find it interesting. But you are correct that once sexual selections takes hold it just runs away with it pretty much.
I remember a TV show or movie. There was an African tribe which considered very hefty women to be the most attractive. There was this one ugly skinny girl who appeared to have very low fitness. However, the Chief had sent his son to the USA, at a fairly early age, to become educated. The son comes home, with PhD, and fixates on the ugly skinny girl. There is general consternation, but eventually the tribe accepts that the son has a different set of criteria for attractiveness, and they live happily ever after.
I do always wonder what the influence of society is in this kind of situation. Do we find something attractive because of a unique personal preference; sexual selection or societal indoctrination? And to what extent can they be separated? After all, society might simply be “formalising” an innate evolutionary desire.
Case studies like the one you raise help cement the idea that sexual selection humans is a lot more complex than many people give it credit for.
In olden times, being fat meant you were wealthy and could afford to eat heavily. It was considered a good thing. It seems natural that in a place like Africa, where being skinny and bony would be the norm, they would find someone capable of becoming fat to be a better mate.
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Hi Adam,
There are a few other approaches to studying sexual selection in humans, including using historical data. I cover a few recent papers in a recent post.
This is an expansive field and I certainly could’ve kept writing my post for a lot longer. I just got a bit lazy.
You’re post is very informative and you seem to pick up on some of the more complex mathematics that flew over my head when I covered one of those papers. Glad to see there’s someone out there who at least knows what they’re talking about.
That’s quite a compliment: I view my studies of science as an unfolding of how much I don’t understand
You, me and most people (from what I gather). Say what you will about Newton, his seashell quote is very apt.
“I was like a boy playing on the sea-shore, and diverting myself now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”
I’ve seen this in documentaries too. Apparently men find an extreme waist-hip ratio attractive (a ratio that cannot exist in nature) even though such a ratio (lower than 0.7) would mean not being able to give birth; however most men still prefer the 0.7 ratio and the preference for extreme ratios was explained as just a by-product or extreme form af a logical sexual preference. Good, neutral reporting, too.
That’s very interesting and it’s certainly important to note that simply being sexually selected for doesn’t mean it is being chosen because the resulting offspring are extra fit. Sometimes creatures can find traits attractive (selecting for them) just because. Assuming there is always a fitness increase is incorrect.
Does age have an effect on sexually selected traits? Can sexually selected traits differ between generations or age difference? For instance, with the chest hair example, you state that chest hair is seen as more attractive. However, the amount of chest hair a man has could potentially be unattractive depending on his age. Younger females would tend to prefer less chest hair if the male was younger. Whereas chest hair could be more attractive to older women looking to mate with older men. Depending on age, chest hair could be seen as very unattractive or attractive. Can sexually selected traits differ with age?
What you are suggesting is not impossible although I suspect it would be unlikely.
The crux of the issue is that sexual selection requires that a trait leads to more reproduction. Since younger women tend to be more fertile then they will be the strongest selection pressure since being attractive to them is more likely to result in babies. As such most sexually selected traits should be attractive to young women, or at the very least not unattractive to them.
In accordance with this 59% of participants in the study which found chest hair to be were <30 year olds.
Do you think chest hair is a discrete trait or do you think it is more of an inheritable continuous trait depending on ones parents.?
Chest hair appears to be continuous, although there hasn’t been much research into the issue.
How do these two traits (chest hair and waist-hip ratio) have to do with fertility? Like in sexual selection in animals, do humans consider the direct/indirect benefits while picking mates based on attractiveness? While obviously individuals choose mates based on directly benefits (what they are attracted to), on the other hand, is there any indication that humans use indirect benefits and pick physical traits based on what they want their offspring to express and benefit from?
If a trait improved the success of offspring then it would be beneficial for the parent to find a mate who would bestow that trait on the offspring attractive. Thus natural selection would include “things that are good for the young” on the list of “things I find attractive,” no real forward planning is needed.
Chest hair isn’t directly correlated with fertility, instead it makes someone appear to be more mature. Since more mature people generally have more resources this is something a woman (who expends a lot of resources during childrearing) would be benefited by.
Waist/hip ratio is correlated with being healthy which is in turn correlated with being fertile.
I am interested to know where you found the research on chest hair. Because there was not that much background information on the findings, I wonder if there were any confounding variables that they weren’t able to control for such as race or age. Do you think that these results could be misleading or skewed in some way?
Furthermore, do you believe that there are ways to conduct experiments on human sexual selection using human behavioral ecology instead of evolutionary psychology since it’s pretty hard to measure reproductive success? I feel that because cultural differences or even the economy can effect the number of children individuals have, this could be a very hard thing to come up with a valid measure for.
Whilst the study does appear to have contained a bias towards younger women it also interviewed women from different countries (Britain and Sri Lanka) which would probably compensate for various social and economic biases in the study.
The big problem with applying behavioural ecology to humans is it takes them quite a while to mate, thus measuring reproductive success is quite difficult. Most studies on the matter have had to look at past populations since you can see birth, death and marriage records allowing you to grab a large quantity of data easily.
However it doesn’t normally record chest hair on marriage certificates so for most of what a HBE would want to study you’d have to do your own research. Which brings me back to the prohibitive nature of the long human reproductive cycle. It is possible and has been done, it’s just difficult to keep the “experiment” running for the necessary length of time. Hence the popularity of evolutionary psychology
Where does women’s preference for a 0.7 WHR on men come from? When I look at Streeter and McBurney, they describe an experiment in which men and women are asked to rate the attractiveness of photoshopped images of a woman. Unless I misunderstood their description, it seems that this would only give us an idea about what both men and women prefer when looking at a woman.
You are correct, I misread the article and have amended the offending section.
If you include a few real Margaritas with that metaphorical grain of salt, I predict more sex being selected for.
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Yeah, this makes sense…. But apparently i have an unusual mindset about what is attractive in women, because I have this crush on a girl at my high school who I find very pretty… She has very pale skin(I don’t think tans look flattering at all on girls), dark brown (almost black) hair(I don’t like blonds anymore), and facial features that look somewhat similar to mine(you’ve probably heard of those studies that show humans are attracted to people with similar features)… But the unusual thing about her is that her hips are very skinny and her w/h ratio looks to be about 0.9, and her overall body frame is sort of masculine looking(except her lady parts, which are properly proportioned)… But I find that quality very sexy… To me it says she is in good shape and has a lithesome body. And before I had my crush on her I was attracted to what men are “usually” attracted to. But it seems that I like her so much ive grown to like all of her physical aspects, and not those of average women… As a matter of fact Im now UNattracted to women with fat and curvy hips… Hmm. And btw I’m quite certain she has a crush on me too. So I guess it just goes to show that I value facial beauty over “hotness”, and then I learn to like the rest of someone. And of course it shows that everyone can have a different opinion of what’s attractive.
Beauty is in the eye of the beholder!
Why do adolescent girls have such pert boobies? According to someone in the thread below, it’s to advertise the fact they haven’t got up the duff yet and still have all their baby making years ahead of them. Is this right or crap?
http://www.boards.ie/vbulletin/showthread.php?t=2056757943
Wow…
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Adam, do you know of any studies to determine the age that men find a woman’s face the most attractive? If so, what’s the answer?
Personally, I would guess about 14.
Whilst there is a general trend to prefer younger women (up to a point, extremely young women – including age 14 – aren’t) there is no specific age at which beauty peaks. There is just too much variation between people. If there is, it seems to be in the 20s at least.
>If there is, it seems to be in the 20s at least.
Nah, take a look at what sells the most in the porn industry: 18yr olds made up to look about 14. They can’t use real 14yr olds like the market wants so what they do instead is use legal 18yr olds and give them the face of a 14yr old.
Because porn is famous for being an accurate description of reality.
No, it shows what men want.
That is a massive oversimplification
Yeah, 14 is about right. The facial template cosmetic surgeons use is based on the facial dimensions of a 14 year old girl because they’re considered to be the most attractive.
Horse whisperer , you are full of nonsense
You’re in denial.
More nonsense
What exactly is nonsense? Explain.
This is great AGAIN! Loved it.
Me, when people have said “what’s your type”, I’ve never been able to describe. I’ve been with Asian, Russian, Aussie average, blonde hair, dark hair. It truly was THE MAN that I got into, not the look of the man (as much). I’ve been with some average looking men, but who were totally engaging in so many other ways.
Your coolimageology pic doesn’t work, by the way.
Cheers – love your stuff!
I got read of the image. In all honesty it was never that cool.
Why do the females in our species become sexual BEFORE they become fertile?
Let’s be realistic. We may try and pretend that girls don’t become sexual until some socially approved age like 16 or 18 but the biological reality is that it happens at about 12. When the hormones kick in they develop an interest in sex and become sexually ornamented with boobs, pubic hair and sharper adolescent facial features that the males soon notice.
Now this is interesting because the typical age that girls would start ovulating and become fertile in prehistory was probably not until about 14. It looks like evolution has engineered girls to become sexual a couple of years before they become fertile.
What purpose does this serve?
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