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.
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).
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!
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|