Mr Muscle: Sexual selection in humans

Aug 27th 2009
From The Economist print edition

The price and privilege of beefcake

WHY are men’s muscles so much bigger than women’s? Partly, of course, because men do the fighting and hunting. But also, perhaps, because women like men who can do these things well, and are thus attracted to muscular men. Both phenomena—competing with members of the same sex and showing off to members of the opposite—are subject to a form of evolution known as sexual selection. It is sexual selection that created the deer’s antlers and the peacock’s tail, and William Lassek of the University of Pittsburgh and Steven Gaulin of the University of California, Santa Barbara, think it explains men’s muscles as well.

The main characteristic of sexually selected features is that they are expensive to maintain. Since, whether competing or attracting, only the best will do, resources get piled into them, almost regardless of the consequences. In a study just published in Evolution and Human Behavior, Dr Lassek and Dr Gaulin show that this crucial characteristic is true for men’s muscles.

Their data came from the National Health and Nutrition Examination Survey, which followed 12,000 American men and women over the course of six years. They found that men require 50% more calories than women do, even after adjusting for activity levels, and that their muscle mass is the strongest predictor of their intake of calories—stronger than their occupation or their body-mass index (a measure of obesity). And there is another cost to being muscly: men’s immune systems are less effective than those of women (which was known before), and become worse the more muscular the men are (which was not).

The benefits, however, were there, as well. The more muscular a man, the more sexual partners he reported, both in the past year and over his lifetime, and the earlier his first sexual experience was likely to have been. This may, in part, be a result of the ability of muscular men to intimidate 97lb weaklings. But in a society where extreme forms of such intimidation are curbed by law, female choice seems as likely an explanation—especially as previous studies have confirmed scientifically the everyday observation that women do indeed prefer men with big biceps and triangular torsos.

Because muscles come at such cost, Dr Gaulin thinks an evolutionary fight is going on between natural selection, which conserves metabolic expenditure and promotes longevity, and sexual selection, which willingly trades both for extra mating opportunities. This may explain why men have such a range of muscularity. In the past, the strong man would have had better mating opportunities in the short term, but the skinny guy who outlived him could have had just as much reproductive success over the course of his longer life.

The irony for the skinny guy is that the laws which protect him from aggression also make it less likely that the hulk will fight himself into an early grave. Modern medicine, meanwhile, means the hulk’s weakened immune system is less likely to expose him to lethal infection. Time, then, to get the weights out, and start pumping iron.

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