Also, why were women usually smaller and lighter than men? Even if women in primitive human societies played nurturing roles rather than aggressive roles, physical strength and height could still be an advantage in defending their young from predators.
I had two clues for an answer. Firstly, men die younger than women. I wondered faintly whether the extra height and bulk that gave men competitive advantages in violence, hunting, construction and so on was weakening the body in other ways.
The second clue came from the dodo.
For most birds there is a competitive advantage in having wings and being able to fly away from danger. The dodo evolved on Mauritius, a small and isolated island with few natural predators, where the competitive advantage of winged flight was lost.
Still, I guessed that there must have been a push in the opposite direction too - that having wings had become a disadvantage in the absence of predators - for dodos to actually lose the power of flight.
Both clues pointed me towards strengths and abilities which are advantageous in some circumstances, but wasteful and disadvantageous in others. After all, wings and muscle need to be fed, and the bigger they are, the more food they require. Then I discovered this research from the University of Pittsburgh:
The beefier the man – measured by total fat-free mass, or arm and leg muscle mass – the more sexual partners he had, Lassek confirmed. The study also showed that more muscled men tended to lose their virginity at a younger age, compared to skinny men.
Yet in the same men, muscles didn't come free. Muscle mass did a better job of predicting caloric intake than body mass index (BMI), age, or activity levels. A larger appetite may not seem like a cost in modern western societies, with a restaurant or grocer on every corner. But ancient humans struggled to get all the calories their bodies needed at times, Lassek says, which could have meant hunky men going hungry more often.
Compared to skinnies, muscular men also tended to produce fewer infection-fighting white blood cells and less of an important immune molecule called C-reactive protein, which helps destroy pathogens.
Beautifully simple. Muscular bodies were at an advantage for attracting female mates, but at greater risk of starvation and disease. This helps explain why humans come in so many shapes and sizes; sexual advantages have high biological costs.
It also explains birds of paradise; extravagant feather displays increase likelihood of attracting a mate by demonstrating the health of the male, but also increase health risks as energy demand rises and flight becomes hampered. The trade off between sexual advantage and health disadvantage strikes a balance between display and performance.
Wealth is health
Still, in developed countries today the risk of starvation and disease have collapsed, which must reduce the long-term health advantage of the skinnier men. Will this mean that more masculine, muscular men will out-breed the others? Will everyone look like He-Man in the future? There are two reasons to believe not.
First, women in rich countries with low risk of disease prefer less masculine features in men, compared with women in poor, disease-ridden countries.
A study of women in 30 countries found they were more likely to choose a masculine-looking partner if their country scored low on a health index based on World Health Organisation mortality figures. By contrast, in countries where people have a longer lifespan, women favoured more feminine-looking men, even though they might not have the healthiest genes available.
High levels of testosterone, associated with muscle and fertility, also weaken the body's immune system, creating the health disadvantage the previous study showed. Rather counterintuitively, this means that in a primitive society if a man is muscular and still active then he "must be healthy and in good condition". In a primitive society, or one with high risks to health, masculinity is "man's way of advertising good genes" - just as complex feather displays are for birds of paradise.
Once again there is a trade-off between these positive traits associated with high testosterone levels and some surprising negative traits. Masculine-looking men with high testosterone levels are good competitors for mates, but it turns out they tend to be bad long-term companions.
In another study of 2,100 Air Force veterans, men with testosterone levels one standard deviation above the mean were 43% more likely to get divorced than men with normal levels, 31% more likely to leave home because of marital problems, 38% more likely to cheat on their wives, and 13% more likely to admit that they hit or hurled things at them.
In a healthy, safe environment women are less drawn to aggressive, masculine men with competitive genes, and instead tend to desire more feminine-looking men - those who look like they will stick around and support the family in the long run.
There is a final reason to doubt a take-over by Rambo types in the future: contraception. Modern family planning technologies have split sexual success from reproductive success and the individual that successfully attracts a sexual partner today may not be passing his genes on to a new generation.
Contraception changes the competition completely; I'm not sure what this means for how humans will evolve into the future.