There's long been a common-sense idea — largely untested by science — that having a surplus of men in a society causes more violence. But now we have evidence that this isn't true. Societies where the population is dominated by women are just as violent as ones dominated by men.

Panel from Y the Last Man, with art by Pia Guerra

A group of anthropologists, Ryan Schacht, Kristin Rauch and Monique Borgerhoff Mulder, pored over a series of studies looking at violence and gender balance in a population. In the latest issue of New Scientist, they report that what they found surprised them: "Out of 20 studies, nine showed violence increasing with more men, but nine showed the opposite. Two were inconclusive."

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They explore possible reasons for this, noting first and foremost that violence is an extremely complicated issue — usually, it's caused by many factors, so you'll likely never find a scenario where it's easy to see a direct relationship between gender balance and violence statistics. That said, studies do show that men are often less violent in cultures where there are more men than women. "When faced with a deficit of women, men can engage in much more positive social behavior to attract and keep a partner," the researchers write.

Still, there is a troubling statistic: "There are higher rates of men killing men and sexual assault in female-biased sex ratios." This flies in the face of the long-cherished notion that men fight less when there are lots of women to go around. The researchers believe that male-male violence may increase in these scenarios, as a result of the "aggressive pursuit of multiple sexual partners." Knowing this might change the ways we attempt to reduce violence. They note:

Incarcerating large numbers of men for non-violent offences sucha s drug crimes, as commonly happens in the US, might actually be contributing to higher rates of violence in society.

Taking men out of their communities, and leaving a majority female population behind, could be creating more problems than it solves.

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More work needs to be done to study the relationship between sex ratios and violence, but one thing is becoming obvious. More women does not mean peace. It just means that people engage in violence for different reasons.

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