More men inevitably means more testosterone-fuelled violence, right? Wrong, according to a comprehensive analysis exploring how a surplus of men or women affect crime rates across the US.
In areas where men outnumber women, there were lower rates of murders and assaults as well as fewer sex-related crimes, such as rapes, sex offences and prostitution. Conversely, higher rates of these crimes occurred in areas where there were more women than men.
Ryan Schacht of the University of Utah in Salt Lake City and his colleagues analysed sex ratio data from all 3082 US counties, provided by the US Census Bureau in 2010. They compared this with crime data for the same year, issued by the US Federal Bureau of Investigation. They only included information about women and men of reproductive age.
For all five types of offence analysed, rising proportions of men in a county correlated with fewer crimes– even when accounting for other potential contributing factors such as poverty. The results suggest that current policies aimed at defusing violence and crime by reducing the amount of men in male-dominated areas may backfire.
According to Schacht, when women are in short supply, men must be more dutiful to win and retain a partner. With an abundance of women, men are spoilt for choice and adopt more promiscuous behaviour that brings them into conflict with other men, and more likely to commit sex-related offences.
“When women are rare they become a valued resource and this gives them more bargaining power over what they expect from a relationship,” says Schacht. “But when women are abundant, men become less committed to single partners and more interested in pursuing multiple relationships,” he says. “This brings men into conflict with each other in response to their more uncommitted, promiscuous mating orientation.”
The upshot, says Schacht, is that men alter their behaviour to suit conditions of “supply and demand”. “In some situations they will be much better behaved, and in others they will be much more prone to nasty behaviour,” he says.
Schacht says the work has implications for crime prevention: “The take-home message for me is that we are overly focused on male excess when we should reorient to places with more women,” says Schacht.
Work in China, where men dramatically outnumber women – especially in rural villages - because of the government’s old one-child-per-family rule, reveals a slightly different pattern, says Therese Hesketh at University College London.
“In China, we didn’t find a relationship between excess males and violence,” she says. “Instead, many single rural men were depressed and introverted. Although they scored slightly higher on measures of aggression, there was no evidence of increased crime or fearfulness among women.”
“Recent work in animals also shows quite similar findings to ours, that when females are abundant and males rare, males are more violently competitive, more promiscuous and less likely to invest in offspring,” says Schacht.
Journal reference: Human Nature, DOI: 10.1007/s12110-016-9271-x