The study, "Too many men: the violence problem?" is in the April issue of Trends in Ecology & Evolution.
Conventional wisdom and scientific arguments have claimed that societies with more men than women, such as China, will become more violent, but a University of California, Davis, study has found that a male-biased sex ratio does not lead to more crime.
Rates of rape, sexual assault and homicide are actually lower in societies with more men than women, the study found. And, evolutionary theories predicting that when males outnumber females, males will compete vigorously for the limited number of mates don't bear out.
"Here, we untangle the logic behind the widely held notion that in human societies where men outnumber women, there will be more violence," said Monique Borgerhoff Mulder, a UC Davis professor of anthropology and co-author of the study.
The anthropologists who conducted this study took their lead from recent developments in evolutionary theory. These new ideas challenge the claim that when in abundance males will necessarily resort to violent competition.
It is true that most perpetrators and victims of violence are men. "It isn't surprising that arguments of more men leading to more violence dominate discussions, this could create such an effect," said Ryan Schacht, a co-author of the paper and doctoral researcher at UC Davis. "But the evidence does not support a relationship between violence and a short supply of women," he said.
The reason for this unexpected outcome, the authors said, has something to do with supply and demand.
"You may actually adjust your behavior according to the circumstances," said Kristin Liv Rauch, a postdoctoral researcher and co-author of the paper. "When men are abundant, rather than rare, they often switch their strategy to compete in nonviolent rather than violent ways. They tend to pursue females in more of a courtship manner that would lead to long-term relationships and marriage, in an attempt to secure a partner in a depleted market."
Schacht, Rauch and Borgerhoff Mulder's study has important policy implications. For example, "tough on crime" policies that incarcerate increasing numbers of men might be contributing to higher rates of violence, rather than alleviating them, the authors said. Future studies might investigate the particular forms of violence that are associated with imbalanced sex ratios, and whether interventions are available that do not further exaggerate asymmetries in the availability of opposite sex mates, the authors said.