Evolutionary biology may explain differences in mortality risks between genders, says Daniel Kruger at the University of Michigan; men take more risks to attract a mate, i.e. to have sex. PhysOrg explains that "Women invest more physiologically in reproduction than men, thus men compete with other men for mating partners and try to make themselves attractive to women. This competition leads to strategies that are riskier for men both behaviorally and physiologically, and these result in higher levels of mortality." According to Kruger's research, mortality levels are more disparate in less egalitarian societies.