From PhysOrg:
Researchers have long known that women outlive men on average, and more recently have discovered that men have higher mortality risks across the entire lifespan. University of Michigan researcher Daniel Kruger offers this explanation: It is all about sex. 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.
“If mating competition is responsible for excess male mortality, then the more mating competition there is, the higher excess male mortality will be,” said Kruger, an assistant research professor in the U-M School of Public Health. In the current study, Kruger shows that two factors related to the level of male reproductive competition contribute to higher rates of risk-taking and mortality. The first factor is polygyny, the social situation in which one man maintains sexual relations with many women (the opposite is polyandry—one women and many men). Several species of primates show high levels of polygyny, where one dominant male mates with most of the females in the group, and other males are left out. Human cultures have varying degrees of polygyny, and Kruger found that the more prevalent the practice, the higher the rate of male mortality.
More here.