The gender gap has widened dramatically in recent years, but it has been on the rise since the 1940s, at least in the US, France, Japan and Sweden, where historical figures are available. The researchers suggest a number of factors that could be to blame for the trend. Population growth and globetrotting have led to a rise in infectious diseases. And improvements in public health and medicine may have benefited women more than men: for instance, far fewer women now die at a relatively young age during childbirth. Technological advances may have played a part, too, by supplying men with more powerful guns and ever faster cars.
Nesse and Kruger say that sexual selection could also partly explain some of the differences. Men generally invest less in their children than women do, and as a result may compete more vigorously with each other for potential mates. This rivalry could be what drives them to take greater risks, with the result that men have evolved greater reproductive success at the expense of longevity. The same may be true for chimpanzees and even fruit flies, says Nesse.