The Washington Post has a fascinating story on new brain research suggesting that our altruistic responses come from the deep old emotional centers of the brain, rather than the newer rational calculation parts.
Other well-known research has shown that women are more likely to use their whole brain in reasoning, whereas men are more likely to use one part or another. Women are more likely to incorporate their emotional responses in their reasoning, whereas men are more likely to separate "cold" reason from their emotions. And everyone knows that men are more likely to commit crimes than women are.
SO, putting these facts together, perhaps men and women have a similar altruistic emotional response to the plight of others, based on empathy. And men and women also make rational calculations about how their self-interest would be served by acting altruistically versus selfishly, even to the point of harming others for my own gain. The difference is that women are more likely to act with a response that tries to integrate the two competing responses, whereas men are more likely to choose one over another. And thus men are more likely to choose a response that suppresses their empathy for others.
Just a first theory. This research on the deep roots of altruism, though, is important and very interesting.