This news story I got to through the TAMU Anthropology in the News is in the same line as research that shows that helping others is one of top contributors to "happiness".
If It Feels Good to Be Good, It Might Be Only Natural
"The results were showing that when the volunteers placed the interests of others before their own, the generosity activated a primitive part of the brain that usually lights up in response to food or sex. Altruism, the experiment suggested, was not a superior moral faculty that suppresses basic selfish urges but rather was basic to the brain, hard-wired and pleasurable."
Regarding the somewhat laughable potential reactions to this finding:
"Even more important, some wonder whether the very idea of morality is somehow degraded if it turns out to be just another evolutionary tool that nature uses to help species survive and propagate."
I wonder how economists would interpret these types of findings. This hard-wired altruism was probably due to selection for those individuals who engaged in strong, generalized reciprocity, or generous tit-for-tat like behavior. The benefit of being nice in leading to reducing risk/variation in food acquisition is the likely ecological factor selecting for this.