If It Feels Good to Be Good, It Might Be Only Natural (Washington Post)

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.

http://www.washingtonpost.com/wp-dyn/content/article/2007/05/27/AR2007052701056.html