This also demonstrates that altruism can be quite selfish. Altruistic acts can lead to very real individual benefits — increased reproductive success, enhanced social stature, or simply feeling good about oneself — but such benefits must be weighed against the potential cost.

At the extreme, altruism is detrimental. The warrior who is killed in combat never gets a chance to enjoy the fruits of his sacrifice. He may still consider the risk worth it, but how can we explain the person who deliberately sacrifices himself to save others, like the soldier who throws himself on a grenade?

In some cases, even such extreme decisions can be selfish, genetically speaking. A suicide bomber, for instance, knows his family will probably be taken care of. A soldier's family gets a government pension and the thanks of a grateful nation. But absent those scenarios, I think such examples demonstrate the power of societal expectations.

People raised in a given society often internalize that society's values. The stronger their attachment to the society, the stronger the internalization. Further, people who live when others die often experience survivor's guilt. Many people talk about how they couldn't live with themselves if they behaved in a way society disapproves of. The cost-benefit analysis is different for every individual, of course, but many people would apparently prefer to risk near-certain death than live with the knowledge that they chickened out or let others die so that they could live.

It turns out the question I posed at the beginning of this article is a bit misleading because, in many cases, being good and being selfish are the same thing. Overall the evidence points to morality and altruism being biologically based, but socially defined. Religion is a part of society and thus contributes to defining society's morality just like any other philosophical system. Religion is also a singularly powerful social tool for enforcing that morality though, like any tool, it can also be used for ill. Morality can flourish absent religion, just like religion can flourish absent morality.