How Can Altruism Exist?
In a sense, evolutionary psychology begins with a paradox, first recognized by Darwin. Darwin's theory of natural selection is, in essence, an extremely selfish theory. Any behavior that enhances the chances that someone else's genes will survive over yours will tend to be selected out. It's long been recognized that close relatives are a special case, so for purposes of this discussion we will restrict the word "altruism" to acts which enhance the fitness of an unrelated individual at some cost to one's own fitness. Risking one's life for a stranger is a classic if extreme case.
In a sense, the central problem of evolutionary psychology is "How can altruism exist?" Of course this is only a problem for those who want scientific and naturalistic explanations. Religious hocus-pocus can find as many explanations as you like.
There are plenty of clues around. For one thing, altruism is almost always restricted in scope. We are more generous to those with whom we feel affiliations (same religion, same home town, fans of the same sports team, etc.) For another, we are more generous when there is social encouragement and pressure to be generous. Some are far more generous than others. Empathy and generosity are closely related.
Boehm makes the altruism paradox central to his book Moral Origins, and makes it clear that a definitive answer is still not completely in place. The most persuasive (IMO) approach to explanation is some type of group selection. Group selection itself is a highly controversial topic in evolutionary theory, for reasons i've talked about before (and which are discussed in the Wikipedia article of the same name.) The problem with group selection is that so-called free riders, members who fail to do their part, can sabotage the whole group to their own advantage.
Again, the most plausible way to deal with this is the punishment of free riders. We know that extant hunter gathering societies consistently have developed mechanisms for this, as have all sorts of organizations in civilization. Most of us feel outrage when we see someone else getting an unfair advantage.
More speculatively, one might guess that the task of identifying and dealing with free riders was sufficiently challenging that it was a major driver of the rapid development of the human brain.
Despite their sometimes speculative nature, these kinds of explanation do have a powerfully predictive aspect. By locating the basis of various behaviors at least partially in biology, they predict the existence of an underlying biological substrate. Empathy is a crucial component of our social behaviors, for example, and it's known to have biological substrates. Certain brain abonormalities abolish empathy.