One explanation is that altruistic acts, although seemingly unselfish, actually benefit those who perform them but in indirect ways. The idea is that unselfish acts are repeated. So those who have been helped go on to help other individuals, ensuring that this behaviour spreads through a group, a phenomenon known as upstream reciprocity.

Eventually, the individual that triggered the altruistic behaviour will be on the receiving end of least one unselfish act, ensuring that, at the very least, he or she doesn’t lose out. In this way, unselfish individuals actually benefit from their altruism.”