"The existence of altruism in nature can be explained by thinking about the replication of genes. We need to descend to the level of the gene rather than the individual in order to see that the gene exists surrounded by copies of identical genes that exist in all its relatives, in particular its siblings who have a half chance of carrying a copy of that particular gene, offspring which also have a half chance, its parents a half chance etc; and so seeing this swarm of genes that exists around a particular one we can then ask what is the behavior caused by this gene that is most likely to cause the obligation of this set of copies in the relatives around it."

W.D. Hamilton