"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, its offspring, which also have a half chance, and its parents. 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 propagation of this set of copies to the relatives around it."

W.D. Hamilton