"There is a hereditary selective advantage to membership in a powerful group united by devout belief and purpose. Even when individuals subordinate themselves and risk death in common cause, their genes are more likely to be transmitted to the next generation than are those of competing groups who lack equivalent resolve.
"The mathematical models of population genetics suggest the following rule in the evolutionary origin of such altruism. If the reduction of survival and reproduction of individuals due to such genes for altruism is more than offset by the increased probability of survival of the group due to altruism, the altruism genes will rise in frequency throughout the entire population of competing groups. Put as concisely as possible: The individual pays, his genes and tribe gain, altruism spreads." - E. O. Wilson, "Consilience: The Unity of Knowledge", 1998 p287