What would it take for you to give your life to save another? The answer of course is two siblings or eight cousins, that is, if you’re thinking like a geneticist. This famous quip, attributed to the British biologist J.B.S. Haldane, is based on the premise that you share on average 50% of your genes with a brother or sister and 12.5% with a cousin. For altruism to be worth the cost it should ensure that you break even, genetically speaking.

This basic idea was later formalized by the evolutionary theorist William Hamilton as “inclusive fitness theory” that extended Darwin’s definition of fitness–the total number of offspring produced–to also include the offspring of close relatives. Hamilton’s model has been highly influential, particularly for Oxford evolutionary biologist Richard Dawkins who spent considerable time discussing its implications in his 1976 book The Selfish Gene. But in the last few years an academic turf war has developed pitting the supporters of inclusive fitness theory (better known as kin selection) against a handful of upstarts advocating what is known as group selection, the idea that evolutionary pressures act not only on individual organisms but also at the level of the social group.