“Mind & Matter,” by Robert M. Sapolsky. The Wall Street Journal, 5/28/2016.
This WSJ column is about the surprising illogic of homicide among family members since family members can pass on many of the genes we ourselves work so hard to pass on. A parent and child, of course, share half of their genes, but so do two siblings. Mathematically, an organism achieves the same genetic success if it reproduces once itself or helps a full sibling to reproduce twice. Sapolsky quotes a quip attributed to a British scientist: “I’d lay down my life for two brothers or eight cousins.”
Evolution thereby favors cooperation and altruism among family members and this explains a whole range of social behaviors, such as why a high-ranking baboon will look after their lower ranking family member. Humans will usually do the same, and the exceptions shock us.