In the 1960's, biologist William Hamilton became interested in the caterpillars of the cinnabar moth. Most caterpillars are heavily camouflaged to protect them from hungry birds.
The cinnabar is an exception. It's covered with yellow and black stripes and it gathers in conspicuous clusters. But it also secretes a chemical which tastes foul to birds.
Survival of the Fittest
If a bird starts eating one cinnabar caterpillar, it quickly realizes how bad it tastes, then leaves the rest of the cluster alone. This method of survival fascinated Hamilton. If evolution involves the "survival of the fittest," why should any cinnabar caterpillar sacrifice its own life to let the others survive and breed?
The key to this mystery, Hamilton found, is that all the caterpillars in a single cluster are from the same set of eggs. Their genes are so similar that it doesn't matter if one caterpillar or another survives to breed. The genes will still be passed down.
Hamilton studied a variety of other species that exhibit self sacrificing behavior, and he found the same thing: The chance of one organism sacrificing itself for another increases mathematically as the degree of kinship increases.
Kinship Across Species
Animals from sea polyps to birds to monkeys all follow Hamilton's rule. Some animals that would die to protect their own young might happily kill the young of a stranger.
Some anthropologists argue that Hamilton's rule applies to humans as well. You're more likely to act altruistically for someone you think of as kin. Perhaps the key to peaceful coexistence lies in recognizing the common kinship of all humankind.