Biologists call a behavior pattern altruistic if it increases the number of offspring produced by the recipient and decreases the number of offspring of the altruist. The altruism of the sterile 'workers', in such insects as ants and bees (pictured opposite), is one undoubted example; here the altruism is extreme, as the workers do not reproduce in some species.
It has been a puzzle to evolutionary biologists to explain how altruism has evolved, since at first it would seem to be impossible that natural selection could ever favor altruistic actions that decrease the reproduction of the actor.
Altruistic behavior often takes place between genetic relatives and, when it does, the most likely explanation is the theory of kin selection.
W.D. Hamilton gives his influential explanation for the existence of altruism.
When does it make sense to be an altruist?