  Though Hamilton himself did not use the term, his idea quickly became known as ‘kin selection’, for obvious reasons. Kin selection theory predicts that animals are more likely to behave altruistically towards their relatives than towards unrelated members of their species. Moreover, it predicts that the degree of altruism will be greater, the closer the relationship. In the years since Hamilton's theory was devised, these predictions have been amply confirmed by empirical work. For example, in various bird species, it has been found that ‘helper’ birds are much more likely to help relatives raise their young, than they are to help unrelated breeding pairs. Similarly, studies of Japanese macaques have shown that altruistic actions, such as defending others from attack, tend to be preferentially directed towards close kin. In most social insect species, a peculiarity of the genetic system known as ‘haplodiploidy’ means that females on average share more genes with their sisters than with their own offspring. So a female may well be able to get more genes into the next generation by helping the queen reproduce, hence increasing the number of sisters she will have, rather than by having offspring of her own. Kin selection theory therefore provides a neat explanation of how sterility in the social insects may have evolved by Darwinian means. (Note, however, that the precise significance of haplodiploidy for the evolution of worker sterility is a controversial question; see Maynard Smith and Szathmary 1995 ch.16, Gardner, Alpedrinha and West 2012.)