Significant progress was made in 1964 when Hamilton formulated the inequality in kin selection known as Hamilton's rule, which showed how eusociality in insects (the existence of sterile worker classes) and many other examples of altruistic behavior could have evolved through kin selection.
Značajan napredak napravljen je 1964. godine kada je Hamilton formulirao nejednakost srodstvene selekcije poznate kao Hamiltonovo pravilo, a koje je pokazalo da su eusocijalnost kod insekata (postojanje sterilnih radničkih klasa) i mnogi drugi primjeri altruističnoga ponašanja mogli evoluirati kroz srodstvenu selekciju.