Hamilton's rule (1964): an altruistic gene spreads when rb > c, where r = coefficient of
relatedness, b = fitness benefit to recipient, c = fitness cost to actor. Altruism can be selected
even when it reduces personal fitness, if it sufficiently benefits genetic relatives.
r values: identical twins = 1.0, siblings = 0.5, half-siblings = 0.25, cousins = 0.125.