Kin selection
This insight led to inclusive fitness and kin selection becoming important theories during the 20th century to help explain eusociality. Inclusive fitness is described as a combination of one's own reproductive success and the reproductive success of others that share similar genes. Animals may increase their inclusive fitness through kin selection. Kin selection is when individuals help close relatives with their reproduction process, seemingly because relatives will propagate some of the individual's own genes. Kin selection follows Hamilton's Rule, which suggests that if the benefit of a behavior to a recipient, taking into account the genetic relatedness of the recipient to the altruist, outweighs the costs of the behavior to the altruist, then it is in the altruist's genetic advantage to perform the altruistic behavior.