Why do related animals sometimes favor their relatives over unrelated group members?

Related animals often favor their relatives over unrelated group members because of kin selection, a form of natural selection that accounts for an individual's inclusive fitness.

While natural selection typically favors traits that increase an individual's own survival and reproduction (direct fitness), it also promotes behaviors that assist the survival and reproduction of relatives (indirect fitness). Because relatives share a portion of the same genes, an animal can effectively pass on its own genetic material by helping kin reproduce or survive, even at a personal cost.12

This phenomenon is explained by Hamilton’s rule, which states that altruistic behavior can evolve when the benefit to the recipient, multiplied by the degree of genetic relatedness, exceeds the cost to the donor. Consequently, animals are more likely to engage in "nepotism"—such as sharing food, forming defensive alliances, or providing care—when the beneficiaries are close kin, as these actions ensure the propagation of shared genes across generations.123