血縁選択説
Kin selection provides the framework for an adaptive strategy by which altruistic behavior is bestowed on closely related individuals because easily identifiable markers exist to indicate them as likely to reciprocate. Kin selection is implicit in theories regarding the successful propagation of genetic material through reproduction, as helping an individual more likely to share one's genetic material would better ensure the survival of at least a portion of it. Hamilton's rule suggests that individuals preferentially help those more related to them when costs to themselves are minimal. This is modeled mathematically as formula_1. Grandmothers would, therefore, be expected to forgo their own reproduction once the benefits of helping those individuals ("b") multiplied by the relatedness to that individual ("r") outweighed the costs of the grandmother not reproducing ("c").