The theory that explains altruistic behavior through benefits to close relatives is kin selection.
Kin selection is an evolutionary process in which natural selection favors traits that increase the reproductive success of an individual’s relatives, even when those traits impose a cost on the individual performing them. Relatives share some genes, so helping them reproduce can indirectly help copies of the helper’s genes persist in the population.
The idea was discussed by Charles Darwin in 1859 as a possible explanation for sterile worker insects that care for a queen’s offspring rather than reproducing themselves. William Donald Hamilton later formalized the logic in 1964 through Hamilton’s rule: an altruistic behavior can spread when the benefit to the recipient, multiplied by genetic relatedness, exceeds the cost to the actor.
Kin selection is closely related to inclusive fitness, which combines an individual’s own reproduction with the reproductive success it helps relatives achieve. It is not the same as reciprocal altruism, where unrelated individuals exchange benefits, or group selection, which emphasizes advantages to entire groups.