Extraordinary siblings: Mole rats, marmosets, and Radcliffe-Brown.

According to the theory of kin selection, an organism that shows some level of altruism toward her kin - lowering her own fitness, raising that of a close genetic relative - may enjoy an evolutionary advantage. Some species show beyond-ordinary altruism toward siblings, and other kin, owing to unusual reproductive biology and/or ecology. Human beings are exceptional in another way: how we treat our kin depends partly on how we feel about them, but also partly on socially enforced norms. This article explores several versions of a simple evolutionary game, the Brothers Karamazov Game, that departs from the standard theory of kin selection to allow for the distinctively human capacity for establishing and enforcing social norms. We discuss possible applications to understanding the "unity of the sibling group" (Radcliffe-Brown) - according exceptional treatment to siblings, and to relatives classified as siblings or linked through siblings. We give special attention to lowland South America, where the sibling relationship is central to social organization.