On the Evolution of Altruistic Ethical Rules for Siblings

This paper explores the evolutionary foundations of altruism among siblings. It is intended as a contribution to the theory of the evolution of preferences, the economics of the family, and the evolutionary biology of kin selection. This paper extends the kin- selection theory of the biologist, William Hamilton, from the special case of additive benefits and costs to a more general class of games between relatives. We show that a population of siblings will resist invasion by dominant mutants if all individuals act so as to maximize a ``semi-Kantian'' utility function with respect to their siblings. We also find a separate condition that ensures that a population will resist invasion by dominant mutants. Conditions similar to ``strategic complementarity'' and ``strategic substitutability'' of strategies determine whether a population resistant to invasion by dominant mutants will also resist invasion by recessive mutants.