Parent-offspring conflict: the full-sib-half-sib fallacy

R. L. Trivers used Hamilton's rule regarding the evolution of altruistic be haviour to demonstrate the occurrence of parent-offspring conflict and conc luded that this would be more severe when future offspring were half siblin gs rather than full siblings of the offspring currently receiving investmen t. Here we show that Trivers' formulation applies in a limited range of cir cumstances and offer an alternative formulation that is universally applica ble. Under this formulation the optimal investment from the offspring's poi nt of view in species with uniparental care and, hence, the extent of paren t-offspring conflict depends on the cost of the caring parent's investment to the future fitness of the non-caring parent.