Archaeological and ethnohistoric data on the Chimu and Inca empires, two prehistoric. Peruvian states that shared a number of organizational features, are used to test the theory of cultural materialism. Materialism explanations of inca expansionism are evaluated; they are shown to be unconvincing in the inca case and inapplicable to the Chimu. An anternative model is proposed that emphasizes the role of a particular legal principle, split inheritance. The presence of split inheritance in the two empires is documented. It is argued that in both cases split inheritance originated through manipulation of traditional elements of Andean idelogy, was the driving force behind imperial expansion, and generated administrative and economic stresses eventually leading to imperial collapse. This model avoids the flaws of the materialist explanations it is intended to replace and the theory of cultural mateiralism is rejected.