Understanding the variation of human behavior in scientific terms requires an explicit mechanistic account of the influence of genes and culture on human phenotypes. In this paper we construct a model of the evolutionary process in humans by assuming that culture is a system of inheritance evolving in parallel with the genetic system. The special properties of the cultural system and its linkage to the genetic system are specified in enough detail to make possible a simple, but reasonably complete and realistic, theoretical analysis of the problem. The central conclusion of this analysis is that, given the known properties of culture and its presumed evolutionary advantages, it is implausible that human behavior can be predicted entirely by considerations of genetic fitness. So long as a cultural system is any genetic advantage at all, a structural constraint is imposed on genetic fitness optimization. A genetically optimal capacity for culture will cause cultural fitness to have an effect on phenotype as well. The magnitude of the cultural fitness effect depends on the degree to which the processes of cultural evolution can be constrained by the 100organic capacity for culture without sacrificing the benefits of the cultural mode of adaptation. Determining the nature of this tradeoff is an empirical problem. The evidence for humans suggests our species' behavioral variation is caused by cultural processes controlled by powerful but rather simple and general genetic constraints.