In theorizing about human evolution, we must include processes affecting culture in our list of evolutionary processes alongside those that affect genes. Culture is a system of inheritance. We acquire behavior by imitating other individuals much as we get our genes from our parents. A fancy capacity for high-fidelity imitation is one of the most important derived characters distinguishing us from our primate relatives (Tomasello, 1999). We are also an unusually docile animal (Simon, 1990) and unusually sensitive to expressions of approval and disapproval by parents and others (Baum, 1994:218-219). Thus parents, teachers, and peers can rapidly, easily, and accurately shape our behavior compared to training other animals using more expensive material rewards and punishments. Finally, once children acquire language, parents and others can communicate new ideas quite economically. Our own contribution to the study of human behavior is a series of mathematical models in the Darwinian style of what we take to be the fundamental processes of cultural evolution (e.g., Boyd and Richerson, 1985). The application of Darwinian methods to the study of cultural evolution was advocated forcefully by Campbell (1965, 1975). Cavalli-Sforza and Feldman (1973) constructed the first mathematical models to analyze cultural recursions (see also Durham, 1991).
The list of processes that shape cultural change includes:
Biases. Humans do not passively imitate whatever they observe. Rather, cultural transmission is biased by decision rules that individuals apply to the variants they observe or try out. The rules behind such selective imitation may be innate or the result of earlier imitation or a mixture of both. Many types of rules might be used to bias imitation. Individuals may try out a behavior and let reinforcement guide acceptance or rejection. Or they may use various rules of thumb to reduce the need for costly trials and punishing errors. The use of a conformist rule of the form “when in Rome do as the Romans do” is an example that is important in our hypothesis about the origins of cooperative tendencies in human behavior.
Nonrandom variation. Genetic innovations (mutations, recombinations) are random with respect to what is adaptive. Human individual innovation is guided by many of the same rules that are applied to biasing ready-made cultural alternatives. Bias and learning rules have the effect of increasing the rate of evolution relative to what can be accomplished by random mutation, recombination, and natural selection. We believe that culture originated in the human lineage as an adaptation to the Plio-Pleistocene ice-age climate deterioration, which included much rapid, high-amplitude variation of just the sort that would favor adaptation by biased innovation and imitation (Richerson and Boyd, 2000).
Natural selection. Because selection operates on any form of heritable variation and imitation and teaching are forms of inheritance, selection will influence cultural as well as genetic evolution. However, selection on culture is liable