This article introduces complementarity theory, which explains the psychology of cultural diversity as a product of evolved social proclivities that enable-and reMy aim in this article is to describe the natural selection of universal psychological mechanisms that result in cultural diversity. Complementarity theory posits that human social coordination is the product of structured psychological proclivities linked to corresponding cultural paradigms. Using innate expectations about the ways in which people encode and conduct social relations, children take primary responsibility in actively searching for the relevant cultural paradigms they need. Putting proclivities together with congruent paradigms, children learn to construct culture-specific coordination devices that enable them to interact in locally meaningful ways. The evolved proclivities and cultural paradigms are complementary: Both are necessary but neither is sufficient to permit complex social coordination. People cannot use either their socially transmitted paradigms or their evolved proclivities independently of each other. Combining them, humans devise and depend on diverse, flexible social adaptations.These coordination systems make possible a second kind of complementarity: the complementarity of the respective actions of the participants. To cooperate, contest, or defect, it is extremely advantageous to know what other people want, judge, feel, think, and will do. Moreover, many kinds of complex mutually beneficial cooperation require this kind of complementarity: communication, exchange, division of labor, joint action, meeting at a known time and place, planning a schedule for the flow of work, conducting a complex ritual, making joint decisions and committing to collective behavior, acting in concert in relation to outsiders, or cooperating to sanction someone-any action whose outcome depends on cooperation based on shared understandings.This complementarity in human interaction is usually made possible by participants' joint use of shared coordination devices to construct their own actions and to interpret others' actions. Moreover, unlike social insects and most eusocial mammals, humans are capable of generating an infinite number of social coordination devices based on each evolved social proclivity. They do this by combining an evolved social proclivity with different cultural paradigms. Furthermore, people can generate endless additional possibilities by combining multiple coordination devices. This generativity makes coordination devices uniquely versatile, permitting humans to use a limited num-
76Personality and Social Psychology Review