A general theory of fertility is derived hypothesizing that the demand for children is primarily an outcome of social psychological processes within the family, subject to certain socioeconomic constraints. Two broad social psychological processes are posited as determinants of fertility. The first suggests that the attitudes or tastes of family members influence the demand for children. The second maintains that the nature of the husband-wife interaction (in terms of power, conflict, decision making, and marital satisfaction) determines family size. Socioeconomic variables, in the form of the normative social structure and social stratification, and economic constraints, such as income and price, are hypothesized to influence fertility through their impact on social psychological processes within the family. The overall theory is tested on two independent samples—one in Ankara, Turkey, the second in Mexico City, Mexico—using a structural equation methodology.