Two concepts of agency, one active and one passive, have dominated attempts to account for fertility change in Western social science since the emergence, toward the end of World War II, of concern with explosive population growth. The passive concept of agency sees people as adhering to conventions or following rules. Human conduct is thus narrowly channeled by norms and institutions. The active concept of agency sees people as deliberately choosing the level of their fertility through some form of abstract rationality. Though the balance between these two forms of agency has shifted, social science accounts of fertility change remain caught between the two poles they define.

In the first versions of demographic transition theory (Notestein 1945, Davis 1945), it was assumed that wherever people exercise deliberate choice or active decision-making with regard to family size they invariably impose sharp limits on their fertility. However, this was thought to occur only in modern societies in which the forces of industrialization, urbanization and education have reduced mortality and freed individuals from the weight of tradition. In premodern and transitional societies, according to this version of transition theory, decision-making remains passive and social and cultural constraints continue to produce high fertility. Most contemporary economic and sociological accounts of fertility change assume that human beings are free to behave rationally in both traditional and modern societies.