Across the industrializing world, birth rates that had held steady for centuries began, in the span of a few generations, a sustained descent. Explaining this fertility transition has divided demographers into two camps whose disagreement is less about the facts than about what kind of cause is doing the work. The first, and long dominant, account is economic. On this view, couples adjust their childbearing to the changing costs and benefits of children. Where children labor on farms and support aged parents, large families pay; where compulsory schooling turns children into expensive, long-term investments and where pensions and markets supply what offspring once did, the calculus reverses. Rising incomes and, crucially, expanding opportunities for women raise the opportunity cost of childrearing, and parents substitute quality - fewer children, each more heavily invested in - for quantity. The transition, on this account, is a rational response to altered incentives, and its timing should track the economic variables that constitute those incentives. The rival account is ideational. Its proponents note that fertility often fell where incomes had scarcely risen, and that within a single country the decline swept across provinces of sharply different economic character while halting at linguistic and religious boundaries that no economic map would predict. What diffused, on this reading, was not a new balance sheet but a new idea: that family size is a matter of deliberate choice rather than fate, and that limiting it is morally permissible. Culture, transmitted through shared language and confessional networks, set the pace. The dispute is not easily adjudicated, in part because the two mechanisms are empirically entangled. Economic change alters the ideas people find plausible, and new ideas about choice and the future reshape economic behavior; an observed correlation between, say, urbanization and smaller families is consistent with either story or with both. Nor can the ideational camp claim the linguistic boundaries as decisive proof, for language marks not only the reach of ideas but the segmentation of labor markets and the networks through which economic information travels. Prudence therefore counsels against a single, universal model. The economic account performs best where markets penetrated deeply and early; the ideational account illuminates the anomalies - the poor regions that limited births before prosperity arrived, the sharp discontinuities at cultural frontiers - that a purely economic model must treat as noise. To insist that one mechanism is everywhere primary is to mistake a family of related transitions for a single event with a single cause. The more defensible conclusion is comparative and conditional: the weight each mechanism carries depends on the setting, and the analyst's task is to specify the conditions under which incentives lead and those under which ideas do. That a phenomenon so uniform in its direction should prove so various in its causes is not a scandal but a caution - a reminder that convergent outcomes need not share a common origin.