This article argues that looking solely for the immediate causes of reproductive change may distort our understanding of policy options by failing to take into account the historical and cultural factors that affect not only the impact of policies and programs but their very nature and existence. The article examines the historical origins and spread of “modern” ideas in Bangladesh and the state of West Bengal in India. It concludes that a colonial history in which education and modernization processes took hold very early among the elites in the larger Bengal region was paradoxically accompanied by a strong allegiance to the Bengali language. This strong sense of language identity has facilitated and reinforced the diffusion of modern ideas both within and between the two Bengali-speaking regions. Thus, to understand the fertility decline in Bangladesh, for example, one needs to look also at cultural boundaries. In this case, the cultural commonality through language facilitates the spread of new ideas across the two Bengals. In turn, the strong sense of language identity has facilitated mass mobilization more easily and intensely within the two Bengals. Shaped by these processes, Bangladesh and West Bengal today are more amenable to social change than many other parts of South Asia and the Middle East.