The spread of developmental idealism's beliefs about how “modern” family practices help achieve a modern prosperous society did not happen spontaneously, especially in societies whose family systems bore little initial resemblance to the “modern” ideal. We examine how Kenya in the 1960s became the first sub-Saharan country to adopt a fertility reduction policy, even though Kenya's leaders and their Western advisers initially had very different population ideologies. The advisers were neo-Malthusians who viewed continued high fertility in the face of rapid mortality decline as a grave threat to Third World development, whereas most Kenyans were traditional mercantilists who viewed a larger family and a larger population as signs of wealth and prosperity. Kenyans' conversion to neo-Malthusianism is often presented as the simple result of education and reason: Kenyans came to be convinced that progress requires slower population growth and lower fertility, achieved through modern methods of fertility control. Our account differs. It recognizes that neo-Malthusianism was a Western export that faced substantial opposition and that its adoption was the result of a coordinated movement by neo-Malthusians that applied pressure on Kenyan elites to change the intimate behavior of their people. We conclude that developmental idealism has spread from its Western origins to ordinary people around the world, but that the process was not simple, inevitable, or uniform.