This article explores a birth control survey conducted by the Japanese government between 1945–1960, and analyses how this research, both as scientific practice and knowledge, resonated with government efforts to manage the population during post-war reconstruction. Focusing on Shinozaki Nobuo, a Ministry of Health and Welfare ‘technical bureaucrat’ (gijutsu kanryō or gikan) specializing in population issues, this article depicts how human agency participated in the at times precarious relationship between policy and practice. It also explores how the epistemological framework inscribed in the resulting scientific knowledge harmonized with the economic and political rationale supporting post-war reconstruction. This article contributes to the study of modern Japan by highlighting the critical, yet thus far underexamined role that population science and technical bureaucrats played in Japanese statecraft. At the same time, it demonstrates that the interaction between state governance of population and the creation of scientific knowledge about the populace was fundamentally complex.