This article traces the origins of China's one-child-for-virtually-all policy to Maoist militarism and post-Mao military-to-civilian conversion. Focusing on the work of Song Jian, leading missile scientist and scientific architect of the strict one-child policy, it shows how during 1978–80 the resources of defence science and the self-confidence of the elite scientist enabled him boldly and arbitrarily to modify the work of the Club of Rome and use that Sinified cybernetics of population to redefine the nation's population problem, create a radical one-child-for-all solution to it, and persuade China's leaders that his “scientific” solution was the only way out. Although the advent of “scientific decision-making” in the population arena helpfully broke a political logjam, allowing China's leaders to adopt a strong policy on population control, the making of social policy by an elite scientist/engineer from the defence world posed dangers for the Party and China's people. The case of population policy is important because it provides rare insight into the way scientists have sometimes shaped elite policy-making and because the social and political consequences of the one-child policy have been so troubling.