Why did Louis Henry create a scientific discipline — historical demography — that dominated population history from the 1950s to the 1980s, and even influenced the École des Annales? Beyond historiography and the history of demographic theories, the answer lies in the history of government, public policy, demographic institutions, and population policies. After 1945, international organizations, most notably the U.N. Population Division, placed analytical demography à la Lotka on a planetary footing. They developed a special interest in fertility. The rich countries’ baby boom undermined the concepts of demographic forecasting and demographic transition, and jeopardized the family allowance systems. World population growth raised the issue of birth control in the developing countries. Demographers wanted to determine “natural fertility” — which they assumed to be the fertility of non-contracepting populations — but were prevented by the dearth of statistical records in the Third World. For Henry, these difficulties could be overcome by the use of parish registers. The relevance of his approach was such that Alfred Sauvy at INED agreed to finance his work, and the leading demographers of his time — including Notestein, Glass, and Hajnal — were convinced from the outset that historical demography provided a major contribution to theoretical demography.