The technique of family reconstitution was devised by Louis Henry primarily as a means of investigating the phenomenon of human fertility; what determined the spacing between births, the phenomenon of teenage subfecundity, the gradual extinction of the capacity to bear children associated with increasing age, and a host of cognate questions. In the years immediately after the Second World War Henry was faced by the instructive and ironic paradox that countries in which birth control within marriage was still largely absent did not collect reliable and sophisticated demographic data, while countries which did collect such data were universally countries where the deliberate restriction of fertility within marriage was common. To study the biological parameters of fertility it was necessary to secure data from a society which did not deliberately restrict fertility within marriage. Henry therefore turned to the past to provide the data which was otherwise out of reach, first making use of the genealogies of the Genevan bourgeoisie and then creating his own genealogies, so to speak, by reconstituting the families of the parish of Crulai in Normandy.

Family reconstitution has proved a most fruitful source of information about the characteristics of fertility, both for the study of the matters which first caused Henry to use historical data, and more generally in providing accurate and detailed pictures of the fertility histories of many communities during the period when parish registers represented the equivalent of a modern state-run vital registration system.