In his study of world population published more than 50 years ago, A. M. Carr-Saunders (1936) observed that the population of western Europe had begun to increase by the early eighteenth century, if not earlier, as a result of a growing gap between births and deaths. The gap was accounted for primarily by the fact that mortality was declining whereas fertility was high and in some countries was even growing. Addressing the reasons for the decline in mortality, he, like others both before and since, classified the causes into several categories:

“For the purposes of this discussion the conditions, of which note must be taken, may be classified into four groups, though the boundaries between them are indefinite and though there is much overlapping: (1) political, that is conditions in relation to the maintenance of external and internal order; (2) social, including the state of knowledge in relation to the production and use of food, and to the making and use of clothing; (3) sanitary, that is conditions relating to housing, drainage, and water-supply; (4) medical, including both the state of knowledge concerning the prevention and cure of disease and its application to the public at large.”

Because Carr-Saunders was dealing with the period after 1700, he thought that the first category – social order – no longer applied because conditions in England and the rest of northwestern Europe were by that time comparatively stable and orderly. Thus, the other three categories were of most value in explaining the mortality decline in northwestern Europe in the eighteenth and nineteenth centuries.