It has long been conventional to subdivide the description and analysis of demographic behaviour under three main heads: fertility, mortality, and nuptiality (a fourth head, migration, is also frequently employed, but has a more uncertain status). Of these, in historical studies, it was mortality which for long attracted the lion's share of attention. There was no analogue in the comparatively even tenor of annual totals of births and marriages for the dramatic, unpredictable surges of mortality which might sweep away a tenth, a quarter, a third of the entire population of a community in a matter of weeks. Pestilence, famine, and the ravages associated with war could bring with them suffering and loss on a scale that challenges even the most vivid imagination and eloquent pen to describe in terms that can do justice to the magnitude of the human disaster involved. Such episodes were not only poignant and eye-catching but were often taken to have dominated population trends.

Until comparatively recently, moreover, both general theorising about population behaviour in pre-industrial societies and also the nature of the techniques available for analysing historical data tended to cause attention to be focused on mortality. It was once commonplace to assume without question, for example, that it was a fall in mortality that initiated the series of related changes often labelled the demographic transition. Population growth rates were envisaged as rising in the later eighteenth century because of declining death rates, followed only after a considerable time-lag by a fall in birth rates, thereby provoking the huge rise in numbers experienced almost throughout Europe during the nineteenth century.