Mortality change has traditionally been considered a necessary prerequisite to any long-term reduction in fertility. In Frank Notestein's original statement of the demographic transition theory, societies went from high to low levels of fertility and mortality by means of a path in which mortality declined first and was followed at a shorter or longer lag by fertility. Subsequently Abdel Omran outlined a number of scenarios of demographic transition, all of which reflected simultaneous or prior declines of mortality as a major factor stimulating fertility reduction. While Omran's view of the epidemiological transition goes on to emphasize many other aspects of mortality decline, his view of it as an agent for fertility decline is similar to Notestein's.

This understanding of the mortality–fertility synergy is based on two very general conceptions of historical reality which both Omran and Notestein share with many people working in the field of historical demography. For one, there has never been a society known to historians in which fertility was high and mortality low, or mortality high and fertility low, except in moments of epidemic or economic or political stress, for relatively brief periods of time, or among very specific historic sub-populations. Normally mortality and fertility have ended up adjusting to each other more or less efficiently so as to ensure moderate rates of population growth. Moreover, high levels of fertility and mortality invariably were replaced by moderate ones during the demographic transition. Secondly, one of the reasons that the demographic transition holds such an allure for most historians is that it was a period of history in which societal traditions appear to have succumbed to the weight of rational choices made by families in order to ensure their best welfare in the face of changing circumstances. The initial instigator of the change, one would reasonably suspect, had to have been a decline in mortality with increasing numbers of children surviving beyond infancy. Faced with this reality, families made the decision to give individual choice priority over social norms and ended up curtailing their fertility. The entire context of social, economic and cultural modernization contributed to people's willingness to adopt new strategies when faced with new realities, but intuitively the point of departure would seem to have been incipient mortality transformation.