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.