In its classic formulation, the demographic transition is schematized as a four-stage process (Figure 5.1). 1 The initial homeostatic demographic regime (Stage I), in which high mortality necessitates high fertility to ensure the survival of the group, is overturned by economic and technological advances which induce a general decline in mortality rates (Stage II). The resulting high rate of natural increase precipitates the perceived need to restrain fertility at the same time that social and economic changes generate changing attitudes regarding appropriate family size and the desirability of large numbers of children (Stage III) . Finally, in Stage IV, as a result of conscious efforts by parents to drastically limit the number of offspring, a new homeostasis is achieved consisting of low birth and death rates and, once again, a moderate rate of population increase. In the discussion which follows, attention will focus directly on the timing, pace, and degree of change in birth and death rates. Since theory predicts and the experience of the Petorca Valley supports the proposition that mortality change occurs prior to the fertility decline, the mortality transition will be discussed first.