Relations Between Individual Life Cycles and Population Characteristics

Populations are collections of individuals and the aggregate of individuals characteristics determines the distribution of characteristics in the population. Despite this seemingly transparent relation between individuals and populations at a moment in time no such identity pertains through time. These discrepancies are produced by demographic conditions. They become particularly important when either the population or the life cycle characteristics are constrained to certain values. In these cases demographic change may force major life cycle or population responses. This paper develops the simple relation between population and life cycle measures and explores their implications for several social processes. Simple relations between life cycle and population measures are developed by reference to steady-state models in which age schedules of fertility mortality and attributes are fixed. Comparison among steady-state models having different morality and fertility conditions permits and demostration of the differential influence of demographic conditions on populations and life cycles. In general fertility differences affect life cycles but not populations. The size of effects is shown to depend upon differences between the mean age of the stationary population and the mean age of persons with the attribute in the stationary population. The most interesting class of models pertains to situations in which population characteristics are constrained and force alterations in life cycles in respose to demopgraphic change.