Age Distributions And Fertility

The three basic processes of fertility, mortality and migration determine observed population age profiles. Grossly simplified age profiles that are stable over time are often used as convenient caricatures of real-world populations (Figure 15.5): (1) an exponentially declining age distribution curve for exponentially growing populations in the Less Developed Countries (LDC) in the developing world, and (2) a uniform age distribution for the Industrialized Developed Countries (IDC); for a number of older industrialized economies, however (3) a more appropriate age distribution is one in which the proportion of older individuals exceeds that of the younger as fertility is below the level necessary for replacing deaths.

Clearly the first of these is unsustainable over the longer term as it would lead to infinite population size and (3), with fertility below replacement level, would lead to population extinction unless supplemented by immigration. More realistically fertility, mortality, and migration evolve over time in response to influences such as better health care, socio-economic factors, and people's expectations about the future (e.g., basic fertility may increase as a result of better health care, but individuals, for a wide range of reasons, may also have, or choose to have, fewer children, the latter trend, in general, appearing to dominate over the long term). Also important is the time lag between birth and reproduction; an increase or decrease in births will lead to an increase or decrease in individuals of reproductive age some two decades later so that the changes observed now in numbers of births will, as those born now reach their reproductive years, be

Figure 15.5. Stylized population age profiles. Two simple population age profiles often employed in modeling: (1) an exponentially declining age profile corresponding to an exponentially growing population (dotted line); (2) a uniform age distribution for a population of constant size (dashed line); and (3) a less simple, theoretical stable age distribution corresponding to a declining population size (solid line).

Age (years)

Figure 15.5. Stylized population age profiles. Two simple population age profiles often employed in modeling: (1) an exponentially declining age profile corresponding to an exponentially growing population (dotted line); (2) a uniform age distribution for a population of constant size (dashed line); and (3) a less simple, theoretical stable age distribution corresponding to a declining population size (solid line).

echoed by increases or decreases in birth numbers two or three decades hence. Thus to reverse the decline of an aging population (i.e., one with an age distribution increasingly skewed towards older ages) simply by means of an increasing per capita birth rate becomes increasingly difficult over the short term (i.e., less than several decades). In these circumstances inward migration becomes the major defense against a declining workforce, which, as migrants may well have had different previous experiences of infection to that of the host population, in turn may bring changes to the age distributions of susceptibility and immunity in the combined population and have implications for epidemiology and the design of control programs.

Pregnancy And Childbirth

Pregnancy And Childbirth

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