D4 Population dynamics and dependency

Topic

The natural growth rate—that is, the difference between the birth rate and the death rate—and the net migration balance are the two independent channels through which the underlying population structure changes. A sustained reversal in the sign of either one—the natural growth rate or the net migration balance—is sufficient to trigger a shift in the demographic mode; it is not necessary for both to reverse sign simultaneously. The underlying population structure can shift direction via natural change, via migration, or through a combination of both.

Birth and death rates together measure the population's natural growth: the extent to which a population grows or shrinks solely due to births and deaths, excluding migration. A sustained reversal in the sign of the difference between the birth rate and the death rate—the natural growth rate mentioned earlier—is one of the two pathways to a shift in demographic mode. It is not enough for the two rates to cross briefly; the difference must be sustained with the reversed sign.

The net migration balance measures the difference between a society's immigration and emigration and represents the other pathway—independent of the natural growth rate—to a shift in demographic mode. A sustained reversal in the sign of the net migration balance directly alters the underlying population structure, even if the natural growth rate does not change sign. This same sustained reversal in the net migration balance can offset fertility levels far below replacement—where sustained immigration compensates for the missing births—or exacerbate the situation, where sustained emigration compounds fertility levels that are already well below replacement. Unlike the natural growth rate and net migration balance, the dependency ratio does not measure a shift in demographic regime but rather the proportion of the dependent population—comprising both the young and the elderly—relative to the working-age population. A sustained rise in this ratio signals an aging anomaly only when driven specifically by the elderly component rather than the youth component; a rise caused by an uptick in the number of young people—linked to an increasing natural growth rate—does not trigger this signal. Instead, the elderly component must be the one driving the dependency ratio upward.

These three indicators allow for monitoring changes in the underlying population structure from different angles: the natural growth rate and net migration balance are the two channels that drive shifts in the demographic regime, whereas the dependency ratio—depending on whether it is driven by the elderly or the young—separately detects the anomaly of aging. This is how "Population"—a theme only partially influenced by the resource regime track—responds to changes in the dominant source of surplus without being entirely determined by them.

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