E3 Population base structure

Topic

The foundational population structure of a society—the substrate of the "Population" component (a theme only partially driven by the resource regime track, unlike "Economy" and "Cliodynamics," which are fully driven by it)—comprises the total population size, its age structure, and its vital rates. As long as these three components remain within typical ranges, the foundational population structure reconstitutes itself in the same way year after year, without the society undergoing a shift in demographic mode.

Two variables trigger a shift in the demographic mode of the foundational population structure: the natural growth rate (the difference between the birth rate and the death rate) and the net migration balance (the difference between immigration and emigration). A sustained change in the sign of either variable—whether the natural growth rate or the net migration balance—is sufficient to cause the foundational population structure to shift its demographic mode; both do not need to change sign simultaneously.

Therefore, tracking the natural growth rate and the net migration balance separately—rather than merely their combined effect—is essential for anticipating a shift in demographic mode. The natural growth rate might reverse its sign while the net migration balance remains stable, or the net migration balance might reverse its sign while the natural growth rate remains unchanged; either scenario is sufficient for the foundational population structure to move beyond its previous demographic mode.

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