A mathematical model (Core Model) is presented that describes the gross dynamic behavior of the demographic transition—falling death rates lead to population increase, temporarily rising birth rates,
temporarily increased population growth, decreased fertility, aging of the population, and finally a steady
population size higher than the initial population. Core Model captures these features. The model is based on three fundamental observations 1) people are born, 2) people die, some at a young age, the
rest at an old age, and 3) people give birth more often when conditions are favorable than not favorable.

In addition to boundary conditions, Core Model has one free parameter, which is associated with the rate at which fertility adapts to changing conditions. Core Model predicts that aging populations are a
natural consequence of the dynamics and the speed at which the population fertility adapts to changing
death rates. The model captures the qualitative features of actual country demographic dynamics with
European countries making up most of the post-transition populations and Sub-Saharan Africa making
up those countries just entering the transition.