Population Change: Fertility, Mortality and the Demographic Transition Model

Population change in any country is driven by the balance of three components: births, deaths, and migration. When we ignore migration and look only at the difference between births and deaths, we get natural increase (or natural decrease). Understanding how fertility and mortality evolve over time is central to demography, and the Demographic Transition Model (DTM) is the framework geographers use to summarize that evolution.

Key measures

Several rates express change. The crude birth rate (CBR) is the number of live births per 1,000 people per year; the crude death rate (CDR) is deaths per 1,000 people per year. These are called "crude" because they take no account of a population's age structure, which is why a wealthy country with many elderly people can have a higher crude death rate t