What are the three main factors that cause changes in population size?

Concept

Population change is governed by three direct demographic flows: births, deaths, and migration.

Over a period, population change equals births minus deaths plus in-migration minus out-migration. Marriage and life expectancy can influence these flows, but they are not separate terms in this accounting identity.

Application

The combination ‘Birth, death and migration’ names all three direct components. Births add people, deaths remove people, and migration changes the population when people enter or leave an area.

Contrast

• Birth, death and marriage: marriage may influence fertility behaviour, but it is not itself a direct inflow or outflow in the population-balance equation.

• Birth, death and life expectancy: life expectancy summarizes mortality conditions; it is not an additional direct component beside deaths.

• More than one of the above: only one listed combination contains all three direct components, so the required count of at least two is not reached.

• None of the above: one listed combination contains all three direct components, so the claim that no combination qualifies does not hold.

Cross-check

Using the balancing identity ΔP = births − deaths + in-migration − out-migration confirms the same three components.

Therefore, the answer is ‘Birth, death and migration’.