The three demographic variables that change the size of a population are

• A. Fertility, mortality, and net migration
• B. Age, education, and income
• C. Mortality, morbidity, and fertility rate
• D. Migration, urbanization, and mortality

Answer: (A)

Births, deaths, and movement of people into and out of a population determine how its size changes. Fertility sets how many births occur, mortality sets how many deaths occur, and net migration accounts for people coming in minus those leaving. In simple terms, population size next period equals current size plus births minus deaths plus net migration. Net migration can add to or subtract from the total depending on whether more people move in than move out. For example, with 100 births, 60 deaths, and 20 people moving in, the population grows by 60 (100 − 60 + 20). The other factors listed influence the rates of these processes or the characteristics of the population, but they do not directly change the size in the straightforward way these three do.