Demography is the study of human populations, in particular their size, composition and spatial distribution, and how they change through fertility (births), migration, and mortality (deaths).
What demographers actually “do” goes well beyond this formal definition. Demography (or, more broadly, population studies) also includes analysis of the social, economic, environmental, and biological causes and consequences of population change. Although demography is a discipline in its own right, it draws heavily on other fields, such as sociology, epidemiology, economics, geography, statistics, history, and political science.