Population growth

Population growth is the change in the number of inhabitants of an area between two points of time. It can be expressed as an absolute number or as a percentage. It can be positive (when births exceed deaths, or people move in) or negative (when births fall below deaths, or people move out).

Three things change a population: births, deaths and migration.

• Natural growth = births − deaths, in a region in a given time.
• Actual growth = births − deaths + in-migration − out-migration.

Births and deaths are measured as rates:

• The crude birth rate (CBR) is the number of live births in a year for every thousand people (measured at mid-year).
• The crude death rate (CDR) is the number of deaths in a year for every thousand people.

Death rates depend on a population's age structure, its social advancement, and its level of economic development. Over the last centuries, better medicine, food supply and sanitation have lowered death rates nearly everywhere, which is the main reason the world's population has grown so fast.