How many people live in a country? How many are born each year? How many die? How many move in or out? Demography is the statistical study of human populations, and it answers these questions with numbers. Birth rates, death rates, migration, and age structure are its core variables.

The field dates to John Graunt, a 17th-century English statistician who analyzed London's mortality records. Today, demographers work for governments, universities, and international organizations. They project population growth, track aging societies, and model the spread of disease.

Key measures

• Crude birth rate: live births per 1,000 people per year
• Crude death rate: deaths per 1,000 people per year
• Total fertility rate: average children per woman
• Life expectancy: average years of life at birth
• Net migration rate: arrivals minus departures per 1,000

The demographic transition model describes how societies move from high birth and death rates to low ones as they industrialize. Many low-income countries are still in early stages. Many high-income countries face shrinking populations and aging workforces.