Where people live, how fast their numbers change, and why they move shape everything else in human geography. This unit covers population density and distribution, population pyramids, the demographic transition, theories and policies about growth, aging, and the causes and effects of migration.
Physical and human factors shape where people live, and the pattern depends on the scale you look at. Arithmetic, physiological and agricultural density each measure a different kind of pressure on the land. Distribution and density affect services, politics and the environment, including carrying capacity.
Age structure and sex ratio vary between places and can be mapped at different scales. Population pyramids show both at once and help predict growth or decline and future demand for goods and services.
Fertility, mortality and migration drive population change. The rate of natural increase and doubling time measure growth. The demographic transition model describes how birth and death rates fall as countries develop, and the epidemiological transition explains why death rates change.
Thomas Malthus argued in 1798 that population grows faster than food supply, so famine, disease and war would check growth. Critics point to technology that raised food output; neo-Malthusians warn that resource limits still matter.
Governments try to change population size and composition. Pronatalist policies encourage births, antinatalist policies discourage them, and immigration policies control who can move in.
More access to education, jobs, health care and contraception has lowered fertility in most of the world, and changing roles for women shape migration too, as Ravenstein noted. Low births and longer lives create aging populations, with political, social and economic consequences measured by the dependency ratio.
Push factors drive people out of a place; pull factors draw them in. Intervening obstacles and opportunities shape the journey. Factors can be cultural, demographic, economic, environmental or political. Migration is forced (slavery, refugees, internally displaced persons, asylum seekers) or voluntary, and voluntary migration takes several forms.
Migration has political, economic and cultural effects on both the places people leave and the places they go: remittances and brain drain, labor supply, ethnic neighborhoods, cultural diffusion, and debates over immigration policy.