Plain-language explanation.
Demography is the statistical study of human populations — their size, composition, and changes over time. Demographers track births, deaths, and migration to understand how populations grow, shrink, or age. Their findings shape healthcare systems, pension policy, housing planning, and immigration policy.
Core concepts and standard treatment.
Core demography covers demographic measures: fertility (crude birth rate — CBR; total fertility rate — TFR — lifetime births per woman; replacement fertility — 2.1; age-specific fertility rates — ASFR), mortality (crude death rate — CDR; infant mortality rate — IMR; under-5 mortality rate — U5MR; maternal mortality ratio — MMR; life tables — survivorship, life expectancy at birth — e₀), migration (net migration rate; push vs pull factors — Lee's theory; international vs internal migration; remittances — $700bn+ globally — World Bank), and population change (demographic balancing equation: P₂ = P₁ + births - deaths + in-migration - out-migration; population momentum — Keyfitz — why population continues to grow even after fertility declines).
Deeper theory, debates and edge cases.
Advanced demography covers the demographic transition model (Warren Thompson 1929; Notestein 1945 — 4 stages: pre-transition high birth/death rates; mortality decline; fertility decline; post-transition low birth/death rates; 5th stage — sub-replacement fertility — Europe, Japan — demographic dividend — 'bulge' cohort in working age), population projections (UN World Population Prospects — medium/high/low variants; cohort-component method; stochastic projections; population pyramids — expansive, stationary, constrictive), population ageing (dependency ratios — youth vs old-age; the silver economy; pension sustainability — PAYG vs funded systems; health system demand; Japan as demographic extreme — shrinking population; China's demographic challenge — one-child policy legacy), and spatial demography (internal migration — counterurbanisation; suburbanisation; shrinking cities — Detroit, Leipzig — Rürup; global cities and migration; refugees and displaced persons — UNHCR — 100+ million forcibly displaced).
How it is applied in practice.
At the chief demographer, ONS statistician, and population policy adviser level, practitioners contribute to Demography and Population Studies; lead national statistics programmes (ONS — UK Census 2021; Integrated Population Statistics; migration statistics reform — Home Office/ONS border data); advise on population policy (UK Population Projections — ONS; immigration impact assessments — MAC — Migration Advisory Committee; pension policy — Triple Lock — OBR sustainability; DWP demographic modelling); apply demography to health service planning (NHS Long-Term Workforce Plan — HEE demand modelling; hospital capacity projections — Kings Fund demographic scenario modelling; geriatric medicine demand — multimorbidity projections); and contribute to global population forecasting (UN DESA Population Division; IHME demographic forecasting — Vollset et al. 'Fertility, mortality, migration, and population scenarios for 195 countries 2017–2100' — Lancet 2020 — predicting population peak then decline for many countries).