How many people now?
Population at time t.
A population count is tied to a place, date and inclusion rule.
Side 44
Demography
A study of how populations change. Demography tracks births, deaths and migration, then follows how those flows reshape age structure, households, labor forces, dependency and future population size.
population→fertility / mortality / migration→structure→projection
05core processes
06population measures
05projection questions
44Side
Population size changes through only a few flows.
Births and immigration add people; deaths and emigration remove them.
How many enter through reproduction?
Fertility + age structure.
The number of births depends on both fertility rates and how many people are in reproductive ages.
How many leave through mortality?
Age-specific mortality.
Population aging can increase deaths even when mortality rates improve.
How many enter or leave geographically?
Inflow − outflow.
Migration can reshape population structure much faster than fertility in some places.
What population remains?
Update the stock.
Demographic accounting connects flows to future population size.
Population balancing equationP₂ = P₁ + births − deaths + immigration − emigration
Fertility is measured by rates, timing and completed behavior.
Raw birth counts cannot be interpreted without considering how many people are exposed to childbearing at each age.
Births per total population.
Easy to compute but strongly affected by age structure.
Births by age group.
Reveals timing patterns hidden by aggregate rates.
Synthetic period measure.
Summarizes current age-specific rates as the births a hypothetical woman would have if those rates persisted.
Timing changes affect period rates.
Postponing births can temporarily lower period fertility even if completed family size changes less.
Birth order matters.
Transitions to first, second or later births can respond differently to social and economic conditions.
Long-run generational replacement.
Replacement fertility depends on mortality and population context and is not a universal fixed constant.
Mortality patterns are age-specific.
Life expectancy compresses a schedule of age-specific death rates into one interpretable measure.
Mortality rate
Deaths relative to exposure.
Age-standardization is often needed when comparing populations with different age structures.
Life table
Follow a synthetic cohort through age.
Life tables translate age-specific mortality into survival probabilities and remaining life expectancy.
Life expectancy
Average years under current mortality rates.
Period life expectancy is not a prediction of one newborn’s exact lifespan.
Infant mortality
Deaths early in life.
Highly sensitive to health, living conditions and measurement practices.
Cause structure
What are people dying from?
Cause composition shifts as infectious, chronic, injury and age-related mortality patterns change.
Migration is selective movement.
People who move are rarely a random sample of either the origin or destination population.
Movement within a country.
Urbanization and regional labor markets can redistribute population without changing national size.
Movement across borders.
Definitions depend on residence duration, legal categories and statistical systems.
Movers cluster in certain ages.
Young adults often move more, altering destination labor force and fertility structure.
Movement is not always one-way.
Return and circular migration complicate simple origin–destination counts.
Prior migration changes later migration.
Family and community ties can reduce information and settlement costs for subsequent movers.
Inflow minus outflow.
A zero net balance can hide very large flows in both directions.
Population structure carries demographic memory.
Past fertility, mortality and migration are visible in the distribution of people by age and sex.
| Structure | Signal | Likely implication | Caution |
|---|---|---|---|
| Wide younger ages | Recent high fertility or young migration | Future school / labor-force growth | Depends on future fertility and migration |
| Narrow younger ages | Lower recent fertility | Population aging | Can be offset by migration |
| Large older cohorts | Past large birth cohorts + survival | Higher old-age dependency | Health and labor participation matter too |
| Working-age bulge | Large cohorts in productive ages | Potential demographic dividend | Requires jobs, skills and institutions |
| Sex imbalance | Migration, mortality or birth imbalance | Household and labor effects | Varies strongly by age |
Population projections are conditional futures.
They are generated by assumptions about fertility, mortality and migration—not observations of what must happen.
Base population
Start with population by age and sex at a known date.
Survival
Apply age-specific mortality assumptions as cohorts move forward.
Births
Apply fertility assumptions to populations at reproductive ages.
Migration
Add and remove people by assumed age, sex and flow levels.
Scenario
Use alternative assumptions to expose which demographic processes drive uncertainty.
Revision
Update projections as new census, vital-statistics and migration evidence arrives.
Demography: Measuring and Modeling Population ProcessesPreston, Heuveline & Guillot · demographic methods
Population: An Introduction to Concepts and IssuesWeeks · broad foundation
Introduction to Demographyfertility, mortality and migration methods
Applied Demographypopulation projection and planning