Demography: Population Size, Composition, and Change Key Concepts

Demography

The scientific study of human populations — their size, composition, distribution, and the causes and consequences of changes in these characteristics.

Size

How many people live in a given area or population.

Composition

The structure of a population by characteristics such as age, sex, race, and ethnicity.

Distribution

Where people live geographically — population spread across space.

Change

How populations evolve over time through fertility, mortality, and migration.

Three Demographic Processes

Fertility (adding people), Mortality (subtracting people), and Migration (moving people).

Fecundity vs. Fertility

Fecundity = biological potential for childbearing; Fertility = actual number of births occurring.

Typical fertility range

Even in high-fertility countries, fertility rarely exceeds 8 children per woman; biological max ≈ 15.

Distal factors affecting fertility

Indirect factors like cultural values, gender roles, women's labor participation, economic conditions, and health.

Proximate determinants of fertility

Direct factors like marriage/sexual union, contraceptive use, infecundity (e.g. breastfeeding), and abortion levels.

Crude Birth Rate (CBR)

(Births / Mid-year population) × 1,000 — measures overall birth intensity.

Total Fertility Rate (TFR)

The sum of age-specific fertility rates for all age groups — average children per woman if she experienced current fertility.

Replacement-level fertility

About 2.1 children per woman — enough to replace parents in the long term.

Completed (cohort) fertility

Average number of children born to a group of women by the end of their reproductive years.

Crude Death Rate (CDR)

(Deaths / Mid-year population) × 1,000 — overall death frequency.

Why CDR can mislead

Depends heavily on age structure — older populations have higher CDR even with good health.

Life expectancy at birth

Average years a newborn is expected to live if current mortality rates remain constant.

Maximum lifespan

Approximate biological upper limit of human life — around 120 years.

Mortality differentials: Age

Infants and the elderly have higher mortality.

Mortality differentials: Sex

Women generally outlive men.

Mortality differentials: SES

Wealthier, better-educated people tend to live longer.

Mortality differentials: Race/Ethnicity

Reflect social inequalities and access to healthcare.

Migration definition

Movement of people across geographic boundaries.

Migration selectivity

Migrants are often younger, more educated, and more ambitious — not a random subset.

International vs. Internal migration

International = crossing national borders; Internal = within a country.

Effects of migration

Alters population size, composition (age/ethnicity), and geographic distribution (urbanization).

Population balancing equation

P₂ = P₁ + (B − D) + M — population at time 2 equals earlier population plus births, minus deaths, plus net migration.

Natural increase

Births − Deaths (ignores migration).

Net migration

Immigration − Emigration.

Why use demographic rates

Standardize demographic events to compare populations of different sizes.

Crude Growth Rate (CGR)

(Net growth / Mid-year pop) × 1,000.

Crude Natural Growth Rate (CNGR)

(Births − Deaths) / Mid-year pop × 1,000.

Crude Migration Rate (CMR)

(Net migration / Mid-year pop) × 1,000.

Relationship between CBR, CDR, and CGR

CGR = CBR − CDR + CMR.

Population composition

Characteristics such as age, sex, race, and ethnicity.

Age structure significance

Determines social and economic challenges — young vs. aging populations.

High fertility + low mortality

Broad pyramid base — young population.

Low fertility

Narrow pyramid base — aging population.

Median age

Indicator of overall population maturity and aging.

Population pyramids: Expansive

Triangle shape — high fertility, rapid growth.

Population pyramids: Stationary

Rectangle shape — low fertility/mortality, stable population.

Population pyramids: Constrictive

Narrow base — low fertility, aging, possible decline.

What pyramids reveal

Birth/death rates, gender balance, historical events, and dependency challenges.

Sex ratio definition

(Males / Females) × 100.

Normal sex ratio at birth

About 105 males per 100 females (≈51% boys, 49% girls).

Dependency ratio

(Population under 15 + over 64) / (Population aged 15-64). Indicates burden on working-age group.

High dependency ratio effects

Economic strain on workers and social welfare systems.

Cohort definition

A group of people sharing a demographic event (e.g., birth cohort of 2000).

Period definition

A specific calendar time (e.g., fertility rate in 2020).

Age definition (demographic)

Time since birth — connects cohort and period.

Lexis diagram axes

X-axis = time, Y-axis = age, diagonals = cohorts.

Purpose of Lexis diagram

Visualizes the relationship among age, period, and cohort in population data.

WPP 2024: Global peak prediction

World population likely to peak within the current century.

Countries that have peaked

About 1 in 4 people live in a country whose population has already peaked.

Change in fertility since 1990

Women today have, on average, one child fewer than in 1990.

Early childbearing effects

Harmful for young mothers and their children's outcomes.

Life expectancy after COVID-19

Global life expectancy is rising again post-pandemic.

Population momentum

Continued growth even after fertility falls, due to many young people entering reproductive ages.

Demographic dividend

Temporary economic boost when working-age population grows faster than dependents.

Demographic dividend conditions

Requires investments in education, jobs, and health to realize benefits.

Global aging trend

By 2080, people aged 65+ will outnumber children under 18.

Role of immigration

For some countries, immigration will drive future growth.

Gender equality & population stability

Empowering women reduces both rapid growth and population decline.

Cohort-component projection method

WPP projects population by 'aging forward' each cohort, applying fertility, mortality, and migration assumptions.

Projection formula

P(t+1) = P(t) + B − D + (I − E).

Regional trend: Sub-Saharan Africa

Fastest growth, TFR > 4.

Regional trend: Asia & Latin America

Rapid fertility decline.

Regional trend: Europe & East Asia

Fertility < 1.5, population shrinking.

Regional trend: North America

Stable growth, largely due to migration.

Global aging impact

Rising median age and dependency ratios worldwide.

U.S. Census purpose

Constitutional count every 10 years for apportionment, redistricting, and federal funding allocation.

Census scope

Counts everyone living in the U.S., regardless of citizenship.

Census frequency

Conducted every 10 years (decennial) since 1790.

de facto enumeration

Counts people where they are found on Census Day.

de jure enumeration

Counts people where they usually live — the method used by the U.S.

Why use de jure method

Improves policy relevance and comparability.

Net undercount definition

(# missed people) − (# counted more than once). Usually positive.

Hard-to-count populations

Young children, racial/ethnic minorities, renters, homeless, undocumented.

Effects of undercount

Misallocation of representation and funding, data bias.

Census data uses: Political

Determines congressional seats and redistricting.

Census data uses: Planning

Informs infrastructure, education, healthcare needs.

Census data uses: Research

Baseline for demographic and economic analysis.

Census data uses: Sampling frame

Base for surveys like ACS, CPS, NHIS.

Census data uses: Private sector

Business location, marketing, housing planning.

Why surveys are needed

Censuses are infrequent and limited — surveys give detailed, frequent updates.

Survey data advantages

Detailed demographic/economic info; annual or monthly updates.

American Community Survey (ACS)

Continuous, detailed annual data about U.S. communities (~3.5M addresses/year).

ACS content areas

Education, income, housing, employment, commuting, ancestry.

ACS uses

Local planning, federal funding, research on inequality and migration.

Current Population Survey (CPS)

Monthly survey measuring labor force characteristics (employment/unemployment).

CPS agencies

Conducted by Census Bureau + Bureau of Labor Statistics.

CPS sample design

~60,000 households/month, rotating panel (4-8-4 months).

CPS main uses

Official U.S. unemployment rate; data on income, insurance, voting.

ACS vs. CPS differences

ACS = large, annual, multi-topic; CPS = small, monthly, labor-focused.

Vital registration systems

Continuous, event-based records of births, deaths, marriages, divorces, and fetal deaths.

Purpose of vital registration

Measure fertility, mortality, and natural increase continuously.

Birth certificate data

Date/place, sex, weight, parents' age, education, marital status.

Uses of birth data

Calculate fertility rates, track maternal and infant health trends.

Death certificate data

Age, sex, race, cause of death, occupation, residence.

Uses of death data

Calculate CDR, ASDR, life expectancy, and causes of death.