AP Human Geography Unit 2 ReviewPopulation and Migration

AP Human Geography Unit 2 is about where people live, why populations grow or shrink, and why people move. Its single biggest idea is that all population change comes from just three things, fertility, mortality, and migration, and each of those is shaped by economic, cultural, political, and environmental forces. You'll use the Demographic Transition Model, population pyramids, and push/pull factors to explain real patterns like China's aging population or rural-to-urban migration in the developing world. Unit 2 makes up 12-17% of the AP exam, tied for the largest weight in the course.

What this unit covers

Where people live and how we measure it

• Population is wildly uneven across Earth. Most humans cluster in East Asia, South Asia, Europe, and eastern North America because of physical factors (mild climate, flat land, fresh water) and human factors (jobs, history, politics, culture).
• The pattern you see depends on scale. At the global scale, climate explains a lot. At the local scale, things like zoning, rent, and job locations matter more.
• There are three ways to calculate density, and each tells you something different. Arithmetic density is total people divided by total land. Physiological density is people divided by arable (farmable) land, which shows pressure on food-producing land. Agricultural density is farmers per unit of arable land, which hints at development level (rich countries have low agricultural density because machines replace farm labor).
• Egypt is the classic example. Its arithmetic density looks moderate, but its physiological density is enormous because nearly everyone lives along the Nile.
• Distribution and density have consequences. Dense populations strain carrying capacity (the number of people an environment can support) and shape how services like hospitals and schools get distributed.

How populations are structured and how they change

• Population composition means age structure and sex ratio. A population pyramid graphs these, and its shape tells a story. A wide base means high birth rates and rapid growth. A narrow base with a bulging top means an aging, possibly shrinking population.
• Pyramids aren't just academic. Businesses and governments use them to predict markets and plan services. A pyramid full of 5-year-olds means you'll need schools; a pyramid full of 70-year-olds means you'll need healthcare and pensions.
• Growth and decline come down to fertility, mortality, and migration. The rate of natural increase (RNI) is the crude birth rate minus the crude death rate, ignoring migration. Doubling time estimates how long a population takes to double at its current growth rate.
• Replacement-level fertility is about 2.1 children per woman. Below that, a population eventually shrinks without immigration.

The big models: DTM, epidemiological transition, and Malthus

• The Demographic Transition Model (DTM) is the backbone of this unit. It links development to population change across five stages. Stage 1 has high birth and death rates (no country is here today). Stage 2 sees death rates fall thanks to sanitation, medicine, and food supply while birth rates stay high, producing explosive growth (think Niger or many sub-Saharan African countries). Stage 3 brings falling birth rates as people urbanize and women gain education (Mexico, India). Stage 4 has low birth and death rates and slow growth (the US). Stage 5 has birth rates below death rates and natural decline (Japan, Germany).
• The epidemiological transition is the DTM's companion. It explains why death rates change by tracking what kills people, from pandemics and famine in early stages to chronic, degenerative diseases like heart disease and cancer in later stages.
• Thomas Malthus argued in 1798 that population grows exponentially while food supply grows arithmetically, so population will outrun food and crash. Critics point out he didn't foresee the Green Revolution, technology, or falling fertility rates. Ester Boserup flipped his logic, arguing population pressure drives agricultural innovation. Neo-Malthusians revive his warning for resources beyond food, like water and energy.

Policy, gender, and aging

• Governments try to steer population growth. Pronatalist policies encourage births (Sweden's parental leave, Singapore's incentives). Antinatalist policies discourage them (China's former one-child policy is the famous case, with consequences like a skewed sex ratio and a rapidly aging workforce). Immigration policies adjust population size and composition from the outside.
• The status of women is one of the strongest predictors of fertility. As access to education, employment, healthcare, and contraception expands, fertility falls. This is happening almost everywhere on Earth.
• Aging populations create a rising dependency ratio, the share of people too young or too old to work compared to the working-age population. That means fewer taxpayers supporting more retirees, pressure on pensions and healthcare, and political debates over immigration as a fix.

Migration: why people move and what happens when they do

• Push factors drive people out of a place (war, drought, unemployment); pull factors draw them somewhere else (jobs, safety, family). Both can be economic, political, environmental, cultural, or demographic. Intervening obstacles (a border wall, an ocean, visa costs) and intervening opportunities (a decent job found along the way) can stop or redirect a move.
• Forced migration includes slavery and crises that produce refugees (people who cross an international border fleeing danger), internally displaced persons (forced to move but still inside their country), and asylum seekers (people requesting refugee protection in another country).
• Voluntary migration comes in flavors you need to know by name. Transnational (across borders, keeping ties to home), internal (within a country), chain (following family or community members who moved first), step (moving in stages, village to town to city), guest worker (legal temporary labor migration), transhumance (seasonal movement of herders with livestock), and rural-to-urban (the dominant migration pattern in developing countries).
• Migration reshapes both ends of the journey. Politically, it fuels debates over citizenship and borders. Economically, remittances (money migrants send home) are a lifeline for many countries, while origin countries can suffer brain drain. Culturally, migrants create ethnic neighborhoods and spread languages, religions, and foods.

Unit 2, Population and Migration at a glance

Why Unit 2, Population and Migration matters in AP HuG

Population is the raw material of human geography. Every later unit is really asking what people do, and Unit 2 establishes who those people are, where they are, and where they're going. The course's recurring themes all run through it.

• Scale of analysis gets its first big workout here. A country can have low arithmetic density nationally but extreme crowding regionally (Egypt), and the exam loves testing whether you notice that.
• The interplay of environmental, economic, cultural, and political factors is the unit's official throughline. Almost every question asks you to sort causes into those categories.
• Demographic change has long- and short-term consequences for economies, cultures, and politics, which is the lens you'll reuse for urbanization, development, and political conflict later.

How this unit connects across the course

• Unit 2 applies the scale of analysis, map types, and data skills you built in Thinking Geographically (Unit 1). Choropleth maps of fertility and population pyramids are exactly the kinds of stimuli Unit 1 prepared you to read.
• Migration is the engine of cultural diffusion in Cultural Patterns and Processes (Unit 3). Relocation diffusion, ethnic enclaves, and language spread all start with the migration flows you learn here.
• Refugees, asylum policy, and immigration debates feed directly into Political Patterns and Processes (Unit 4), where borders, sovereignty, and centrifugal forces depend on who moves where.
• Malthus, carrying capacity, and physiological density set up Agriculture (Unit 5), while rural-to-urban migration is the origin story of Cities (Unit 6). The DTM's link between development and demographics pays off again in Industrial and Economic Development (Unit 7), where it lines up with development models.

Key thinkers and models

• Thomas Malthus: Predicted in 1798 that exponential population growth would outpace arithmetic food growth, ending in famine and crisis.
• Ester Boserup: Argued the opposite of Malthus, that population pressure pushes societies to innovate and grow more food.
• Neo-Malthusians (e.g., Paul Ehrlich): Updated Malthus for the modern era, warning that population growth strains resources like water, energy, and the environment, not just food.
• Warren Thompson: Developed the Demographic Transition Model linking economic development to falling death rates, then falling birth rates.
• Abdel Omran: Created the epidemiological transition, explaining how causes of death shift from infectious disease to chronic illness as countries develop.
• E.G. Ravenstein: Wrote the laws of migration, including that most migrants move short distances, move for economic reasons, and that women are more likely than men to migrate internally.
• Wilbur Zelinsky: Proposed the migration transition model, tying migration patterns to a country's DTM stage (stage 2 countries send emigrants, stage 4 countries receive immigrants).

Unit 2, Population and Migration on the AP exam

Unit 2 is 12-17% of the exam, tied for the heaviest weight of any unit, so this content is guaranteed to show up repeatedly.

• Multiple-choice questions lean hard on stimuli. Expect population pyramids you have to match to a country or DTM stage, dot maps and choropleth maps of distribution, and data tables of birth rates, death rates, or dependency ratios. The skill is reading the graphic, not memorizing it.
• Free-response questions reward category thinking. Prompts often ask you to identify a pattern and then explain its economic, political, or cultural causes or consequences, sometimes one of each. Aging populations, pronatalist/antinatalist policies, and the effects of migration on origin and destination countries are classic FRQ territory.
• Scale matters. A frequent move is asking how a pattern at one scale (national fertility decline) looks different at another (regional or local variation). Name the scale you're describing.
• Use models actively. The DTM and Malthusian theory show up as tools you apply to a country or dataset, including their limitations. Knowing a critique of Malthus is as valuable as knowing the theory.

Essential questions

• Why do humans cluster where they do, and what happens when too many people depend on too little land?
• What causes a country's population to boom, stabilize, or shrink, and can governments actually change that trajectory?
• Why do people leave home, and how does their movement reshape both the places they leave and the places they join?
• Does population growth doom us to resource crisis, or does human innovation keep pace?

Key terms to know

• Carrying capacity: The maximum population an environment can sustainably support with its available resources.
• Rate of natural increase (RNI): Crude birth rate minus crude death rate, showing growth without counting migration.
• Doubling time: The number of years it takes a population to double at its current growth rate.
• Total fertility rate (TFR): The average number of children a woman is expected to have, with about 2.1 as replacement level.
• Dependency ratio: The number of people under 15 and over 64 compared to the working-age population that supports them.
• Population pyramid: A bar graph of a population's age and sex structure used to assess growth, decline, and future needs.
• Pronatalist policy: A government policy encouraging births, like paid parental leave or baby bonuses.
• Antinatalist policy: A government policy discouraging births, like China's former one-child policy.
• Push and pull factors: Conditions that drive people away from a place or attract them to a new one, in economic, political, cultural, environmental, or demographic form.
• Intervening obstacle: A barrier, like a border, ocean, or visa requirement, that blocks or redirects a migration.
• Refugee: Someone forced to flee across an international border because of danger or persecution.
• Internally displaced person (IDP): Someone forced from home who remains within their own country's borders.
• Chain migration: Migration that follows family or community members who moved to a destination first.
• Remittances: Money migrants send back to family in their home country, a major income source for many origin countries.

Common mix-ups

• Refugee vs. IDP vs. asylum seeker. Refugees have crossed an international border. IDPs have not. Asylum seekers are applying for refugee status in the country they've reached, but it hasn't been granted yet.
• Physiological vs. arithmetic density. Arithmetic uses all land; physiological uses only arable land. A desert country like Egypt can have a low arithmetic density and an extremely high physiological one.
• Falling RNI does not mean a shrinking population. Population momentum keeps a country growing for decades after fertility drops, because a huge cohort of young people is still entering childbearing years.
• Stage 2 vs. Stage 3 of the DTM. In stage 2 only the death rate falls (growth explodes). In stage 3 the birth rate finally starts falling too (growth slows). Don't say birth rates drop in stage 2.