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Demography

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Demography is the statistical study of human populations. It looks at how many people live in a place, how old they are, where they live, and how these things change through births, deaths, and migration.

What demography studies

A population changes in only three ways. People are born, people die, and people move. The field draws on statistics, sociology, economics, geography, and public health.

Population change is slow, so the near future is easier to forecast than most trends.[1]

Core measures and how to read them

  • Crude birth rate and crude death rate. Births or deaths per 1,000 people in a year. Crude means they ignore age, so an older population shows a higher death rate.
  • Total fertility rate. The births a woman would have in her life if current rates at each age held steady. Replacement level is about 2.1, where each generation roughly replaces itself.[1]
  • Life expectancy. The years a newborn would live if current death rates held steady.
  • Age structure. The share of people at each age. A population pyramid draws it as bars, with a wide base for many children.
  • Dependency ratio. People under 15 and over 64, divided by people aged 15 to 64. Definitions vary, so check the source.
  • Net migration and growth rate. Net migration is immigrants minus emigrants.[1] Add the net migration rate to the birth rate and subtract the death rate, all per 1,000. Divide by 10 for the growth rate in percent.
  • Doubling time. Divide 70 by the growth rate in percent. At 2 percent, doubling takes about 35 years.

The demographic transition

The demographic transition is a shift from high birth and death rates to low ones, toward longer lives and smaller families.[1] The classic model has four stages.

  1. Birth and death rates are both high. The population grows slowly.
  2. Death rates fall because of better food, clean water, and medicine. Births stay high, so growth speeds up.
  3. Birth rates fall. Growth slows, and the share of working age people rises.
  4. Birth and death rates are both low. Growth is near zero.

Some versions add a fifth stage, with a shrinking population.

Major data sources

  • National censuses count every person, often every ten years. The United States Census Bureau publishes US results and some international data.
  • UN World Population Prospects gives estimates and projections for all countries.[1]
  • The UN Data Portal, the World Bank, and Our World in Data offer tables and free charts. Eurostat serves the European Union.

The United Nations projected that the world passed 8 billion people on 15 November 2022.[2] The 2024 revision expects a peak near 10.3 billion in the mid-2080s, up from 8.2 billion in 2024, then a slow fall to 10.2 billion by 2100.[1]

  • Falling fertility. The global rate was 2.25 births per woman in 2024, down from 3.31 in 1990.[1]
  • Longer lives. Global life expectancy reached 73.3 years in 2024, up 8.4 years since 1995.[1]
  • Aging. By the late 2070s, about 2.2 billion people will be 65 or older, more than the children under 18.[1]
  • Shrinking populations. In 63 countries and areas, with 28 percent of the world, the population has already peaked. They include China, Germany, and Japan.[1]

Migration does not change the world total, since a mover leaves one country and enters another. Immigration is projected to be the main source of growth through 2054 in 52 countries and areas, including Australia, Canada, and the United States.[1] The page Mexicans in the United Kingdom shows one small migrant community. A December 2008 article in The Independent tells of a plan to persuade Chinese farmers to emigrate.[3]

Why demography matters for policy

  • Pensions and health care. Longer lives raise demand for health care and long-term care, and they raise the cost of public pensions.[1]
  • Education and housing. Falling child numbers can close schools, and rising numbers need new ones. Household counts shape housing demand.
  • Labor. A larger share of working age people can speed growth, which is called the demographic dividend.[1]

Some people ask whether a basic income could ease the strain, and Basic income experiments around the world lists pilots. Post-scarcity ideas ask what follows if automation lifts output while the workforce shrinks. Longevity escape velocity asks what follows if medicine adds more than a year of life each year.

Learning, teaching, and research

How a student can start. Learn the core measures, then look at one country on Our World in Data. The math is percent, ratios, and rates per 1,000.

Classroom activities.

  • Build a population pyramid. Download an age and sex table from a census, chart it in a spreadsheet, and name the stage it suggests.
  • Compute a growth rate. A town grows from 20,000 to 22,000 people in 5 years. The ratio is 1.1, and its fifth root is about 1.019, so growth is 1.9 percent a year.
  • Explore the UN Data Portal. Chart fertility for three countries.

Project ideas.

  • Place one country on the transition model and defend the choice.

A caution about projections. A projection is a what-if built on assumptions. The UN publishes prediction intervals that widen with time, and future births are a main source of doubt.[1] Treat any single number for 2100 with care.

Discussion questions

  • Why can a country with falling births still grow for decades?
  • Which policy matters most for an aging population, and why?

Essay ideas

  • Explain why population change is easier to forecast than economic change.

AI prompt ideas

  • Explain the demographic transition to a high school student, then quiz me with five questions.
  • Help me build a population pyramid from census data, and ask me questions where my steps are vague.

Readings

Readings from Wikipedia

See also

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 World Population Prospects 2024: Summary of Results, United Nations
  2. ↑ Day of Eight Billion, United Nations
  3. ↑ China's new export: farmers, The Independent, December 2008
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