Topic 2.3 Notes – Population Composition

What Population Composition Is

Population composition means the demographic makeup of a population. Two countries can have the same total population and still function very differently if one is full of children and the other has many elderly residents.

In this topic, AP Human Geography narrows the focus to:

• Age structure = how people are distributed across age groups
• Sex ratio = how many males there are compared with females

That matters because composition shapes demand:

• lots of children → schools, childcare, pediatric care
• lots of working-age adults → jobs, housing, transportation
• lots of elderly residents → pensions, assisted living, geriatric care

Scale matters too. A national average can hide local patterns like:

• college towns with lots of 18 to 24 year olds
• retirement communities with older residents
• military bases, mining towns, or migrant labor destinations with male-heavy working ages

A common exam move is simple. Spot the pattern first, then connect it to likely social or economic needs.

Age Structure and Sex Ratio

Age structure

Age structure shows the distribution of people across age groups.

The three broad groups are:

• Young dependents 0 to 14
• Working-age population 15 to 64
• Older dependents 65+

You’ll also see five-year cohorts like 0 to 4, 5 to 9, 10 to 14. Those are more precise and make bulges or gaps easier to see.

Typical patterns:

• Youthful structure = lots of children, often tied to high fertility; common in many sub-Saharan African countries
• Stable or slow-growth structure = age groups are more similar in size
• Aging or declining structure = smaller younger groups and larger middle or older groups; Japan is the standard example

Sex ratio

Sex ratio is the number of males per 100 females.

$$\text{Sex ratio}=\frac{\text{number of males}}{\text{number of females}} \times 100$$

• Above 100 = more males
• Below 100 = more females
• At birth, the ratio is usually around 105 male births per 100 female births

The ratio usually drops with age because males have higher mortality and women tend to live longer. That’s why older cohorts often have more women.

What changes age and sex composition

These patterns come from population processes:

• Fertility shapes the base. High fertility widens younger cohorts; low fertility narrows them.
• Mortality and life expectancy shape the top. Lower mortality means more people survive to older ages.
• Migration often changes working-age groups most.
  • Persian Gulf states often show male-heavy labor migration.
  • Local examples include military bases, mining towns, and resource-frontier settlements.
• Social disruption like war, famine, or epidemics can leave small cohorts or sex imbalances.
• Cultural preference and policy can matter too. Son preference and sex-selective abortion contributed to male-skewed younger populations in China and India, and China’s one-child policy intensified that pattern.

Mapping Population Composition

Geographers map composition to show where these patterns cluster.

Common mapped variables include:

• percent under 15
• percent 65+
• median age
• working-age share
• total sex ratio
• sex ratio by age group

Median age is a good example because it quickly shows where populations skew older or younger across the world.

World median age by country, 2017

On a choropleth map, use percentages, rates, or ratios, not raw totals. Raw counts tell you size. Composition maps compare shares.

In this example, the main takeaway is the broad regional pattern of older populations in much of Europe and Japan and younger populations across much of sub-Saharan Africa.

Patterns to look for:

• clustering
• core-periphery
• urban-rural contrast
• isolated concentrations

A U.S. county map might show older populations in parts of Florida and Arizona, while university counties show big 18 to 24 populations. Aggregation can hide this. A country may look balanced overall but still contain male-heavy labor districts or female-heavy retirement communities.

Reading Population Pyramids

A population pyramid is a horizontal bar graph showing age and sex.

Setup:

1. Age cohorts go on the vertical axis, youngest at the bottom.
2. Males are usually on the left and females on the right, but always check the legend.
3. The horizontal axis shows counts or percentages.
4. Check the title, place, year, and units before you interpret it.

Three overall forms:

• Expansive = wide base, narrow top, rapid growth
• Stationary = more rectangular, slow or stable growth
• Constrictive = narrow base, broader middle, aging and possible decline

Features to spot:

• wide base = high recent fertility
• narrow base = low recent fertility
• broad middle = large working-age population
• broad top = many elderly, longer life expectancy
• asymmetry = male and female sides differ
• bulge = unusually large cohort
• indentation = unusually small cohort

The classic example is the U.S. baby boom from 1946 to 1964. That large cohort moves upward over time, shifting demand from schools to housing to retirement and health care. Remember, a pyramid shows a pattern, but the cause could be births, deaths, migration, or conflict.

What Population Composition Tells You

Composition helps you infer future growth and likely needs.

• Large younger cohorts suggest continued growth.
• Smaller younger cohorts than middle cohorts suggest future decline unless immigration offsets it.

You can also estimate dependency with the dependency ratio:

$$\text{Dependency ratio}=\frac{\text{population 0-14}+\text{population 65+}}{\text{population 15-64}} \times 100$$

• youthful populations → high child dependency
• aging populations → high old-age dependency

Follow cohorts through time:

• many children → schools, childcare, pediatric care
• many young adults → universities, jobs, housing, transportation
• many elderly → pensions, geriatric care, accessible housing