Demographic Structure of Society

5 min read Updated Apr 19, 2026

Demographics is the statistical study of populations - age, sex, race, income, geography, and similar characteristics. This section covers the major demographic dimensions.

Age

Cohorts and Generations

A cohort is a group of people who share a time-based life experience - typically birth within a specific span of years. Generational cohorts (Baby Boomers, Gen X, Millennials, Gen Z) are defined by shared cultural and economic experiences at similar ages.

Bar chart titled 'U.S. Living Adult Generations' showing the projected 2019 size and the original numbers born for the Silent Generation (1928-1945), Baby Boomers (1946-1964), Generation X (1965-1980), and Millennials (1981-1996); Boomers were the largest birth cohort but Millennials are now the largest living adult cohort — U.S. living adult generational cohorts. Birth cohorts shrink as members die; immigration can add to a cohort over time. Note that Millennials have surpassed Boomers in size despite a smaller original birth count. *Credit: Nerd271 via Wikimedia Commons (CC BY-SA 4.0).*

Age Structure and Pyramid

A population pyramid shows the distribution of ages and sexes in a population. Shapes reveal:

Expanding (pyramid-shaped). Many young people, few elderly. Typical of high-birth-rate developing countries.
Stationary (column-shaped). Even distribution. Typical of stable industrialized countries.
Contracting (inverted-pyramid). Few young people, many elderly. Typical of low-birth-rate, aging societies like Japan, Italy.

Population pyramid for Niger in 2020 with a steep triangular shape: each successive age band from infancy upward is dramatically narrower, with hundreds of thousands of children at age 0–2 tapering to almost no one above age 70 — Niger 2020: a textbook expanding (pyramid-shaped) age structure. High fertility produces a huge base of children; few people survive into old age. Typical of high-birth-rate, low-life-expectancy countries early in the demographic transition. *Credit: Sdgedfegw via Wikimedia Commons (CC BY-SA 4.0).*

Population pyramid for the United States in 2020 with a relatively rectangular shape: bands from age 0 to age 60 are roughly similar in width, with a slight bulge in the 25–34 and 55–64 cohorts and a clear taper above age 70 — United States 2020: a near-stationary (column-shaped) structure. Bands are roughly similar from infancy to early sixties, with a Boomer bulge near the top and a Millennial bulge in the middle. Characteristic of stable industrialized countries. *Credit: CIA World Factbook 2020 (Public Domain) via Wikimedia Commons.*

Population pyramid for Japan in 2018 with an inverted pyramid shape: the widest bands are in middle age (40s, 60s) and the youngest age bands at the bottom are noticeably narrower than the bands above them — Japan 2018: a contracting (inverted-pyramid) structure. The youngest cohorts are smaller than the working-age and older cohorts above them. This shape signals an aging population and an above-1 dependency ratio in the future. *Credit: CIA World Factbook 2019 (Public Domain) via Wikimedia Commons.*

Dependency Ratio

Dependency ratio = (population < 15 + population > 64) / (population 15-64). The ratio of dependents to working-age people. High dependency ratios strain social services; aging populations are a growing concern in most developed countries.

Aging Populations

Most wealthy countries are aging. Implications:

Strained pension systems.
Rising healthcare costs.
Labor shortages in care industries.
Political conflict between generations over entitlements.

Race and Ethnicity

Race. A socially constructed category, typically based on perceived physical differences. Biologically weak concept; socially powerful reality.
Ethnicity. Shared cultural heritage, including language, religion, ancestry, traditions. More overtly cultural than racial.

Race and ethnicity often overlap but aren’t identical. A person can share ethnic heritage (Italian-American culture) without being assigned the same “race” by societal categorization.

Racial categories change over time. Italian, Irish, and Jewish Americans were not always considered “white” in the United States. Latino/Hispanic categories were formalized in U.S. census only in 1980. These categorical shifts illustrate the social construction of race.

Immigration and Emigration

Immigration. Moving into a country.
Emigration. Moving out of a country.
Migration. The broader term for either.

Push and Pull Factors

Push factors - reasons to leave the home country: war, persecution, economic collapse, climate change, political instability.
Pull factors - reasons to move to a destination: economic opportunity, family reunification, political freedom, educational or cultural attractions.

Most real migration decisions involve both push and pull.

Black-and-white historical photograph of about a dozen Italian immigrant men in flat caps and three-piece suits standing with battered suitcases and bundled belongings against a stone wall at Ellis Island in the early 20th century — Italian immigrants at Ellis Island, early 20th century. Push factors (rural poverty, political instability) and pull factors (jobs, family already in America) drove one of the largest migration waves in modern history. Notably, Italians were not initially considered "white" in U.S. racial classification - a textbook example of race as a socially constructed, shifting category. *Credit: mákvirágok via Wikimedia Commons (Public Domain).*

Assimilation vs. Multiculturalism

Two models for how societies absorb newcomers:

Assimilation. Immigrants adopt the dominant culture, shed distinct identity. The classic “melting pot” model in the U.S.
Multiculturalism. Immigrants retain their cultural heritage while participating in broader society. “Salad bowl” metaphor - distinct flavors coexisting.

Most actual societies mix elements of both. Second- and third-generation immigrants often navigate layered identities.

Sex, Gender, and Sexual Orientation

The MCAT distinguishes these carefully:

Sex - typically biological categorization based on chromosomes, anatomy, and hormones (male, female, intersex).
Gender - a social construction relating to behaviors, roles, and identities associated with being a man, woman, or other gender.
Gender identity - a person’s internal sense of their gender.
Gender expression - how a person outwardly expresses their gender.
Sexual orientation - the pattern of attraction to others (heterosexual, homosexual, bisexual, asexual, etc.).

These dimensions are independent. A person’s sex, gender identity, and sexual orientation can align in any combination.

Transgender and Cisgender

Cisgender. Gender identity matches sex assigned at birth.
Transgender. Gender identity differs from sex assigned at birth.

Transgender individuals face significant health disparities related to discrimination, access to gender-affirming care, and mental health outcomes.

Horizontal flag with five stripes from top to bottom: light blue, pink, white, pink, light blue, in equal proportion — The transgender pride flag (Monica Helms, 1999). The blue and pink stripes flank a central white stripe representing nonbinary and intersex identity, signaling that gender identity exists independently of sex assigned at birth. The MCAT consistently distinguishes biological sex, gender identity, gender expression, and sexual orientation as separate axes. *Credit: SVG by Dlloyd, design by Monica Helms, via Wikimedia Commons (Public Domain).*

What is the dependency ratio?

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(Population < 15 + Population > 64) / (Population 15-64). The ratio of dependents to working-age people. High dependency ratios strain pensions, healthcare, and labor markets.

Distinguish push from pull factors in migration.

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Push factors: reasons to LEAVE the home country (war, economic collapse, persecution). Pull factors: reasons to MOVE TO a destination (jobs, freedom, family reunification, education). Most migration decisions involve both.

What is the difference between sex and gender?

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Sex is typically the biological categorization (chromosomes, anatomy). Gender is the social construction - roles, behaviors, and identities associated with being a man, woman, or other gender. Gender identity is a person's internal sense; sexual orientation is the pattern of attraction. These dimensions are independent.