# Population: Structure and Dynamics

Sociology I · Foundations and the Great Themes · https://tryals.app/learn/sociology-i/population-structure-and-dynamics

## Counting a Population, and What the Counts Hide

A population changes through four events: births, deaths, immigration, and emigration.

Demographers measure these events using distinct metrics:
- A **crude birth rate** counts births per 1,000 people. It is sensitive to age structure.
- The **total fertility rate** (TFR) sums age-specific rates for a single year. It estimates children per woman.
- **Replacement level** in low-mortality countries is about **2.1** children per woman.

Recent figures provide context:
- Spain's TFR runs between **1.1 and 1.3** (INE), among the lowest in the European Union.
- The EU average is roughly **1.4 to 1.5** (Eurostat).
- Spanish life expectancy is around **84 years**, among the highest in the Union.
- Roughly **one person in five** in Spain is aged 65 or over.
- World population passed **8 billion** in late 2022.

## Structure and Transition

The **demographic transition** explains long-term population changes across four stages.

| Stage | Birth rate | Death rate | Growth | Age structure |
|---|---|---|---|---|
| 1 | High | High | Near zero | Wide base, few old |
| 2 | High | Falling | Rapid | Very wide base |
| 3 | Falling | Low | Slowing | Base narrows |
| 4 | Low | Low | Near zero | Rectangular |

Mortality falls before fertility drops. This lag drives rapid growth. **Malthus** famously predicted widespread famine, but technology and falling fertility proved him wrong.

Age distribution dictates dynamics:
- A **population pyramid** plots population by age and sex.
- The **dependency ratio** compares those under 15 and 65 or over to working-age adults.
- **Population momentum** keeps births high when past cohorts of mothers are large.

> **Common pitfall:** What looks like decline may not be one. A sub-replacement fertility rate does not mean an immediate drop in Population. Momentum and immigration have grown Spain's population since 2000 despite low fertility.

## Practice questions

10 of this lesson's 15 practice questions, with answers. The full set is in the app.

### 1. Complete the account of the basic demographic measures.

**Answer:** A crude birth rate is expressed per 1,000 of the **whole** population, which makes it sensitive to age structure. The total fertility rate avoids that by summing **age-specific** rates for a single year. In a low-mortality country the **replacement** level is about 2.1 children per woman, and the ratio of the young and the old to those of working age is called the **dependency** ratio.

**Why:** The word "crude" is doing real work: it is a warning label. Two countries with identical childbearing behaviour can show crude birth rates that differ by half simply because one has more women of childbearing age, which is why any comparison across countries or decades should reach for the age-specific measure.

Page: https://tryals.app/practice/sociology-i/population-structure-and-dynamics/complete-the-account-of-the-basic-demographic-measures

### 2. Crude birth rates are sensitive to age structure, whereas the total fertility rate isolates cohort childbearing. Why does this distinction matter when comparing an ageing nation with a younger one?

A. Younger populations automatically register lower crude birth rates in all cases
B. Crude birth rates overstate fertility in societies that have many older citizens
C. The total fertility rate varies according to the overall size of the population
D. Ageing populations may record fewer births despite higher individual fertility

**Answer:** D. Ageing populations may record fewer births despite higher individual fertility

**Why:** Crude rates reflect the whole denominator, so a country with a high proportion of elderly people can show a low crude birth rate even if women of childbearing age have relatively large families. TFR removes this age-composition distortion.

Page: https://tryals.app/practice/sociology-i/population-structure-and-dynamics/crude-birth-rates-are-sensitive-to-age-structure-whereas-the-total

### 3. Spain’s fertility has been below replacement since the early 1980s, and its population is nonetheless larger now than it was then. Which mechanisms explain why that is possible?

A. A total fertility rate below 2.1 that nevertheless implies a growing number of births
B. Population momentum, since many women were already of childbearing age when fertility fell
C. Net immigration, which has exceeded natural decrease in most years since 2000
D. Rising life expectancy, which reduces deaths at every age

**Answer:** B. Population momentum, since many women were already of childbearing age when fertility fell; C. Net immigration, which has exceeded natural decrease in most years since 2000; D. Rising life expectancy, which reduces deaths at every age

**Why:** Momentum, migration and falling mortality all add to the total while fertility subtracts from it. The first option is the error the lesson is built around: a TFR below replacement means a cohort does not replace itself, and it says nothing on its own about whether the total population rises or falls this year.

Page: https://tryals.app/practice/sociology-i/population-structure-and-dynamics/spains-fertility-has-been-below-replacement-since-the-early-1980s

### 4. A country whose fertility has been below replacement for four decades must by now have a shrinking population.

**Answer:** False

**Why:** **False.** Spain is the counter-example in this lesson: below replacement since the early 1980s, and substantially larger today. Fertility describes the reproduction of a cohort; total change is the sum of four separate terms, and any of the other three can dominate for decades.

Page: https://tryals.app/practice/sociology-i/population-structure-and-dynamics/a-country-whose-fertility-has-been-below-replacement-for-four-decades

### 5. Arrange the demographic transition in the order the changes actually occur.

**Answer:**

1. Births and deaths are both high, and population barely grows
2. Mortality falls while fertility stays high, and growth accelerates
3. Fertility begins to fall and growth slows
4. Births and deaths are both low, and growth returns to near zero

**Why:** The population explosion is not caused by people suddenly having more children. It is the gap between two declines: deaths fall first, because clean water, food supply and disease control act quickly, while the fertility decline waits on changes in schooling, women’s employment and the cost of raising a child.

Page: https://tryals.app/practice/sociology-i/population-structure-and-dynamics/arrange-the-demographic-transition-in-the-order-the-changes-actually

### 6. In a low-mortality country, what total fertility rate is taken as replacement level? Give the figure to one decimal place.

**Answer:** 2.1 (within ±0.01)

**Why:** **2.1.** Two children replace two parents; the extra tenth covers girls who die before reaching childbearing age and the fact that around 105 boys are born for every 100 girls. In a high-mortality country the replacement figure is higher, sometimes well above 2.5.

Page: https://tryals.app/practice/sociology-i/population-structure-and-dynamics/in-a-low-mortality-country-what-total-fertility-rate-is-taken-as

### 7. Sort each feature by whether it characterises stage 2 of the demographic transition or stage 4.

**Answer:**

- Stage 2: Death rates fall sharply while birth rates remain high, The age pyramid has a very wide base and a narrow top, The fastest population growth of the whole sequence
- Stage 4: Both rates are low and growth is close to zero, Public debate concentrates on ageing and pension funding

**Why:** The last pair is the political consequence of the demographic one. A stage 2 society argues about feeding and schooling a young population; a stage 4 society argues about funding an old one. Both arguments are usually conducted as though the age structure were a policy choice rather than the arithmetic of decisions made forty years earlier.

Page: https://tryals.app/practice/sociology-i/population-structure-and-dynamics/sort-each-feature-by-whether-it-characterises-stage-2-of-the

### 8. Estimate the percentage of Spain’s population aged 65 and over in recent years.

**Answer:** 20 (within ±3)

**Why:** **About 20 per cent**, on recent INE figures, and projected to rise steeply as the large cohorts born in the 1960s and early 1970s reach 65. The exact figure moves a few tenths a year, which is why it is worth remembering as a fraction rather than a decimal.

Page: https://tryals.app/practice/sociology-i/population-structure-and-dynamics/estimate-the-percentage-of-spains-population-aged-65-and-over-in

### 9. Arrange these moments in the study and history of population, earliest first.

**Answer:**

1. Malthus publishes the Essay on the Principle of Population
2. Thompson sets out the sequence later called the demographic transition
3. Notestein gives the demographic transition its name
4. Spain’s total fertility rate falls below replacement level
5. World population passes eight billion on the United Nations estimate

**Why:** The gap between Thompson and Notestein is worth noticing: the pattern was described from the data before anyone had a name or a causal theory for it, which is the usual order in demography. Naming it turned a regularity into a framework that was then applied, often too confidently, to countries whose transitions have not followed the European path.

Page: https://tryals.app/practice/sociology-i/population-structure-and-dynamics/arrange-these-moments-in-the-study-and-history-of-population

### 10. Malthus’s prediction failed chiefly because food production grew far faster than he expected while fertility also fell without the famines he anticipated.

**Answer:** True

**Why:** **True.** Agricultural output grew at a pace he had ruled out, through new crops, fertiliser and mechanisation, and fertility declined for reasons — schooling, urban living costs, contraception, women’s employment — that had no place in his model. Being wrong twice is why the theory survives as a name for a worry rather than as a tool.

Page: https://tryals.app/practice/sociology-i/population-structure-and-dynamics/malthuss-prediction-failed-chiefly-because-food-production-grew-far