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.