AP® Environmental Science · Unit 3 · Topic 3.9
Demographic Transition
Demographic transition is the pattern of birth and death rates changing as a country develops, moving through four stages from high-high to low-low. The lag between falling death rates and falling birth rates drives huge population growth in the middle stages.
Picture two curves on a graph: birth rate and death rate, both plotted over time as a country develops. In Stage 1, both are high, so population barely grows; this described almost all of human history. Stage 2 starts when sanitation, medicine, and food security improve, so death rate drops fast while birth rate stays high out of habit, culture, or lack of access to contraception. That gap between the curves is where population explodes. In Stage 3, birth rate finally starts falling too, as education, urbanization, and family planning spread, so the gap narrows. By Stage 4, both rates are low and the curves sit close together again, giving slow or stable growth.
Age structure tracks this story. A Stage 2 country has a wide-based population pyramid, loaded with children, because lots of babies are born but few people die young. By Stage 4, the pyramid turns into more of a column or even an inverted shape, with fewer children and a larger share of elderly people, since birth rates dropped and life expectancy stayed high. This connects straight into the age-structure and carrying-capacity ideas from earlier in the unit: a bulging youth population in Stage 2 is tomorrow's momentum for continued growth, even after birth rates start to fall, because those children haven't reached reproductive age yet.
No country is guaranteed to finish the march to Stage 4. The transition only happens when the real conditions behind it show up: girls staying in school, healthcare reaching rural areas, contraception being available and accepted, and economies shifting away from needing child labor. Poverty, conflict, or weak institutions can freeze a country in Stage 2 for generations, which is exactly why the gap between birth and death rates, and the population spike it causes, can last far longer in some places than others.
Key terms
- Demographic transition
- The shift in a country's birth and death rates as it develops, typically described in four stages.
- Stage 2
- The phase where death rates fall quickly but birth rates stay high, causing rapid population growth.
- Age structure
- The distribution of a population across different age groups, often shown as a population pyramid.
- Population momentum
- Continued population growth caused by a large proportion of young people who haven't yet reached reproductive age.
Watch it explained
- How it worksWalk me through the four stages of the demographic transition as two crossing curves.
- Going furtherWhat happens to a country's age structure as it moves from stage 2 to stage 4?
- Common mistakeWhy isn't every country guaranteed to march through all four stages?
- Ties togetherHow does the gap between the two curves in stage 2 explain the global population spike?
Check yourself
End of topic.