Early Expanding Phase: Population Explosion in Demographic Transition

Population doesn’t grow steadily over time. It moves in waves, and the steepest wave of all sits right in the middle of the demographic transition: the second stage. This is where a society shakes off centuries of high death rates but holds on to its traditional preference for large families. The result is a striking imbalance between births and deaths, which demographers describe as a population explosion. Understanding this stage is key to making sense of why some countries doubled in size within a generation, and why others are still grappling with the social, economic and environmental aftershocks of rapid growth.

What the second stage actually means 🔗

The Demographic Transition Model, first proposed by Warren Thompson in 1929 and later expanded by Frank Notestein, describes how societies move from high birth and death rates to low ones as they modernise. The second stage, often called the early expanding phase, is the dramatic middle chapter of that story. Here, death rates fall rapidly while birth rates remain stubbornly high, and the widening gap between the two produces explosive population growth.

The trigger is not a baby boom. People are not suddenly having more children. What changes is that far fewer of them are dying, especially infants and young children. Better food supply, cleaner water, basic sanitation, vaccines, antibiotics and improved maternal care all chip away at mortality. According to Our World in Data, the population pyramid during this stage develops a very broad base because so many children are now surviving past their first five years, the age window that used to claim the most lives in pre-industrial societies.

Why births stay high while deaths fall 🔗

Mortality responds to technology and public health relatively quickly. Fertility, on the other hand, is tied to culture, religion, economic structure, gender norms and the perceived value of children. In rural and agrarian societies, children are often considered an economic asset who contribute to farm labour and provide old-age security. Add limited access to contraception, early age at marriage and low female education, and you have a system that produces large families even after death rates have collapsed. This lag between falling deaths and falling births is the engine of stage two.

Characteristics of the early expanding phase 🔗

A few features show up consistently in any society passing through this stage. Recognising them helps separate stage two from the stages before and after.

Sharply declining mortality 🔗

The most visible change is the steep drop in the crude death rate. Infant and child mortality fall first, followed by improvements in adult life expectancy. The drivers usually include improved medicine, better sanitation and a more secure food supply. In historical Europe, the Industrial Revolution and the agricultural improvements that accompanied it created the conditions for this transformation. In today’s developing world, public health interventions such as immunisation, oral rehydration therapy and antibiotic access have done much of the same work, often in a much shorter time.

Persistently high fertility 🔗

Even as deaths fall, families continue to be large. Total fertility rates of five, six or seven children per woman were common in nineteenth-century Europe and were still common across much of Asia and Africa in the mid-twentieth century. Cultural preference for sons, low contraceptive prevalence and low female autonomy all reinforce this pattern.

Rapid population growth 🔗

The arithmetic is simple. When births stay around 35 to 45 per 1,000 people while deaths fall from 30 to 10 per 1,000, the natural increase can climb to historically unprecedented levels. Annual growth rates of 2.5 to 3.5 percent are common during this phase. At 3 percent annual growth, a population doubles in roughly 23 years, which is why this stage is described as a population explosion.

A young age structure 🔗

Because the survival of children is what is driving the change, the population pyramid becomes increasingly youthful. The decline in death rates during stage two mainly reflects the increasing survival of children, which makes the age structure of the population progressively younger. Children under fifteen often make up 35 to 45 percent of the total population, and the median age can fall below twenty. This shapes everything from school enrolment pressure to the future labour market.

Early second stage versus late second stage 🔗

The second stage is not a single moment but a long passage that can last 40 to 100 years. Demographers often split it into two phases because the dynamics shift noticeably as a country progresses.

The early phase 🔗

In the early part of stage two, mortality is just beginning to decline while fertility remains essentially untouched. Birth rates can sit close to their pre-transition levels of 40 or more per 1,000. Public health gains drive most of the change, but the broader social transformations such as urbanisation, female education and modern contraception are still in their infancy. Population growth begins to accelerate, but the full force of it is yet to come.

The late phase 🔗

By the later half of stage two, mortality has dropped sharply and is approaching the levels found in industrialised countries. Fertility, meanwhile, has started to inch downward as urbanisation spreads, more girls attend school and family planning programmes take root. The growth rate is at its peak in this phase because the gap between births and deaths is at its widest. Many countries spend decades here, and some sub-Saharan African nations have been described as showing signs of being stuck in late stage two due to stagnant development and the lingering impact of HIV/AIDS.

Contemporary examples 🔗

The early expanding phase is not a historical curiosity. Several countries today are still working through it, each in its own way.

Kenya 🔗

Kenya is a textbook example of a country in the later portion of stage two. Fertility in Kenya was above seven children per woman in the 1950s, but it has been falling for decades and is now around 3.3. Life expectancy has improved from around 50 years in the 1970s to the late sixties today, despite a serious setback during the HIV/AIDS epidemic. The country still has a very young population, with more than 40 percent under the age of 15, which means strong demographic momentum will keep the population growing for decades even as fertility continues to decline.

Bangladesh 🔗

Bangladesh is one of the most remarkable demographic stories of the last fifty years. Between 1966 and 2010, total fertility fell from 6.7 to 2.6 children per woman, while life expectancy rose from about 53 years to over 69 years. What makes the case unusual is that this transition happened in a poor, predominantly agricultural country with relatively low urbanisation. Aggressive family planning outreach, expanded female education and the work of community-level female health workers drove much of the change. Bangladesh has now largely moved into stage three, but its transition through the early expanding phase is one of the most studied in modern demography.

Nepal 🔗

Nepal is moving through stage two more recently and at its own pace. Mortality has improved substantially, and fertility has dropped from over five children per woman in the early 1990s to close to replacement level today. However, mountainous terrain, regional inequality and significant out-migration of working-age men complicate the picture. Infant mortality in Nepal has fallen, though gains have been uneven, with rural and hilly regions still lagging behind urban centres.

Consequences of the population explosion 🔗

The second stage is not just a demographic fact. It reshapes economies, environments and politics for generations.

Pressure on services and infrastructure 🔗

A rapidly growing, youthful population demands schools, healthcare, housing and eventually jobs at a pace that even well-functioning governments find hard to match. When provision lags, the result can be overcrowded classrooms, strained clinics and rising youth unemployment. Failure to invest in education, healthcare and job creation during this phase can lead to social unrest and economic strain, which is why the second stage is sometimes described as a high-stakes window for development policy.

Demographic momentum 🔗

Even when fertility finally starts to fall, the population keeps growing for decades. This is because the very large cohorts of children born during stage two eventually become parents themselves. So a country can reach replacement-level fertility and still see its total population continue to expand for another 50 to 70 years. This momentum is one reason the global population is still growing despite falling fertility almost everywhere.

The demographic dividend, if used well 🔗

A large working-age population relative to dependents can be a tremendous economic opportunity. The reduction in fertility allows resources to be reallocated from the quantity of children toward their quality, enhancing human capital formation and labour productivity. Countries that invest in education, health and employment during this window can capture what economists call the demographic dividend. Those that do not risk turning a youth bulge into a source of instability.

Why the second stage matters today 🔗

Most of the world has already passed through the early expanding phase, but the countries still in it account for a large share of future global population growth. The decisions they make about family planning, female education, healthcare and economic policy will determine whether the population explosion of the second stage becomes a foundation for prosperity or a burden their institutions cannot bear. The stage is short on the timeline of human history, but its effects ripple forward for a very long time.

What do you think? Looking at countries currently in the early expanding phase, what single intervention – improved female education, wider contraceptive access, better child healthcare or job creation – do you think would do the most to ease the pressures of rapid population growth? And how should a country prepare its economy to absorb a youth bulge before it turns into a demographic challenge?

References

1. https://en.wikipedia.org/wiki/Demographic_transition
2. https://ourworldindata.org/demographic-transition
3. https://transportgeography.org/contents/chapter8/transportation-urban-form/demographic-transition/
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2781829/
5. https://pages.uwc.edu/keith.montgomery/Demotrans/demtran.htm
6. https://www.buddinggeographers.com/demographic-transition-model-dtm/
7. https://ourworldindata.org/global-decline-fertility-rate
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC3657819/
9. https://www.tandfonline.com/doi/full/10.1080/17441730.2022.2028253
10. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5223552/
11. https://fiveable.me/ap-hug/key-terms/stage-two-early-expanding
12. https://pmc.ncbi.nlm.nih.gov/articles/PMC4116081/