Key Components of Global Population Growth: Birth, Death, and Migration

Every second, roughly four babies are born somewhere in the world and two people die. Add the millions who pack up and move across borders each year, and you have the three forces that decide whether a country grows, shrinks, or stays the same. Demographers call them the components of population growth, and understanding how they interact is the first step to making sense of headlines about ageing nations, shrinking villages, and crowded megacities.

The basic equation behind population change 🔗

At its core, population growth follows a simple accounting rule. The size of any population changes because of three things: births add people, deaths remove them, and migration moves people between places. Put together, demographers express it as:

Population change = (Births − Deaths) + (Immigration − Emigration)

The first part, births minus deaths, is called natural increase. The second part, immigrants minus emigrants, is called net migration. A country can have a positive natural increase but still lose people if too many emigrate, and a country with more deaths than births can still grow if it attracts enough migrants. The United Nations World Population Prospects 2024 uses exactly this framework, called the cohort-component method, to project the future of every country on Earth.

Birth rates: the engine of growth 🔗

Births are usually the strongest driver of population growth, especially in younger societies. Demographers measure births in two main ways. The Crude Birth Rate (CBR) counts live births per 1,000 people per year. The Total Fertility Rate (TFR) estimates the average number of children a woman would have over her lifetime if current age-specific rates stayed the same.

The replacement-level fertility is about 2.1 children per woman. Below this, a population eventually starts shrinking without migration. Globally, fertility has fallen sharply, from roughly 5 children per woman in the 1950s to around 2.3 today. According to analysis of the latest UN data, more than half of all countries are already below replacement, and nearly one in five, including China, South Korea, Italy, and Spain, are in “ultra-low” fertility territory with fewer than 1.4 births per woman.

The Indian story 🔗

India crossed a major threshold recently. The Sample Registration System (SRS) Statistical Report 2023, released by the Office of the Registrar General of India, recorded a TFR of 1.9, below the replacement level for the first time. The Crude Birth Rate fell to 18.4 in 2023, almost half of the 36.9 recorded in 1971.

But the national figure hides huge regional contrasts. Bihar still records a TFR of 2.8 and a CBR of 25.8, the highest in the country. At the other end, Delhi has a TFR of just 1.2, comparable to Japan or South Korea. Rural India had a birth rate of 20.3, while urban areas were down to 14.9. These differences reflect uneven progress in female education, age at marriage, contraceptive use, and economic opportunity.

Why fertility falls 🔗

Fertility decline is rarely an accident. It follows a familiar set of social shifts: girls staying longer in school, women joining the workforce, later marriages, urbanization, better access to contraception, and lower infant mortality (which reduces the need to have “spare” children). The demographic transition theory captures this idea well: as societies modernize, birth rates eventually catch up with falling death rates, and rapid growth slows down.

Death rates: the quiet revolution 🔗

Deaths are the second component, and the story of the last century is largely a story of fewer deaths. The Crude Death Rate (CDR) counts deaths per 1,000 people per year, and life expectancy at birth is the average number of years a newborn is expected to live.

Globally, life expectancy has risen from about 30 years in 1900 to over 72 years today, one of humanity’s most remarkable achievements. India’s own death rate dropped from 14.9 in 1971 to just 6.4 in 2023. The Infant Mortality Rate, a particularly sensitive indicator of public health, fell from 129 deaths per 1,000 live births in 1971 to 25 in 2023.

What drives mortality down 🔗

Several forces work together to push death rates down. Vaccination campaigns and antibiotics control infectious diseases. Clean water and sanitation prevent outbreaks of cholera and diarrhoea. Better nutrition strengthens immunity. Maternal healthcare reduces deaths in childbirth. Rising incomes mean people can afford treatment and live in safer housing. The Population Reference Bureau notes that since 1970, death rates in less developed countries have actually fallen faster than birth rates, which is precisely what keeps growth high in many regions.

The demographic transition gap 🔗

When death rates fall faster than birth rates, a country experiences explosive growth. This gap is the central engine of the population boom seen across Asia and Africa in the second half of the 20th century. India’s population multiplied roughly four times between 1951 and 2024 largely because modern medicine cut mortality decades before fertility began its serious decline. Today, with both rates falling together, growth is slowing but not yet stopping.

Migration: the third and trickiest component 🔗

Migration is the most volatile of the three components because it can change almost overnight in response to wars, economic shocks, or policy decisions. Immigration means people moving in; emigration means people moving out; net migration is the difference between the two.

At the world level, migration does not change the total population, since one country’s gain is another’s loss. But at the national and regional levels, it can be decisive. The UN’s 2024 projections estimate that in 50 countries, immigration is now expected to partly or fully offset population decline caused by very low fertility. Germany, Italy, and Japan are all examples where migrants are slowing what would otherwise be a sharp drop in numbers.

Internal and international migration in India 🔗

India is a country shaped by movement, but most of it happens within its borders. Internal migration from villages to cities, and from poorer states like Bihar and Uttar Pradesh to richer ones like Maharashtra, Gujarat, and Karnataka, is reshaping local population pyramids. Cities like Delhi, Mumbai, and Bengaluru grow far faster than their natural increase alone would allow.

Internationally, India has a small negative net migration rate, with more people leaving than arriving. According to demographic estimates, the net migration rate is roughly −0.68 per 1,000 people. The Indian diaspora, spread across the Gulf, North America, the UK, and Australia, is among the largest in the world, and remittances from these workers form a major source of foreign income for the country.

Why migration matters more than the numbers suggest 🔗

Migration is selective. The people who move are typically young, working-age, and often more educated than average. So when a region loses migrants, it loses not just bodies but productive years, tax revenue, and future parents. Conversely, regions that receive migrants gain a workforce and often see a temporary boost in birth rates. This is why aging European nations are increasingly debating immigration not just as a humanitarian issue but as a demographic necessity.

Population momentum: the hidden fourth force 🔗

There is a fourth ingredient that often gets overlooked: population momentum. Even when fertility falls to replacement level, a country can keep growing for decades simply because it has a large cohort of young people entering reproductive age. The UN estimates that this momentum, built into today’s youthful age structure, will account for around 79 per cent of global population increase through 2054. In other words, most of the growth still ahead is already “baked in” by past births, not by future fertility decisions.

How we know all this: data sources 🔗

None of this analysis would be possible without reliable data. Population statistics come from three main pillars.

The census 🔗

A census is a complete count of every person living in a country at a specific moment, conducted by the government, usually once every ten years. India’s last completed census was in 2011, and the next one has been rescheduled to 2027. The census is the gold standard for demographic data because it captures age, sex, education, occupation, religion, and migration history for the entire population.

Civil Registration and the Sample Registration System 🔗

Between censuses, countries rely on continuous registration. India’s Civil Registration System (CRS) records every birth and death under the Registration of Births and Deaths Act, with informants required to report events within 21 days. Because the CRS still has coverage gaps, India also runs the Sample Registration System (SRS), a large dual-record survey covering about 1.5 million households. The SRS produces the annual estimates of CBR, CDR, IMR, and TFR that researchers and policymakers use every year.

The UN World Population Prospects 🔗

For global trends, the gold standard is the UN World Population Prospects, updated every two years by the Population Division of the UN Department of Economic and Social Affairs. The 2024 revision draws on 1,910 censuses, 3,189 surveys, and civil registration data from 169 countries to produce estimates and projections for 237 countries from 1950 to 2100. It uses the cohort-component method, which means it ages each group of people forward year by year while applying assumptions about fertility, mortality, and migration.

Why projections are not predictions 🔗

It is worth remembering that projections are conditional, not fixed. The UN now expects the global population to peak at about 10.3 billion in the mid-2080s before slowly declining. Compare this to the 2017 projection, which expected continuous growth past 2100. The change happened because fertility in Africa and South Asia fell faster than anyone had predicted. Projections shift as data improves, which is exactly why high-quality censuses and registration systems matter so much.

Putting it all together 🔗

Births, deaths, and migration are not isolated levers; they interact. A region with high fertility and falling mortality grows explosively. A region with low fertility and stable mortality stagnates and ages. A region with low fertility but high immigration can keep growing despite a negative natural increase. Looking at any one component alone gives a misleading picture; population studies is really the art of reading the three together against the backdrop of age structure, education, and policy.

For India, the next two decades will be a fascinating test case. Fertility has dipped below replacement, mortality continues to fall, and internal migration is rewriting the geography of the country. The total population is still rising thanks to momentum, but the growth rate is slowing, and ageing is already on the horizon. Whether this transition leads to a “demographic dividend” or a “demographic drag” depends on how well the country invests in jobs, health, and education for its still-young workforce.

What do you think? If your home state were a separate country, how would its birth, death, and migration profile compare to India as a whole? And do you believe immigration should play a bigger role in solving the population challenges of ageing economies, or are there better alternatives?

References

1. https://population.un.org/wpp/assets/Files/WPP2024_Summary-of-Results.pdf
2. https://ourworldindata.org/un-population-2024-revision
3. https://www.drishtiias.com/pdf/1757151696.pdf
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4436256/
5. https://www.prb.org/resource/human-population/
6. https://www.ined.fr/en/everything_about_population/demographic-facts-sheets/focus-on/2024-les-nations-unies-publient-de-nouvelles-projections-de-population-mondiale
7. https://en.wikipedia.org/wiki/Demographics_of_India
8. https://www.dataforindia.com/crs-srs-explainer/
9. https://www.un.org/development/desa/pd/world-population-prospects-2024