Demographic Processes Explained: Fertility, Mortality and Migration

Populations are not static numbers. They grow, shrink, age, and shift shape because of three demographic engines: fertility, mortality, and migration. Together, these processes explain why India’s population looks the way it does today, why some states are ageing faster than others, and why cities keep pulling in more people every year. Understanding how each engine works, and how they interact, is the starting point for demographic anthropology.

Fertility: more than just having babies 🔗

In everyday language, fertility and the ability to have children sound like the same thing. In demography, they are not. Fertility is the actual production of children, a real, countable event. Fecundity is the physiological capacity to reproduce, the biological ceiling that exists whether or not it is ever used. A woman can be fecund for decades without ever becoming a mother, and fertility research uses this distinction to separate what is biologically possible from what actually happens. Studies comparing the two concepts show that demography has traditionally focused on fertility as the outcome, while fecundity is treated as the underlying biological constraint that shapes it.

This gap between capacity and outcome exists because childbearing is never purely biological. Decisions about when to marry, whether to use contraception, how many children a couple wants, and how long they space births are shaped by economic pressure, education, religion, and family expectations. A demographer studying fertility has to hold both realities in view at once: the biological limits of reproduction and the social forces that decide how close a population comes to those limits.

Two ways to study the same number 🔗

Micro fertility analysis looks at individuals. It tracks how many children a specific woman or couple has had by a certain age, how many they eventually have by the end of their reproductive years, and how births are spaced across the life cycle. This approach is useful for understanding personal reproductive histories and family-building patterns.

Macro fertility analysis zooms out. It measures the rate at which births occur across an entire population or a defined sub-population over a period of time. This is the level at which demographers calculate the Total Fertility Rate, compare fertility against mortality, and track how reproductive patterns change across regions, communities, or income groups. Both approaches answer different questions, and both are needed to build a full picture of how a population reproduces itself.

What actually moves fertility up or down 🔗

Fertility responds to biological, social, and environmental pressure at the same time. Education is one of the strongest predictors: women with more years of schooling tend to marry later, delay childbearing, and end up with fewer children overall. Socio-economic status works the same way, higher income households generally have fewer children than lower income ones. Place matters too. Urban fertility is consistently lower than rural fertility, largely because cities bring later marriage, better access to contraception, and higher costs of raising children.

India’s own numbers reflect this pattern. The Total Fertility Rate fell to 2.0 children per woman according to the fifth National Family Health Survey, down from 2.2 in the previous round, meaning the country has reached what demographers call replacement-level fertility, the point at which each generation is just large enough to replace itself. National averages, however, hide sharp state-level differences, with some states still well above replacement and others well below it.

To explain how these social forces actually reach the human body, sociologists Kingsley Davis and Judith Blake proposed in their influential 1956 framework that every social or economic factor affecting fertility must pass through a small set of biological intermediate variables. They grouped these into three stages: exposure to intercourse, exposure to conception, and gestation through to a successful birth. In simple terms, no matter how strong the cultural or economic pressure to have more or fewer children, that pressure can only act on fertility by changing behaviour at one of these three biological stages. This model remains one of the most widely taught frameworks in population studies because it links abstract social causes to concrete reproductive outcomes.

Mortality: how long we live, and why 🔗

Mortality refers to the frequency of deaths in a population. Demographers separate two ideas that often get confused. Life span is the maximum number of years a human being can theoretically live, a fixed biological ceiling. Life expectancy is the average number of years a person born into a specific population is expected to live, calculated from current death rates. Life span barely changes across generations. Life expectancy changes constantly, rising with better nutrition, sanitation, and healthcare, and falling during famines, epidemics, or conflict.

India tracks these patterns through its Sample Registration System, which produces abridged life tables used to estimate life expectancy at the national and state level. This data consistently shows that death is certain for everyone, but how long life lasts, and how it ends, is not evenly distributed. Higher social classes tend to live longer than lower ones, and married individuals tend to outlive unmarried individuals of the same age, patterns that point to the role of social support, healthcare access, and lifestyle in shaping survival.

Why people die: it’s rarely just biology 🔗

The causes of death are enormously varied, ranging from infectious diseases like tuberculosis, pneumonia, and HIV/AIDS to non-communicable conditions like cancer and diabetes, alongside famine and conflict in some regions. But mortality is not purely a medical story. Personal behaviour, including smoking, diet, and engagement in hazardous work, plays a direct role in how long a person lives. Environmental and social conditions such as climate, housing quality, population density, sanitation, and access to medical facilities push mortality rates up or down independent of individual choices.

As societies industrialise, the causes of death also shift. Later stages of development see a rise in deaths linked to social pathologies, alcoholism, road accidents, suicide, and homicide, gradually replacing the infectious diseases that once dominated mortality. This shift is itself a demographic signal, showing how a population’s health profile changes as its economy and society change.

Migration: population change without a birth or a death 🔗

Fertility and mortality change a population through biological events. Migration changes it through movement. It is the transfer of people from one place to another, either within a country or across its borders, adding to the population of one area while subtracting from another.

Movers and migrants are not the same thing 🔗

Demographers make a careful distinction here. A mover is anyone who changes residence, even within the same city. A migrant is someone who crosses a political or administrative boundary in the process, a district line, a state border, or a national frontier. Census data in India identifies a person as a migrant if their place of enumeration differs from their place of birth, or from their place of last residence, giving demographers two separate ways to measure the same movement. Migration also brings changes far beyond an address: a new school, a new job, a new place of worship, and a new set of social ties. Unlike birth and death, which happen exactly once in a lifetime, migration can happen many times, or not at all, which makes it the most unpredictable of the three demographic processes.

Internal migration: moving within the country 🔗

Internal migration involves a permanent change of residence within national borders, but still across a political boundary such as a district or state. Demographers describe this from two directions at once: in-migration, the movement of people into a destination area, and out-migration, the movement of people away from an origin area. In the Indian context, this pattern shows up in the steady flow of workers from states with limited local employment toward industrial and urban hubs, gradually reshaping the population balance between sending and receiving regions.

International migration: crossing national borders 🔗

International migration occurs when people move between countries. This includes immigration, the movement of people into a country for permanent residence, and emigration, their permanent departure from a country. Statistical agencies typically treat immigration and emigration as two sides of the same process, combining them to calculate net international migration for a country or region. For a country like India, both directions matter, workers, students, and professionals moving abroad, and people of Indian origin or foreign nationals moving in, each leaves a measurable mark on population size and composition.

Of the three demographic processes, migration is often described as the single most significant one for shaping the population of a specific community, precisely because it decreases numbers in areas people leave and increases them in areas people move to, at a pace that can outstrip anything fertility or mortality can achieve on their own.

Reading the three processes together 🔗

No single demographic process tells the full story on its own. A falling fertility rate combined with rising life expectancy produces an ageing population, a shift already visible in India as the country approaches, and in some states has already crossed, replacement-level fertility. Recent estimates put India’s population at around 1.46 billion, even as fertility has fallen below replacement level, a combination that will eventually stabilise and then shrink the population over coming decades unless migration patterns offset it. Reading fertility, mortality, and migration together, rather than in isolation, is what allows demographers to explain not just how many people live somewhere, but who they are, how old they are, and where they are likely to be a generation from now.

What do you think? As India’s fertility rate keeps falling, do you think migration, both within the country and abroad, will play a bigger role in shaping where India’s population grows in the coming decades? And do you notice any of these three processes, fertility, mortality, or migration, shifting visibly in your own city or state?

References

1. https://pubmed.ncbi.nlm.nih.gov/17502290/
2. https://www.pib.gov.in/PressReleasePage.aspx?PRID=1847431&reg=48&lang=2
3. https://www.journals.uchicago.edu/doi/abs/10.1086/449714
4. https://censusindia.gov.in/nada/index.php/catalog/45558
5. https://iasp.ac.in/uploads/journal/10.%20Migration%20in%20India%20trends%20and%20characteristics-1669206793.pdf
6. https://www.census.gov/topics/population/migration/about.html
7. https://www.deccanherald.com/india/indias-population-touches-146-billion-fertility-drops-below-replacement-rate-un-report-3578930