The Three Main Components of Population Dynamics: Fertility, Mortality, and Migration

Every second, somewhere in the world, a baby is born and a person dies. Somewhere else, a family packs up and moves to a new city in search of work. These three events – births, deaths, and movement – are the foundation of how populations change. In demography, we call them fertility, mortality, and migration, and together they explain why some regions are bustling and overcrowded while others are quietly emptying out. Understanding these three forces is the first step to making sense of everything from housing prices to pension policies to school enrolment trends.

The basic equation of population change 🔗

Population change in any region follows a surprisingly simple equation: the population grows when births and in-migration exceed deaths and out-migration. Demographers call this the balancing equation of population. Births and deaths together produce what is known as the natural increase, while migration adds the spatial dimension of people physically moving between places. Even though the formula looks straightforward, the social, economic, and cultural forces behind each component are anything but simple.

Fertility: The engine of population growth 🔗

Fertility refers to the actual number of children born to women in a population. It is typically measured using the Total Fertility Rate (TFR), which represents the average number of children a woman is expected to have over her reproductive years (15-49 years). When TFR is exactly at the replacement level of 2.1, each generation just replaces itself. Anything above leads to growth; anything below eventually leads to decline.

According to the Sample Registration System (SRS) Statistical Report 2023, India’s TFR has fallen to 1.9, dropping below the replacement level for the first time at the national scale. This is a dramatic shift from 5.2 children per woman recorded in 1971. The decline reflects deep changes in society – but the pace of change is uneven. Bihar still records a TFR of 2.8, while Delhi sits at just 1.2.

Socio-economic factors shaping fertility 🔗

Income, urbanisation, and women’s workforce participation are the biggest economic levers of fertility. As families grow wealthier and women enter paid work, the cost of raising children – both financial and in terms of foregone earnings – rises sharply. Couples respond by choosing smaller families. Urban TFR in India stood at just 1.5 in 2023, compared to 2.1 in rural areas, as SRS data shows.

Education, particularly of women, is perhaps the single strongest predictor of lower fertility. Educated women marry later, have fewer children, and are more likely to use contraception. Kerala, with its high female literacy and education levels, has consistently maintained low fertility for decades, while states with lower female education tend to have higher TFRs.

Cultural norms and the age at marriage 🔗

Cultural expectations around marriage, family size, and gender preferences strongly influence reproductive behaviour. In many parts of South Asia, early marriage and the social pressure to have a son still push fertility upward. The duration of effective married life matters here – the earlier a woman marries, the longer her reproductive window, and the more children she is likely to have. Religious teachings, joint-family structures, and the cultural value placed on large families all play their part too.

Access to contraception and family planning 🔗

Access to modern contraception is a game-changer. The National Family Health Survey-5 documented a sharp rise in family planning use across nearly all Indian states, with Goa and Bihar showing some of the steepest increases. The expansion of public family-planning programmes, free contraceptives at primary health centres, and the involvement of ASHA workers at the community level have all contributed to bringing fertility down. Yet unmet need for contraception still exists, especially among younger women and in less developed regions, showing that supply alone is not enough – awareness and autonomy matter equally.

Mortality: The other side of natural change 🔗

Mortality refers to deaths in a population, and it is just as important as fertility in shaping demographic outcomes. The most commonly used measures are the Crude Death Rate (CDR) – deaths per 1,000 people in a year – and the Infant Mortality Rate (IMR), which captures deaths of children under one year of age per 1,000 live births. IMR is considered one of the most sensitive indicators of a country’s overall health and development.

India’s CDR has fallen dramatically from 14.9 in 1971 to just 6.4 in 2023. Even more striking is the drop in IMR – from a heartbreaking 129 per 1,000 live births in 1971 to 25 in 2023, an 80% decline over five decades. Life expectancy at birth has climbed to around 68.5 years for males and 72.5 years for females.

Standard of living and nutrition 🔗

The single greatest reducer of mortality in modern history has been improvements in the standard of living. Access to clean drinking water, sanitation, nutritious food, and safe housing dramatically reduces the spread of infectious diseases – historically the biggest killers of children. The Swachh Bharat Mission’s push for toilets and improved sanitation, along with food-security schemes like the Public Distribution System, have all contributed to falling mortality.

Healthcare access and quality 🔗

The expansion of healthcare infrastructure has been central to mortality decline. Immunisation programmes have shielded children from measles, polio, and diphtheria. Antibiotics and improved obstetric care have made childbirth far safer. Schemes like Janani Suraksha Yojana have promoted institutional deliveries, while Ayushman Bharat has expanded access to secondary and tertiary care for lower-income households. Still, gaps remain – rural IMR (28) is significantly higher than urban IMR (18), and states like Madhya Pradesh and Uttar Pradesh continue to record IMR levels of around 37.

Education and awareness 🔗

Education affects mortality just as much as it affects fertility. Educated mothers are more likely to recognise warning signs in their children’s health, seek timely care, and follow vaccination schedules. They are also more likely to space pregnancies adequately – which directly lowers infant deaths. Education spreads health literacy through families and communities, multiplying its effect well beyond the individual.

It is also worth noting how the causes of death have shifted. India is undergoing what demographers call an epidemiological transition – moving from a burden dominated by infectious diseases to one increasingly driven by non-communicable diseases like heart disease, diabetes, and cancer. As people live longer, the shape of mortality changes too.

Migration: The spatial reshuffle 🔗

While fertility and mortality determine the natural change in a population, migration redistributes people across space. It can rapidly transform the size, age structure, and economic character of a region in ways that births and deaths alone never could. The 2011 Census recorded 45.6 crore migrants in India – nearly 38% of the population – a number that grew 45% between 2001 and 2011 even as overall population growth was only 18%.

Types of migration 🔗

Migration is usually classified by where people move from and to. Internal migration happens within a country’s borders, while international migration involves crossing national boundaries. In India, internal migration dominates overwhelmingly – about 99% of all migration is internal, with international migrants making up just 1%.

Internal migration is further classified into four streams: rural-to-rural, rural-to-urban, urban-to-urban, and urban-to-rural. As per the 2011 Census, rural-to-rural was the largest stream at 54% of classifiable internal migration, dominated by women moving for marriage. Migration is also categorised as intra-state (within the same state) and inter-state – with intra-state movement accounting for 88% of all migration. Uttar Pradesh and Bihar are the largest source states for inter-state migrants, while Maharashtra and Delhi are the biggest receivers.

Push and pull factors 🔗

Migration is usually explained through push factors (what drives people away from a place) and pull factors (what attracts them to a new location). Push factors include unemployment, poverty, agrarian distress, conflict, and natural disasters. Pull factors include better jobs, higher wages, educational opportunities, healthcare, and safety. The classic example in India is the steady stream of workers from Bihar and Eastern UP heading to construction sites and factories in Mumbai, Delhi, and Bengaluru.

Gender, marriage, and migration 🔗

Migration in India has a strong gender dimension. The Migration in India 2020-21 survey by the National Statistical Office found women’s migration rates (around 48%) were far higher than men’s (around 6% in rural areas). This is largely because women traditionally move to their husband’s residence after marriage – a pattern unique to Indian and South Asian societies that significantly inflates female migration numbers.

Effects on regional population dynamics 🔗

Migration reshapes regions in profound ways. Receiving cities experience a rejuvenated workforce, but also overcrowding, pressure on housing, and strain on services. Source regions, on the other hand, often lose their young, productive adults, leading to a demographic vacuum where children and the elderly are left behind. This affects everything from local agriculture to family structures. International migration adds another layer – Indian workers in the Gulf, for instance, send back substantial remittances that boost local economies in states like Kerala.

Migration also interacts with fertility and mortality. Migrants often bring different reproductive behaviours from their place of origin, though over generations these tend to converge with local patterns. Migration can also indirectly affect fertility by delaying marriage, separating spouses, or altering economic incentives for childbearing.

How the three components interact 🔗

Fertility, mortality, and migration do not operate in isolation. India’s experience over the last 50 years is a perfect illustration of how they unfold together. As mortality fell faster than fertility in the mid-20th century, the population exploded. As fertility caught up and began to fall, growth slowed. Now, with TFR below replacement, the country is heading toward a stage where migration and ageing will play larger roles in shaping demographics. The southern states, with their low fertility and aging populations, are already attracting workers from the high-fertility northern belt – creating a new internal demographic dynamic.

This three-way interaction is captured by the Demographic Transition Theory, which describes how societies move from high birth and death rates to low ones, passing through stages where population growth accelerates and then slows. India is currently considered to be in the later phase of this transition.

What do you think? If India’s fertility has already fallen below replacement level, what kinds of policy challenges might the country face over the next two or three decades? And in your own state or district, which of the three components – fertility, mortality, or migration – do you think is doing the most to reshape the population today?

References

1. https://www.drishtiias.com/daily-updates/daily-news-analysis/sample-registration-system-srs-statistical-report-2023
2. https://www.business-standard.com/health/india-s-birth-death-rates-halve-in-50-yrs-infant-mortality-at-record-low-125090500478_1.html
3. https://prsindia.org/policy/vital-stats/national-family-health-survey-5
4. https://www.dataforindia.com/infant-mortality/
5. https://prsindia.org/theprsblog/migration-in-india-and-the-impact-of-the-lockdown-on-migrants
6. https://ruralindiaonline.org/en/library/resource/migration-in-india-2020-2021/