Human Population Size

Eight billion people: how and why the world population has grown

The world population reached one billion people at about 1800, and it passed eight billion in November 2022. Growth of this kind is exponential growth: the population increases by a roughly constant percentage each year, so the number added each year gets bigger and a graph of population against time curves upwards instead of rising in a straight line. Two figures decide how fast a country grows. The birth rate is the number of live births per 1000 people per year; the death rate is the number of deaths per 1000 people per year; and natural increase is the birth rate minus the death rate.

The population of a country changes through natural increase and through migration. Net migration is the number of immigrants minus the number of emigrants, so the population change in a year is natural increase plus net migration. Almost all of the growth in the world today comes from natural increase, because death rates fell long before birth rates did: more babies survive, adults live longer, and each generation is larger than the one before even where each woman has fewer children than her mother did.

Doubling time is how long a population takes to double at its present rate of growth. A quick estimate is the rule of 70: divide 70 by the annual percentage growth rate. A country growing at 2 per cent a year doubles in about 70 / 2 = 35 years, while one growing at 4 per cent a year doubles in about 70 / 4 = 18 years. Doubling time matters because the difference between a small and a large growth rate decides how many people a country has to house, feed, educate and employ within a single lifetime.

The world growth rate has already passed its peak. It was highest at a little over 2 per cent a year in the late 1960s and has fallen since, because birth rates fell in most of Asia and much of Latin America. The population is still rising even so, because the growth rate is still positive and because of population momentum: the very large numbers of young people now reaching the age at which they have children. The United Nations projects a world population of around 10 billion in the 2080s, and most of the remaining growth will be in sub-Saharan Africa and South Asia.

• Natural increase = birth rate - death rate, both measured per 1000 people per year.
• Population change in a year = natural increase + net migration.
• Doubling time is roughly 70 divided by the annual percentage growth rate.
• The time taken to add each billion people has fallen to about eleven years.

Why death rates fall: medicine, sanitation, clean water, food supply and vaccination

The death rate is a measure of how many people die each year per 1000 of the population, and in the poorest countries most of those deaths are of babies and young children. That is why the death rate responds so strongly to measures that keep children alive. Life expectancy at birth is the average number of years a baby born today would live if the death rates of today stayed the same, so it rises steeply whenever child deaths fall.

Clean water and sanitation cut deaths from cholera, typhoid and diarrhoeal disease, which are among the largest causes of death among young children worldwide. Medicine works on three fronts: vaccination against measles, polio, diphtheria, tuberculosis and other infections; antibiotics against bacterial disease; and oral rehydration, which turns a fatal case of diarrhoea into a treatable one within hours. Better care in pregnancy and childbirth reduces maternal deaths and the deaths of newborn babies.

Food supply matters just as much. The Green Revolution from the 1960s brought high-yielding varieties of wheat and rice, irrigation and fertiliser to India, Pakistan and other countries, so that food production grew faster than the population and famine became far less common in Asia. A population that is adequately fed resists infection, and children who are not malnourished survive the diseases that would otherwise kill them.

Not every country follows the same path downwards. Where health spending is low, where conflict destroys clinics and water systems, or where HIV/AIDS and malaria are widespread, the death rate can stop falling or even rise again, and life expectancy in parts of southern Africa fell during the worst years of the AIDS epidemic before treatment became available. Once the death rate has fallen, it stays low and the population keeps growing: a falling death rate does not make a population smaller, it makes it larger.

• Clean water and sanitation: fewer deaths from cholera, typhoid and diarrhoeal disease.
• Vaccination: children survive measles, polio, diphtheria and tuberculosis.
• Antibiotics and oral rehydration: infections and severe diarrhoea become treatable.
• Maternal and newborn care: fewer deaths in pregnancy, childbirth and the first month of life.
• Food supply: irrigation, fertiliser and high-yielding varieties mean fewer famines and better resistance to disease.
• Where health systems break down - through war, poverty or epidemic disease - the death rate can stop falling.

Why birth rates fall, and why they stay high

The fertility rate is the average number of children born to each woman in her lifetime, and it is usually measured for the whole country. A country where women have two children each has a birth rate of roughly 20 per 1000, and a country where they have six has a birth rate above 40 per 1000. Replacement level fertility is about 2.1 children per woman: two children to replace the parents, and a little more because some children die before they reach adulthood and not every woman has children, which is why a rate of 2.1 keeps a population stable rather than growing.

Several conditions keep the birth rate high. Where infant and child mortality is high, parents have more children than they want because they expect some to die. Where there is no contraception or family planning service, and no information about it, couples cannot choose how many children they have. Where women have little education and little chance of paid work, early marriage and childbearing are the expected path. Religion and tradition can place a high value on large families and on sons in particular. In a farming community a child costs little and earns from an early age, and children are the only support in old age for parents who have no pension, so children are an economic asset rather than an expense. Women who have no access to healthcare face risk in pregnancy, so families may start earlier.

The birth rate falls when those conditions change. A fall in infant mortality gives parents confidence that their children will survive, so they have fewer. Education, especially of girls, is the single most powerful factor: each extra year of schooling tends to delay marriage and first birth, and to raise a woman's expectations of paid work. Access to contraception lets couples act on the smaller family size they already want. Women in paid work and urbanisation raise the cost of children, because in a city a child needs school fees, uniforms, transport and childcare, and cannot earn a living by helping in the fields. Later marriage shortens the years in which a woman can have children, and a government may add its own policy, either encouraging or discouraging large families.

The critical point is the timing. The birth rate falls after the death rate, and usually decades after, because the reasons for smaller families - education, contraception, secure old age, confidence that children survive - take much longer to build than a vaccination programme. That gap is what makes the middle of the transition the period of fastest growth.

• Keeps the birth rate high: high infant mortality, no contraception, little female education, religion and tradition, children as workers and as support in old age.
• Pushes the birth rate down: lower infant mortality, girls staying in school, contraception, women in paid work, later marriage, and the high cost of raising children in cities.
• Children per woman is the figure to quote: about 2.1 keeps a population stable, six or seven produces very rapid growth.
• The birth rate falls after the death rate, which is why growth is fastest in the middle of the transition.

The demographic transition model

The demographic transition model describes how the birth rate and the death rate of a country change as it develops. Time runs along the horizontal axis and the rates per 1000 people run up the vertical axis, so two lines are drawn. The birth rate line controls how many people are added, the death rate line controls how many are lost, and the gap between the two lines is natural increase - so the gap decides the shape of the population curve, which rises slowly, then steeply, then flattens off into an S shape.

Stage 1 has a high birth rate and a high, fluctuating death rate, so the population is fairly stable and grows only very slowly. Deaths come from disease, famine and poor sanitation, and births are high because families expect losses. Stage 2 is where the death rate falls rapidly, because clean water, sanitation, vaccination and medicine arrive, while the birth rate barely moves. The gap between the lines is at its widest, natural increase is at its greatest, and the population grows faster than at any other time.

In stage 3 the death rate continues to fall but more slowly, and the birth rate begins to fall as girls stay longer in school, contraception spreads, more women take paid work and fewer children die. The gap narrows, so growth slows - but the population is still rising, sometimes by a great deal, because births still outnumber deaths. In stage 4 both rates are low and roughly equal, the population is stable, and life expectancy is high, so the population is much older than in stage 2. Stage 5 is the stage that was added once countries began to shrink: the birth rate falls below the death rate, so the population declines and continues to age.

Treat the model as a model, not a law. It was built from the experience of western European countries as they industrialised, and it assumes that every country will follow the same path at roughly the same pace. Countries are not obliged to do so: some have moved through stages 2 and 3 very quickly, others have been held in stage 2 or 3 by poverty, conflict or disease for decades, and migration is not shown on the model at all, even though a country such as the United Arab Emirates has grown almost entirely through immigration rather than through births. The model is nevertheless the framework examiners expect, because it explains why a population grows in a particular period and what is likely to happen next.

Migration, rural and urban migration, and urbanisation

Migration is the movement of people from one place to another to live, and it changes the size of a population just as births and deaths do. Emigration is leaving a country and immigration is arriving in one, and net migration is immigrants minus emigrants. Migration may be international, as when workers move from the Philippines or Bangladesh to the Gulf states, or internal, as when families move from a village to a city inside the same country. People who are forced to move by war, persecution or disaster are refugees, and they move whether or not they want to.

Migration is driven by push factors, which drive people away from a place, and pull factors, which attract them to a new one. Push factors in the countryside include low farm incomes, a shortage of land as holdings are divided between sons, poor services such as schools and clinics, drought, soil erosion, and conflict. Pull factors in the city include jobs and higher wages, education, healthcare, electricity and clean water, entertainment, and relatives who have already moved and can help with a place to stay.

Rural to urban migration is the movement of people from the countryside to towns and cities, and urbanisation is the rise in the proportion of a country's population that lives in towns and cities. The world crossed the point where more than half of all people lived in urban areas at about 2007, and the proportion is still rising, fastest in Asia and Africa. Three things make a city grow: in-migration from the countryside, natural increase among the young migrants who arrive, and the reclassification of farmland as the built-up area expands. Migration is usually the largest of the three in the early years, but in the longer term natural increase in the city contributes a great deal, because the new arrivals are young and starting families.

Migration has advantages as well as costs, and both sides need to be stated. For the receiving city, migrants supply the labour that builds and serves it, and they pay taxes; for the family left behind, remittances sent home pay for school fees, medicines and housing, and in countries such as Nepal and the Philippines these transfers are a large share of national income. The costs fall on the places that are losing people, where schools and clinics close and only the old remain, and on the migrants themselves, who may work in the informal economy without a contract, and who can be sent home if the economy of the host country turns down. Settlement is also a pressure on the receiving country, because the city has to house and serve the new arrivals in the years before they can support themselves.

The impacts of rapid growth: megacities, informal settlements and overloaded services

A megacity is a city with more than ten million people in its built-up area, and there are now more than thirty of them, most of them in Asia. Megacities are centres of economic opportunity and creativity, so they attract more people, which makes them bigger still. The problem is not the size itself but the rate of growth: where a city adds several hundred thousand people a year, the water mains, sewers, power lines, roads, schools and clinics cannot be built quickly enough to keep up, and the shortfall falls on the newest arrivals.

Informal urban settlements, often called slums or shanty towns, are the visible result. They are built without legal title to the land, from whatever materials are available - corrugated iron, cardboard, timber, plastic sheeting - on land that nobody else wanted, such as a steep hillside, a flood plain, a rubbish dump or the strip beside a railway. Residents may have no security of tenure, so they risk eviction and have no reason to improve a house they may lose. Services are missing or shared: one standpipe or one water vendor for hundreds of families, water bought by the litre from a tanker at several times the price paid by wealthier districts, a few shared latrines, and rubbish tipping into a river or a drainage channel because no collection reaches them.

Water supply and sanitation are where the impacts become dangerous. Where a water supply is intermittent, the mains lose pressure and sewage seeps in through cracks, so the supply becomes contaminated and diarrhoeal disease and cholera spread. Where there is no sewer, waste enters the drains, the river and the groundwater, which contaminates the sources that other people drink from. Waste management is overwhelmed: the volume of refuse rises with the population, collection vehicles cannot reach narrow unplanned streets, and uncollected waste blocks drains, worsens flooding and is burned, which adds air pollution. Availability of land falls as the city spreads over farmland and flood plains, and building on the flood plain turns a heavy rainfall into a disaster.

Facilities and infrastructure - power, roads, water, drainage, schools, hospitals, transport - are all under strain at once, and so is the environment. Rapid growth produces air pollution from traffic, industry and the burning of rubbish and fuel; water pollution from untreated sewage and industrial effluent; land pollution from dumping and uncontrolled waste; noise pollution from traffic and machinery; light pollution from street lighting and advertising; and visual pollution from advertising, overhead cables and sprawling unplanned building. Traffic congestion costs hours each day and worsens air quality, because road building rarely keeps pace with the number of vehicles. Employment changes as well: agriculture declines as a share of jobs while manufacturing, construction and services grow, and many new workers are absorbed into the informal economy of street trading, repair work and casual labour, with low pay, no contract and no protection. Others, especially young people, remain unemployed, because the supply of workers grows faster than the jobs.

Pressure on resources, and what happens when a population declines

A growing population needs more of everything: food, water supply, energy, land, timber, minerals and building materials. Demand rises both because there are more people and because incomes rise, and the second effect can be the larger one - one person in a high-income country typically uses many times more energy, water and materials than one person in a low-income country, so a small rich population can have a larger environmental impact than a large poor one. Rising energy use is the most visible pressure: more households with electricity and more vehicles mean more fuel burned, so more emissions of greenhouse gases and more air pollution. Whether the Earth has fixed limits is a matter of argument. The pessimistic view, going back to Thomas Malthus, is that population will always outrun food supply, and there is evidence for it in famines, water shortages and farmland lost to erosion. The optimistic view is that technology, trade and better management keep raising the ceiling, as the Green Revolution did. In a management answer, say what the pressure is, how far it can be eased, and what limits the measure.

A declining population happens where the birth rate has fallen below the death rate and stays there, which is the situation in Japan, Italy, Germany and much of eastern Europe. It is not the same problem as rapid growth, but it is still a problem for management. The working-age population shrinks, so there are fewer people paying tax and producing goods, while the number of older people rises. The dependency ratio - the number of people of working age compared with the number of dependants, young and old - worsens, so pensions, healthcare and long-term care consume a growing share of national spending. Employers cannot fill vacancies, rural schools and clinics close as the villages empty, houses stand empty in some regions, and the country has to find more productivity from fewer workers.

There are benefits on the other side, and a good answer gives them. Fewer people mean less demand for water, food, energy and land, less traffic congestion, lower emissions and less pressure on farmland, forests and fisheries. Some workers gain, because a shortage of labour pushes wages up, and governments gain room to raise spending per head on education and health. For the environment, a falling world population would ease the pressures described in every other topic in this course.

Countries try three kinds of response, and each has a limitation. Pro-natalist policy offers baby bonuses, long parental leave, free or subsidised childcare and cheap housing to encourage larger families. It is expensive, and it usually produces a small and temporary rise in the birth rate rather than a return to replacement level, because the number of women of childbearing age is itself falling and because the effect is often to bring forward births that would have happened anyway. Immigration is the fastest way to raise the number of workers, and it has been used by Germany and Canada; its limitations are political and social, and migrants may return home when the economy weakens. Raising productivity, through a higher retirement age, automation and longer participation by women, keeps the economy running without more people; it works, but it changes expectations and can be unpopular. The judgement to aim for is that a rate of change matters more than the direction: a country or city can absorb steady growth of one or two per cent a year with planning, while faster growth or a sudden decline overwhelms the infrastructure, the services and the tax base that pay for them.