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Demography is a science with few general theories, yet the two that
dominate seem to be diametrically opposed. First came the ideas of English
clergyman and economist Thomas Malthus, who proposed in 1798 that rising
prosperity increases the demand for workers, so stimulating birth rates.
Then, early this century came the theory of ‘demographic transition’ – that
as people become prosperous, they want more comfort and have fewer children.

It is now clear that these two ideas are like the blind man’s description
of the elephant: they describe the same phenomenon, but approach it from
different ends. With increased prosperity, traditional farming communities
first expand in size. But further improvements in comfort, education and
wealth encourage restraint, and the population stabilises. The size of a
population at equilibrium, depends on the size of birth and death rates
and how fast they change.

If a society wants to restrict the growth of its population, it should
adopt policies that speed its passage through this demographic transition.
How can this be done? Evidence suggests that low birth rates are not merely
the result of Western-style industrialisation and wealth. Demographers
now realise that improvements in healthcare, literacy, birth control and
the status of women are equally important. It is such measures that governments
meet to discuss next week at the International Conference on Population
and Development in Cairo.

To understand global population change, the phenomenon must be considered
in a historical context. During the millennia when human societies were
predominantly agrarian, a large number of children were essential for survival.
Population size was governed by an unstable equilibrium between a high
birth rate and a high rate of mortality linked to violent crises, especially
wars, famines and epidemics. As a result, populations grew very slowly.
But from the mid-18th century to the 1960s there was a spurt in population.
Why populations started increasing at that time is still open to debate
(see ‘Was the population explosion inevitable?’). Now we are in the central
phase of the global demographic transition – growth rates are still huge,
but are starting to slacken.

The current global population explosion can be explained by considering
patterns of development as individual societies go through the demographic
transition. The early stages are characterised by decreases in death rates.
First, the threats of famine and epidemics are reduced. Then technological
progress allows infectious diseases to be controlled and a food supply
to be secured. Almost all the world’s nations have now been through these
stages. Everywhere, average life expectancy is at least twice as high as
it was in the past. In India during the 19th century, for example, repeated
famines and epidemics meant that life expectancy at birth averaged only
20 years. Now it is close to 60 years.

Initially, while death rates decrease, birth rates remain at the level
previously required to ensure the survival of society – typically, five
or six children per woman. This period of lower mortality and high fertility
explains the population explosion in Europe in the 19th century and now
seen in the world’s developing countries. But people eventually adapt to
compensate for increased life expectancy. Birth rates fall. One major impetus
for such an adjustment is the economic strain that increasing population
puts on a nation. Other factors include urbanisation, industrialisation
and improvements in education, all of which increase the cost of children,
while reducing their value as workers. Under these circumstances, birth
rates may well fall below the level needed to maintain the size of a population.

This equilibrium now characterises Western Europe, North America and
newly industrialised countries such as Japan, Taiwan, Singapore and South
Korea. Other countries are at diverse stages of transition between the traditional
equilibrium of high birth and death rates, and the modern balance of low
birth and death rates.

But while the principle that populations make this transition appears
to be universal, the rates at which this process occurs are not. The rate
depends primarily on the country’s access to technology, cultural traditions
and degree of openness .

Variations on a theme

For example, countries that are late in starting the process of transition
can benefit from medical advances elsewhere, making the decline in their
death rates sudden and rapid. This can result in a larger final population.
So can a long delay between a decrease in death rate and a fall in birth
rate – a common pattern in many Islamic countries and Sub-Saharan Africa,
where women’s social status has remained low. Furthermore, a certain ‘demographic
inertia’ occurs, as the number of women of child-bearing age increases.
So, even if women have fewer babies, the population may still rise.

The result of all these factors is that now, peak rates of population
growth in developing countries are on average twice as high as they were
in 19th-century Europe. In Europe, the initial drop in death rate was not
very steep and people took around 25 years to start cutting birth rates.
When France, the first country to begin demographic transition in 1785,
reached equilibrium in 1970 its population was not even double the starting
value. Sweden, which also started early, had a slower onset of decline
in birth rates, but the final population was still only 3.8 times bigger
than it had been.

At the opposite end of the spectrum, countries such as Syria and Kenya,
where the transition began more recently, will probably experience 15 or
20-fold increases in their populations by the time their numbers stabilise,
though birth rates are now starting to fall. In Mexico, where transition
started in 1920, mortality has fallen four times faster than it did in France.
Yet Mexican birth rates have fallen more slowly than European birth rates
– with a lag of 50 years. As a result, it is estimated that when Mexico’s
population reaches equilibrium, it will be up to ten times larger than it
was before demographic transition.

Like Mexico, most countries now going through this process will end
up with huge population increases. But this pattern is not inevitable. The
experience of China shows that governments can intervene to limit growth.
There, demographic transition started in 1930 and by the time it finishes
– around 2000 – the population will have expanded just two and a half times.
Although most governments have not gone as far as China, and strictly limited
family size, they can manage the growth of their populations in other ways
if they understand the forces that influence falls in birth rates.

Most developing countries started their demographic shift after the
Second World War, when modern medicines and agricultural practices where
introduced suddenly from outside. But they were unprepared for the rapid
increase in life expectancy and their response – in terms of limiting birth
rates – has been slower than that of most European countries. One of the
factors most crucial for the transition to equilibrium is attitudes to family
planning.

This has become clear as the pattern of developing countries’ transition
to a smaller average family size has emerged: this is remarkably similar
to the pattern of the earliest demographic shifts in Europe. In France,
birth rates began to fall around 1770, a century before similar declines
in neighbouring countries. By this time death rates had already been declining
for 50 years, putting a strain on the capability of the available farmland
to feed the growing population. As the situation became untenable, couples
started marrying later, so sharing fewer childbearing years. The result
was that the population growth slowed to a new equilibrium point.

A lifetime of celibacy

A similar pattern emerged across Europe. The average age of women at
marriage in Western Europe rose from 18 to 25 years. Lifetime celibacy also
increased, reaching a peak of around 20 per cent. Similar, though less dramatic
trends occurred in Eastern Europe at the same time; the average age at marriage
increased to 20 years. Across Europe population growth slackened. But it
wasn’t until the 1920s, with the availability of modern contraceptives,
that populations started to reach equilibrium.

Today, the world’s developing countries are going down a very similar
demographic road. Data from the Chinese census of 1982, for example, shows
that women were first marrying at an average age of 22.5 years, compared
with 17.5 in 1930. In India, too, the average age has increased from 13
in 1930 to 17.7 in 1971, when birth rates began to fall. Other parts of
Asia have experienced even more marked changes. In Taiwan, Singapore, Hong
Kong, South Korea, Japan and Sri Lanka, women tend to marry at the age of
25 or over. Meanwhile, the pattern has been repeated throughout Latin America
and Africa since the early 1960s, though these women have traditionally
married later, rendering the effect less pronounced than in Asia. Similar
changes are being seen in Islamic countries.

Later marriages and fewer children go hand-in-hand, and both are linked
with increasing status for women. There is still some debate as to whether
this is a cause or an effect of demographic transition. There is no doubt,
however, that governments can manipulate the trend towards lower birth rates
by helping women in their long climb towards higher status and wider-ranging
rights. Female literacy is crucial. Even in countries such as India, China
and Indonesia, where incomes are very low, there have been dramatic falls
in birth rates following increases in literacy of young women. Better health
education and, with it, greater access to a range of family planning services,
also hastens the move to smaller family size. Marriage is latest in the
countries that invest most in women’s education.

The other crucial influence on the transition to equilibrium is economic
growth. When Europe started its demographic transition, it was at the forefront
of global industrial development. The region could, therefore, adapt to
booming populations by creating labour-intensive industries, while the congestion
of overpopulation was eased by mass emmigration to colonies. By the time
Europe’s population stabilised in the middle of this century, its peoples
were among the world’s most prosperous. Prosperity, in turn, accelerated
the decline in birth rate. It was this experience that led demographers
to believe that Western-style industrialisation was essential before family
size could decrease.

Family fortunes

This view is now being reassessed. Historically, several societies have
been through demographic transition – post-war Japan is one, 18th-century
France another. In both cases controlling family size was a way of adapting
to poverty rather than to wealth. A similar motivation may soon prevail
in some developing areas – particularly sub-Saharan Africa and the Middle
East – where population pressure is limiting survival.

One reason why much of the developing world is facing demographic crisis
is that it relies for its technology on countries that have already passed
through the transition. Such technologies tend to be capital intensive;
they are not geared to the needs of nations where labour is abundant and
cheap while investment capital and energy are not. The result seems likely
to be an explosion in urban unemployment which will be extremely difficult
to manage. Investment in labour-intensive industrial or agricultural growth
might prevent such a disaster. But the political discontent this is likely
to produce means that rich and poor governments alike are keen to find alternatives.
This is one reason why interest in the Cairo meeting is running high.

If governments are to have any hope of stemming rising populations,
they will have to recognise the factors that hasten a country’s passage
through transition. Representatives meeting in Cairo next week must be aware
that they can curb birth rates by promoting better healthcare and education,
later marriage, increases in income and higher status for women. Whether
they have the political will to do this remains to be seen.

Jean-Claude Chesnais is a demographer based at the Institut National
D’etudes Demographiques in Paris.

* * *

Was the population explosion inevitable?

The demographic transition is humanity’s response to population growth.
But what causes the growth in the first place?

A fall in death rates is the first step on the road to population explosion.
This is often attributed to advances in medicine, hygiene and farming. But
there had been few if any such advances in Europe and China in 1750, when
population growth there soared from 0.2 per cent each year to as much as
1.5 per cent. By 1850, this trend had spread to Japan, India and the Middle
East. By 1950, it was universal. What explains the growth?

William McNeill, a historian at the University of Chicago, says it is
not because of better medicine, but because most of us are now so frequently
in contact that we have become immune to each other’s infections.

Formerly, different diseases arose in different regions of the world,
depending on the local environment. Some, such as measles or smallpox, induced
immunity in survivors. Without new susceptible people to infect, these diseases
would have died out; but in cities, where population density was great enough,
they thrived. Over time such infections ceased to be catastrophic. They
reached most people in early childhood, resulting in a constant, low level
of child mortality, and a population of immune adults.

Let these adults contact people from outside the ‘disease pool’, however,
and such diseases caused murderous plagues among the non-immune. Millions
in the Roman Empire died of mysterious new diseases shortly after the open-ing
of the Silk Road to China. A third of Europe died after bubonic plague arrived
from Asia, and as many as 90 per cent of native Americans died after contact
with Europeans.

But once travel and contact became frequent enough, everyone encountered
and became immune to each other’s infections. ‘It seems plausible to connect
the modern surge of population growth with this changed incidence of exposure
to lethal infections,’ says McNeill. In 1750, the communication networks
were most extensive within Europe and China, so the surge started there.
By 1850, says McNeill, ‘further improvements of transport affected all the
inhabited Earth, allowing the modern growth of population to become genuinely
global’. By this theory, population explosion was the inevitable result
of human migration and contact.

Debora MacKenzie