Aisha, Miriam and Akhi are three young factory workers in Dhaka, the capital of Bangladesh.  They are poorly educated and badly paid.  But, like millions of other young women, they relish their freedom from the stultifying conformity of rural life, where women are at the constant beck and call of fathers, brothers and husbands.

There is something else.  The three women together have 22 siblings.  But Aisha plans three children, Miriam two and Akhi just one. They represent a gender revolution that many see as irrevocably tied to a reproductive revolution. Together, the changes are solving what once seemed the most difficult problem facing the future of humanity: growing population. 

Almost without anyone noticing, the population bomb is being defused. It is being done without draconian measures by big government, without crackdowns on our liberties—by women making their own choices. 

Family planning experts used to say that women only started having fewer children when they got educated or escaped poverty. Pessimists feared that if rising population prevented the world's poor from advancing, they would get caught in a cycle of poverty and large families. The poverty trap would become a demographic trap. 

But the reality is proving very different. Round the world, women today are having half as many children as their mothers did. And often it is the poorest and least educated women who are in the vanguard. Women like Aisha, Miriam and Akhi.

There are holdouts, in parts of the Middle East and rural Africa. But more than 60 countries—containing approaching half of the world's population—already have fertility rates at or below the rate needed to maintain their populations long-term. The club now includes most of the Caribbean islands, Japan, South Korea, China, Thailand, Sri Lanka, Iran, Turkey, Vietnam, Brazil, Algeria, Kazakhstan and Tunisia.  Within 20 years, demographic giants like Indonesia, Bangladesh, Mexico and India will in all probability also have below-replacement fertility. 

Now is this happening?  For one thing, more and more women are leading independent working lives, rather than succumbing to a life of child bearing and raising. In many countries, women are staying single through their 20s and beyond. In 1960, two thirds of American women in their early 20s were already married; today the figure is less than a quarter. The habit is spreading fast.  As recently as 1980, a UN study found that nuptials were "near universal" across Asia, and half of Asian women were married by the age of 18. No more. In Japan, half of all 30-year-old women today are unmarried. In South Korea, the figure is 40 per cent.  

The trend is especially marked in cities. In Bangkok, a fifth of all women are single at 45.  Manila, Singapore and Hong Kong are not far behind. And, again largely unremarked, it is women who are heading the urbanization of the planet. Whether sweatshop workers in Dhaka, bar girls in Bangkok, office workers in Shanghai, students in Delhi or maids in Caracas, there are more young women than men in almost every city in the world. 

Academics debate whether they are there for the jobs or the men (there are more rich educated men in cities, but more women of all sorts). But whatever the motive, they are there: in the bars, shoe shops, gyms and clubs. With jobs but often no dependants. In Japan, they get called wagamama, or "single parasites." No matter; it's better than changing daipers. 

How have they gained their freedom? Some say liberation allowed women to make new choices about their lives. Equally, however, it has been the dramatic improvement in the survival rate of infants that for the first time has freed women from the social obligation for a lifetime producing and rearing babies. 

Women are having smaller families and grabbing a new life outside the home because, for the first time in history, they can. In the 20th century, the world largely eradicated the diseases that used to mean most children died before growing up. Mothers no longer need to have five or six children to ensure the next generation. So they do not. Two or three is enough.

Rich or poor, educated or illiterate, socialist or capitalist, Muslim or Catholic, secular or devout, with tough government birth control policies or none, most —most families—tell the same story. 
Scottish sociologist John MacInnes at the University of Edinburgh calls this the reproductive revolution. Until the 18th century, half of children died before entering their fertile years, and many more before they completed them. Most women spent almost all their (often rather short) adult lives bearing and rearing children.  

The patrician societies that have dominated the world for millennia were designed to ensure women fulfilled this role.  The regulation of child production was done "through church and state, the norms surrounding sexual activity and sex roles, illegitimacy, cohabitation and marriage, family and kinship obligations and property law," says MacInnes. 

In much of the world, the drive to maintain fertility institutionalized arranged marriages, often of very young girls, maintained brutal sanctions against female adulterers or girls who would not accept their lot, and ostracized any form of homosexuality. 

The reproductive revolution is kicking all this away, because it is simply no longer needed to sustain populations. Feminism is not a new idea. And some individuals have always broken free.  But, for most women, the reproductive revolution has "taken it from the realm of utopia to practical possibility," says MacInnes. 

The global collapse in fertility rates and rise of feminism is not the slow diffusion of a new idea, or a mechanistic response to aid workers handing out condoms. This is the breaking of a logjam. The logjam of a patriarchy that has suddenly lost its purpose. 

British demographer Tim Dyson of the London School of Economics sees the change even in rural India.  India still adds about 19 million people to its population each year—a quarter of total global growth. But its fertility is falling fast, now averaging 2.8 children per woman.  Dyson remembers that back in the 1960s, sociologists compared women's lives in India and the U.S. "In India they married at 17, had seven kids, the last one at 43 years old, and died typically at 46. In America, women typically married at 18, had two kids quickly and then more than 40 years of life after childbearing. Now, Indian women are grabbing that life too.  Sterilization is the main form of contraception in India, and the average age of sterilization is 26 years old."

Where is this taking us? Through most of history, women have had between five and eight live births each. For a while after childhood death rates fell, they continued to do so. That's why world population quadrupled in the 20th century. But today the global average is 2.6 births, half the figure even a generation ago. The figure is falling fast towards the current global replacement rate which, allowing for girls who do not reach adulthood, is 2.3 children per woman. 

Smaller families do not immediately cut population growth. There is rising life expectancy to account for, along with a legacy of 20th-century baby boomers who remain fertile. But the trend is clear. And in the past 40 years, the world's population growth rate has fallen from 2.1 per cent to below 1.2 per cent.

Percentages are not the same as absolute numbers, of course.  But since 1987, the number of additional people on the planet each year has fallen from 87 million to 78 million.   The downward trend will accelerate. Growth may, on current trends, be zero or even negative by mid-century. Some countries are already shrinking. In 2008 there were 26 of them, headed by Russia, Montenegro, Bulgaria, Zimbabwe, Ukraine, Latvia and Swaziland. Others are only sustained by inward migration.

It took around 130 years, from about 1800 to 1927, for the world to get from one billion people to two billion, but only another 33 years to reach three billion, which happened in 1960. Reaching four billion took just 15 years to 1975. The fifth billion came in 12 years, in 1987, as did the next billion, achieved in 1999. 

The next billion people will take a little longer than the last, probably 14 years if we reach seven billion as expected in 2013. And getting to eight billion will take another 20 or more years. And we may never reach 9 billion. 

Peak population is probably much closer than most people think. Virtually all countries that have brought their fertility rates down from five, to four and three have gone on down below the replacement level. Much of Europe is below 1.5. By 2100, on current fertility trends, Germany could have fewer natives than today’s Berlin, and Italy’s population could crash from 58 million to just 8 million. 

Once a negative trend has set in, it may prove very hard to break.  As well as having ever fewer potential mothers, societies may get out of the habit of having babies.  Children will be rare, exotic and unusual. We can see this already. Only a few years ago, going to a cafe in Italy would see you surrounded by noisy children. Now you will likely only see adults, including many young latte-sipping men and women who would once have been surrounded by kids. 

Other repercussions of the baby bust will play out over the coming decades. One of the most controversial is the rising tide of migration. It is created in part by the income differentials round the world, but even more by current record differences in fertility. When women in some countries have more than six children, while others have barely more than one, trading people is an obvious safety valve for both sides. Europe and North America already badly need foreign hands to keep societies and economies functioning. We should stop pretending otherwise.

The other critical change is aging. As fertility falls, the world is becoming older. Our species has never lived in societies where there were more old people than children. But soon we will. This is truly terra incognita. 

Some say we will never be able to afford to look after all those old people. But there is, to coin a phrase, a silver lining. The old are human capital, sources of wisdom and experience.  We have to harness that capital better. The old will have to work longer—but they will also expect to be valued more. 

Take health. All the things I have been talking about have happened because the 20th century eradicated the killer diseases that wiped out most children before they could grow up. But in the 21st century there is a new priority—to help the old stay fitter for longer. So they can have better lives, of course. But also so they can contribute more.

We are coming through the greatest surge of human population numbers in history. It has already changed us profoundly and the end game will change us even more.  The reproductive revolution unleashed huge forces of economic activity, social dislocation and liberation—for women in particular.  But well before the end of this century, Homo sapiens—the brash, go-getting, hormone-driven, young naked ape of the 20th century—will be older and will likely be more conservative, less innovative, more boring even. But perhaps also wiser, less frenetic, and more caring of each other and the environment. Older, wiser, greener.

The tribal elders may take center stage once more. But this time they will not just be revered, they will be the largest group in society. And in all the probability, they will be dominated by women. Demography is destiny. There is no going back.

Excerpted from The Coming Population Crash: And Our Planet’s Surprising Future by Fred Pearce. Copyright © 2010. Reprinted with permission by Beacon Press.