How many people does it take to Change the World

For most of the 50,000 years mankind has walked the earth, life was fragile and hard. Whenever humanity progressed, famine and epidemic seemed to push it back. War brought more misery.

Then, in the last 500 years - a few ticks of the anthropological clock - the world changed. Calamity receded, harvests grew. In French villages and English shires, people began to take control of their environment and their lives. The result: a worldwide explosion of people that has transformed societies, gobbled up resources, and changed the course of history.

Whatever happens next, population experts say, this passing millennium - and especially the 20th century - will be remembered as unique in the history of humanity.

"The global population explosion of the past half millennium is a unique phenomenon in human history," says George Moffett, author of "Critical Masses," a book examining the population explosion. "Because of roughly simultaneous modern revolutions in mass communications, contraceptive technology, and women's rights, it is a phenomenon that will certainly never be repeated."

If the millennium contains a central demographic theme, it's humanity's increasing dominance over a premature death. In AD 1000, nature held the upper hand. Crop failures could spark mass starvation. Epidemics could brutalize populations. Even under the best of circumstances, childbirth proved risky for both mother and child. The world population, which stood at roughly 300 million in AD 1, had barely budged 1,000 years later.

Growth rates improved a little after that. But in the 14th century, a series of plagues ravaged Europe and China, wiping out a third of their populations. By most estimates, the world had fewer people in 1400 than in 1250.

As devastating as it was, the Black Death marked the last time that natural disaster would stop population growth worldwide. By 1600, agricultural advances, such as crop rotation and fertilization, and very rudimentary health measures, had brightened humanity's prospects. Population began to increase in Europe.

Interestingly, the growth didn't come about because of rising birth rates, but rather because of lower death rates, especially among children. Even in pre-industrial Europe, mothers began to see more of their children survive into adulthood, thanks to better crops and hygiene. Those children, in turn had their own offspring, and very gradually the compound effect began to boost Europe's population.

The Industrial Revolution accelerated the process by boosting incomes and making food cheaper. Even when famine hit one region, trains began to bring in excess grain - something that would have been too expensive for pack horses in medieval times. Mortality rates fell even faster when cities adopted elementary sanitation practices, such as treating sewage and providing clean drinking water. Individuals could afford soap and their industrially made cotton clothes proved far easier to clean than their pre-industrial woolens.

"Certainly by the late 1800s, there was a sense that body washing, washing clothes, was hygienic and helpful," says Samuel Preston, a dean at the University of Pennsylvania in Philadelphia and former director of the Center on Population Studies. The hygienic theories of Louis Pasteur and other scientists spread to the developed nations and, eventually, the world, setting the stage for the dramatic drop in mortality that took place in the 20th century.

The average person born today enjoys a life expectancy nearly four times greater than what demographers believe existed 1,000 years ago.

"The overarching fact is that we've had this extraordinary explosion of health," says Nicholas Eberstadt, a demographer at the American Enterprise Institute, a Washington, D.C., think tank. "It's not because people started breeding like rabbits. It's that they stopped dying like flies."

If trimming the death rate was historic, Europe had another surprise in store. Sometime around 1780, just a few years before Thomas Malthus published his famous book warning that population growth would outstrip food production, families in pre-Revolutionary France began cutting back the number of children they had. Such reductions, already seen sporadically in England a century earlier, spread throughout Europe and created something unheard of on a continental scale. For the first time, families raised fewer children not because of disease or famine but because they chose to.

Demographers can only guess why this happened. Families possibly came to the realization that the more children survived, the more expensive they became. This proved doubly true once families moved from farms into the burgeoning cities of the era. Extra labor wasn't needed. To better children's lives, families would have to educate them - an expensive proposition.

Family limitation spread throughout Europe and the United States, pioneering what population experts call the demographic transition. In the first phase, countries experience high birth and death rates. Then advances in such practices as sanitation reduce the death rate and populations surge. Finally, people voluntarily lower the birthrate and growth decelerates.

Such actions don't immediately trim the number of people in the world. Like braking a speeding train slowing population growth can be difficult. Even after growth rates decline, total population can still grow for decades, even more than a century later. Nevertheless, the pattern was set and continues to this day. Fertility rates in the developed world have dropped so low that policymakers now have a new worry: decline.

Unless something changes dramatically, estimates by the United Nations suggest that by 2050 Sweden will have ceased to grow, Japan will lose a quarter of its population; Italy will shrink 28 percent. Such losses carry huge implications, demographers say.

"What accompanies decline in population is aging of the population," says John Bongaarts, vice president for policy research at the Population Council in New York. "And that requires adjustments of the pension system and healthcare system that, frankly, have not been worked out."

If the millennium has proved amazing, the 20th century has been its showstopper. Following Europe's lead, the rest of the world has plunged into the demographic transition with a vengeance. It took 50,000 years for humanity to reach the 1 billion mark, around 1800. In the 20th century alone, population has grown from 1.6 billion to 6.1 billion. Every week, the world adds enough people to create a new Philadelphia; in little more than a year, a new Germany. And more than 95 percent of that growth is in less developed nations.

Once again, falling death rates have proved decisive. Average life expectancy has soared to 65 around the world, an increase of 20 years since 1950, thanks in large part to progress in the developing world. Demographers credit not only the spread of public sanitation, but also the introduction of antibiotics and other medicines, particularly after World War II.

As a result, population boomed so much that many demographers in the 1960s warned that the world was sitting on a time bomb. But then, in equally dramatical fashion, many developing countries entered the next phase of the demographic transition and began limiting family size.

In the late 1960s, the average woman in Asia produced 5.7 children; today, that's down to 2.6. Latin America has witnessed similar declines. In China, South Korea, Taiwan, and Thailand, the rate is less than the 2.1 break-even rate needed to maintain population size long term. Often, governments force such moves through sterilization. But technology has played its role, too. Demographers point to the widening acceptance of modern contraception for cutting fertility at breakneck speed.

"It took the US 200 years to go from seven babies per family to two," says Carl Haub, a demographer at the Population Reference Bureau in Washington, DC. "Bangladesh has [nearly] done that in 20." Iran has more than halved its fertility rate in a decade.

A big question mark now hangs over Africa. Even the spread of AIDS, which has caused more than half as many fatalities as World War I, has only slowed that continent's growth. While several nations there have substantially reduced fertility rates, central and West Africa have not. The average woman in Zambia or Nigeria still has more than six babies in her lifetime.

If such nations don't take the next step in the demographic transition, they will quickly overwhelm their resources and, perhaps, the world's, demographers warn. That's because small changes in fertility create huge impacts. If average world fertility falls to 2.1 children per woman in the next 30 years and maintains that level over the next century and a half - the United Nations' moderate growth scenario - world population will reach 9.7 billion around 2150 and begin to level off.

But if each woman averages just half a child more, population will total 24.8 billion people by 2150 and continue climbing sharply. If she averages half a child less, population will fall to 3.2 billion - nearly half today's total. (See box left: Will population level off?)

Even achieving the UN's moderate scenario will require a sea change in attitudes in large swaths of the world, population experts say. The trick is to offer women access not only to family planning but to means of empowerment.

"Overpopulation is one of the few global problems we know how to solve," says Dr. Moffett, the author. "There's a crucial connection between a woman's productive role - the improved legal, educational, and economic opportunities that are the source of empowerment - and a woman's reproductive role."

The future remains up in the air.

"There's been a demographic revolution," says Dr. Preston of the University of Pennsylvania. "But it's a revolution whose end is not in sight."

(c) Copyright 2000. The Christian Science Publishing Society