China's similarly notorious one-child policy has done a better job of slowing population growth but not without problems. In a country that values boys over girls, one-child rules have led to abandonments, abortions and infanticides, as couples limited to a single offspring keep spinning the reproductive wheel until it comes up male. "We've learned that there is no such thing as Ôpopulation control,'" says Alex Marshall of the U.N. Population Fund. "You don't control it. You allow people to make up their own mind."

That strategy has worked in many countries that once had runaway population growth. Mexico, one of Latin America's population success stories, has made government-subsidized contraception widely available and at the same time launched public-information campaigns to teach people the value of using it. A recent series of ads aimed at men makes the powerful point that there is more machismo in clothing and feeding offspring than in conceiving and leaving them. In the past 30 years, the average number of children born to a Mexican woman has plunged from seven to just 2.5. Many developing nations are starting to recognize the importance of educating women and letting them — not just their husbands-have a say in how many children they will have.

But bringing down birthrates loses some of its effectiveness as mortality rates also fall. At the same time Mexico reduced its children-per-mother figure, for example, it also boosted its average life expectancy from 50 years to 72 — a wonderful accomplishment, but one that offsets part of the gain achieved by reducing the number of births.

When people live longer, populations grow not just bigger but also older and frailer. In the U.S. there has been no end of hand wringing over what will happen when baby boomers-who owe their very existence to the procreative free-for-all that followed World War II-retire, leaving themselves to be supported by the much smaller generation they produced. In Germany there are currently four workers for every retired person. Before long that ratio will be down to just 2 to 1.

For now, the only answer may be to tough things out for a while, waiting for the billions of people born during the great population booms to live out their long life, while at the same time continuing to reduce birthrates further so that things don't get thrown so far out of kilter again. But there's no telling if the earth-already worked to exhaustion feeding the 6 billion people currently here-can take much more. People in the richest countries consume a disproportionate share of the world's resources, and as poorer nations push to catch up, pressure on the planet will keep growing. "An ecologist looks at the population size relative to the carrying capacity of Earth," says Lester Brown, president of the Worldwatch Institute. "Looking at it that way, things are much worse than we expected them to be 20 years ago."

How much better they'll get will be decided in the next half-century (see chart). According to three scenarios published by the U.N., the global population in the year 2050 will be somewhere between 7.3 billion and 10.7 billion, depending on how fast the fertility rate falls. The difference between the high scenario and the low scenario? Just one child per couple. With the species poised on that kind of demographic knife edge, it pays for those couples to make their choices carefully.

— Reported by William Dowell/New York, Meenakshi Ganguly/ New Delhi and Dick Thompson/Washington