About 30 years ago, a group of economists at the College of Mexico saw some grim handwriting on the wall. If trends continued, Mexico's population would nearly triple from 51 million to 148 million by the close of the century, with dire consequences for the environment and economy.

At around the same time, the world was being bombarded with doom-and-gloom scenarios prompted by a landmark book, Paul R. Ehrlich's "The Population Bomb," which focused global attention on the inability of world resources to sustain billions more people.

With these threats in mind, the economists launched a campaign of serious arm-twisting to get Mexico to curb its population growth. Their work was not easy; Mexico had a president who declared that good government meant promoting large families. But by 1974, he changed his mind and Mexico adopted one of the world's most aggressive family planning programs. His decision has helped cut the country's birth rate in half, resulting in today's population of about 98.5 million.

The taming of Mexico's population growth is emblematic of a global success story being celebrated around the world on Tuesday, the day the United Nations has fixed as the occasion to mark the birth of the world's 6 billionth person. The event is being heralded with new statistics showing that, contrary to the alarmist predictions of three decades ago, global population is expected to start leveling off at about 8.9 billion in 2050 and stabilize at about 10 billion around 2200.

"This is one of the 20th century's remarkable success stories," said Robert Engelman, chief of research at Washington-based Population Action International. "We've virtually conquered infant and child death. That's one of the main reasons population has blossomed . . . and there's been no planetary disaster, no worldwide famine."

At the same time, according to Engelman and others, the gradual slowing of population growth and its eventual stabilization has created different problems with new challenges for the next century, particularly in creating new jobs and reducing social and economic inequality.

"More than 95 percent of future growth will happen in the poorest countries of the world, which are least capable of dealing with rapid change, which often have weak governments and weak institutions and economies that are not strong," Engelman said.

But without the family-planning efforts of the last three decades, according to John Bongaarts, vice president of the New York-based organization Population Council, the world would have passed the 6 billion mark several years ago, leading to a higher growth trajectory that could have added as many as 4 billion more people to the planet's population -- meaning that it would stabilize in 2200 with 14 billion people instead of 10 billion.

For some, hitting 6 billion is a dubious achievement; for others it is a historic milestone. But for everybody, it focuses the mind: The world entered the 20th century with about 1.65 billion people, and in just 100 years it has added more than 4 billion more.

The increase reflects the remarkable advances of humankind, especially in medicine over the last 50 years. In 1950, the population of Mexico stood at 16.5 million, compared to almost 100 million today and the 146.6 million projected for 2050. In 1950, life expectance at birth was 50.6 years, women had about seven children on average, and about one in eight infants died at birth. Today, life expectancy is about 72, women have an average of about 2.7 children, and one in every 32 babies dies during birth.

Given these reasons for population growth, the key to controlling it was lowering the birth rate -- which resulted from a complicated set of dynamics that has been repeated in numerous industrial and developing countries around the world.

Generally, as it industrialized, Mexico began to spend more on education, and the population began shifting more toward urban areas in search of greater economic opportunities. As they became more urban and educated, Mexicans found less incentive to have more children, who instead of contributing to a family's wealth by working in the fields became a drag on a couple's standard of living. More children crowded the family home, and keeping them in good health was expensive.

Taking a critical step, the Mexican government adopted a constitutional amendment in 1974 giving individuals the right to determine how many children they would have and when to have them -- declaring, in effect, that the government would take no coercive measures to control family size. Studies show that, given education and access to birth control, women everywhere choose to have fewer babies; in Mexico, the results were immediate and extraordinary.

"We cut the birth rate in half in 15 years -- it was phenomenal -- through a voluntary family-planning system," said Victor L. Urquidi, who at the time was president of the College of Mexico and was instrumental in creating the study of demographics here. "This was amazing, and it signified a huge cultural change."

Many countries and regions, especially in the developing world, have yet to achieve a significant drop in birth rates, and their population growth is expected to be explosive. By 2050, the population of Africa, for instance, is expected to balloon from 785 million -- 12 percent of the globe's population -- to more than 1.7 billion -- 17 percent of the world's population. Experts say the key to slowing such growth rates is the same as it was in Mexico: Educate women and give them access to contraceptives and employment.

But even so, experts say, the huge number of young people in the current population will spur a "population momentum" that will produce a steady growth in numbers for decades to come, raising a whole new set of problems of the sort that Mexico and many other countries are facing today.

Chief among those is the so-called aging or "graying" of a national population that occurs after lower birth rates become the norm. A larger proportion of the population becomes older, requiring changes in priorities and expenditures; instead of new elementary schools, for example, governments and their people will have to accommodate the growing number of elder citizens with new jobs and services.

This graying effect is now underway in Mexico, where 17 was the median age in 1965 but is projected to be 31 in 2020 and 45 in 2050, according to U.N. and Mexican government statistics.

"Every three or four years there's an economic crisis that pushes back our progress, so we have to have 10 to 15 years of sustained economic development," said Rene Zenteno, who heads a demographic study center at the Monterrey College of Technology in Guadalajara. "We have to add 1.1 million jobs a year. If we don't do this, we are going to be a poor, old country."

And while experts still worry about the world's diminishing resources, they now focus more on how resources are distributed. Alejandro Cervantes-Carson, a sociologist at Guilford College in Greensboro, N.C., argues that the problem for Mexico and other countries is not a lack of resources, but social and economic inequities that concentrate them in the hands of the wealthy.

"There's no way to claim population is the bad guy in this story anymore," he said. "The problem is not that Mexico does not produce enough food to feed its citizens. It does. The problem is that so few are well fed."

POPULATION MOMENTUM

The world population is still rising, although birth rates have declined recently in many countries. China and Mexico are among the countries that have been particularly successful in controlling their population growth.

More than half of world population growth will occur in Asia, including 25% in India and China. Population in developed countries has been stable for several years.

Projected population growth in less developed countries, 2000-2050

Africa: 34%

Other Asia: 31%

India: 18%

Latin America: 10%

China: 7%

SOURCES: U.N. Population Division, Population Reference Bureau

CAPTION: Most of the population growth this century has been in less developed, countries:

(This graphic was not available)

CAPTION: WORLD POPULATION IN BILLIONS

(This graphic was not available)