Sari-clad Nafis Sadik, a United Nations population expert, had only to visit a Chinese maternity ward recently to appreciate why ''you don't see so many children around in China today.''
Of the 25 women in the ward, only one had two children at home. The other 24 who were expecting either had only one child - or no child at all. What is more, all the women were determined not to have any more babies in a country now set on one-child families.
In Sri Lanka, another UN official, Tarzie Vittachi, landed by helicopter in a coconut tree-rimmed village where he immediately corralled the village men in a discussion of family planning.
''How many children do you have?'' he asked of a young married man who had submitted himself to a vasectomy.
''Three,'' the man replied. ''Two girls and a boy.''
''Aren't you worried if you lose your son and you can have no more?'' the official asked, conscious of the great stress placed on sons in the developing world. ''But I have two daughters,'' the villager replied proudly.
A social revolution is taking place in the world today - a noticeable shift in cultural attitudes and economic priorities - that goes some way to explain why for the first time in recorded history world population rates have been dramatically slowed. Population is still rising, but not so fast.
China and Sri Lanka are striking examples of this trend in world population, a trend that has taken population experts by surprise.
By the year 2000, world population will be as much as 20 percent less than had been projected in the late 1950s and 1960s, according to widely accepted UN projections.
That means that there will be 1.4 billion fewer mouths to feed at the turn of the century than had been previously anticipated. Instead of a global population of 7.5 billion by year 2,000, the UN is settling on the more modest figure of 6. 1 billion.
The effect is to muffle those shrill and alarmist voices raised 20 to 30 years ago that predicted an imminent demographic catastrophe in the world.
According to Mr. Vittachi, a UN population expert now attached to UNICEF, what is taking place in much of the world is not so much a population boom, but an aspiration boom.
''When aspirations are substantially met, population birthrate falls steeply, '' he said.
Helping this aspiration boom is an improved status for women in many parts of the developing world where birthrates traditionally have been highest.
In those developing countries such as Sri Lanka and Thailand where women are getting more control of their lives because of greater equality, higher status, more economic clout, birthrates are slowing. These women are more easily able to defer the age of marriage, perhaps by as much as seven years, which is a considerable factor in producing fewer babies. In many Islamic countries - particularly Iran, Iraq, Bangladesh, and Pakistan - the very seclusion of women builds a barrier to getting family planning information to them.
''I'm convinced,'' says Vittachi, ''that the population problem will never be resolved in the uterus, but in the human mind.''
While the slowing of the rates of growth delights and surprises both population experts and government planners, there is no cause for complacency.
Even with a noticeable dip in the rates of growth, densely populated countries like Mexico (current population: 72 million), Bangladesh (89 million), Nigeria (79 million), and the entire continent of Africa (486 million) will still double their populations within 20 years because of net annual population growth.
Put another way, while world population is growing at a slower rate than before, it is neverthless still growing. And since it builds from such a substantial base, the number of people added to the world's population is as much as 77 million each year. Or approximately the population of Nigeria today.
In practical terms, it means that for every two people on the globe today an additional person will be added by the year 2,000 - compounding the problems of finding food and shelter in a world of shrinking resources.
Unless current trends are reversed or arrested, supercities will emerge. Mexico City, with an estimated population of 31 million by the end of the century, will have the dubious distinction of being the world's most densely populated conurbation. Behind it will come another Latin American city - Sao Paulo in Brazil - with 25.8 million. Tokyo-Yokohama is expected to be the world's third-largest city with 24.2 million.
This is the result not only of continuing world population growth, but of rapid urbanization.
Many like Edwin Martin of the Population Crisis Committee in Washington see the slowing of the growth rate as ''a remarkable shift.'' But he remains concerned about the future.
The slowdown, he says, ''is not good enough. If there are going to be as many children born in the year 2,000 as there are today, we will have desperate problems.''
According to Mr. Martin, a former US ambassador and assistant secretary of state for economic affairs from 1962 to 1963, ''We still have a very substantial way to go when it comes to the urban explosion, the unemployment situation, hunger, environmental deterioration, and all the other problems of population we still haven't got under control.''
Lester R. Brown of Worldwatch Institute, the Washington-based global monitoring service, cautions: ''In our excitement and satisfaction at having slowed the population growth, we have overlooked the absolute size of the world population.''
Moreover, a significant decline in per capita amounts of such key commodities as wood, oil, fish, beef, and grain compounds the problems of increasing world population. What is more, the larger world population helps to cause the per capita commodity decline.
According to Worldwatch, per capita amounts of wood in the world peaked in 1967. Fish catches crested in 1970; beef in 1976; grain in 1978; and oil in 1973 . Since then, per capita amounts of fish have declined a further 13 percent, beef 10 percent, and oil 20 percent. The future of grain is uncertain.
''This reversal - this per capita decline - is, I believe, the dominant explanation of the extraordinary difficulties the world is having now,'' Mr. Brown says.
On the other hand, inflation, which is one such difficulty, is ''acting as a kind of contraceptive,'' he says. The reasoning is that while individuals may feel they have no control over the price of oil or food, they can at least control the number of mouths that have to be fed.
Despite these sober observations all the experts who were interviewed agree without exception that the slowing of the rate of population is a significant breakthough and that at last the trend is in the right direction. A march through time shows why:
It took most of the time mankind has been on this planet, perhaps as long as 4 million years, to reach its first billion in 1800; 130 years to reach its second billion in 1930; 30 years to reach its third in 1960; 15 years to arrive at its fourth billion, and only an estimated 12 years to rack up its fifth billion in 1987. The UN expectation is that the world population will stabilize at 10.5 billion in the year 2110.
The UN's efforts to control world population have been fraught with obstacles. There has been ''much opposition,'' concedes Rafael M. Salas, executive director of the United Nations Fund for Population Activities.
Mr. Salas, who was described by Werner Fornos of the Population Action Council in Washington ''as having almost singlehandedly brought the population issue to light,'' said in his UN office that, in order to survive predators, man throughout time has been pro-birth. ''And survival has been reinforced by all sorts of injunctions against curtailing birth.''
But economic imperatives and the UN's neutral political posture have apparently persuaded as many as 59 developing countries to overcome their religious, racial, cultural, and ideological objections and accept UN family planning programs.
''The most important thing is the realization by the developing countries that they can do something about population,'' says Mr. Salas.
The willingness of governments to adopt family planning programs is often proving crucial to its effectivess. To Gayl D. Ness, professor of sociology and director of the program in population planning at the University of Michigan in Ann Arbor, the readiness of so many countries to undertake population control measures represents ''a very profound policy revolution on the part of governments.''
Governments throughout history, he points out, have been pro-natalist: ''You work them; you tax them.''
Professor Ness goes so far as to say, ''Population is important, but far more important are strong political and administrative systems.'' The success of any population program is almost directly dependent on how much political muscle the government has in carrying these programs out.
Weak governments, as in the states of Bihar and Uttar Pradesh in India, result in weak population programs in those states. But states like Kerala and Maharashtra, which have good administration, have strong family planning programs, and much lower fertility rates.
''Its very clear that China uses very strong administrative pressures amounting to coercion at times, which has successfully brought down population rates. They have no qualms about getting people to have abortions,'' says Ness.
One of the key determinants in whether world population can be restrained is the success of China (with 22 percent of the world's population) in pushing forward with one-child families.
China has consistently brought down its rates of population growth. After Cuba - which had the largest decline in birthrate, 47 percent between 1965-70 and 1975-1980 - China was the next most successful country in curbing the number of births. It registered a 34 percent decline in the same period.
Its aim is zero growth by the turn of the century. But a recent editorial in China's official Communist Party newspaper, People's Daily, said the nation's policy of one couple, one child was going unheeded.
Traditional customs left over by the force of thousands of years - such as ''more children, more happiness'' and ''boys rather than girls'' - the editorial said were impairing the family planning work.
As in China, so in many parts of the developing world, there is a growing realization that the answer to world population lies not so much in medical advances or economic strategies. But, as a UN report put it recently, in ''millions of individual decisions taken by millions of individual couples.''