People are pouring into the world at spectacular speed. In the minute it takes to read these words, 150 babies will be born. In one year they will be joined by 80 million others -- the equivalent of another Mexico. In just 16 years world population will swell to 6 billion people, concentrated mostly in the poorer, overcrowded two-thirds of the world where life is already extremely difficult -- Chinese cities, Bangladeshi villages, Kenyan slums, Nigerian hovels , Latin American shantytowns, and on the open, teeming streets of Calcutta and Bombay.

What is to be done? Can third-world men and women be given the same opportunities to have fewer children that adults in richer nations have long taken for granted? Can the status of women be raised and their chances for education be increased? Can the fear of infant mortality and of old age without large numbers of sons be overcome?

As the second United Nations World Conference on Population opens today in Mexico City. the Monitor begins a five-part series that asks, and tries to answer, these vital questions based on research in the villages and capitals of nine countries on four continents.

He Li-Lian in Peking knows it. Her daughter has one child, ''and if she came to me saying she was to have another,'' she says flatly, ''I would order her not to.''

Saroj Bala, a tiny figure in a blue and yellow sari in a New Delhi slum, knows it. Married to a poor picture-framer, she is in her early 20s but has limited her family to two children.

Neneng Nasir in Jakarta, knows it. She has three daughters, ''and three is enough,'' she says.

Yet many people in the affluent Western world still don't know it - or they've heard it so many times that they have tuned out:

The already overcrowded third world - Asia, Africa, and Latin America, where some 3.6 billion people, or three-quarters of the globe live - is still adding so many people at such a dangerous speed that the quality of life there is seriously threatened.

Much has already been done to combine economic development with available family-planning methods and information. Rates of growth for the world as a whole dropped from 2 percent to 1.7 percent a year in the decade to 1984.

Yet that fall was mainly in the Western, developed world and China.

Everywhere else, much, much more remains to be done.

A sheer and growing weight of numbers is combining with other factors to thin out food and water supplies, to swamp schoolrooms, to pour ever more rural illiterates into the teeming shantytowns of mushrooming cities, to heighten tensions, and to contribute to restlessness and violence.

Each year the world as a whole is adding the equivalent of another Mexico (almost 80 million people) - and 73 million of them are in the third world.

The rate of growth in poorer countries since World War II is unprecedented in history, as the World Bank Development Report for 1984 points out.

By 1990 the globe will be adding another Nigeria (90 million) a year: that means 250,000 daily, or some 10,417 new people every hour.

The population in the third world alone, the World Bank says, is expected to soar to 5 billion by the year 2000 (more than the entire population of the world today) and to 7 billion by 2025.

Already about half of the third world is aged 16 or less. Forty percent (1.4 billion) is under 14. These youngsters will soon be bearing children and looking for jobs.

It took Europe hundreds of years to achieve the kind of growth that is occurring in the third world today. When Europe was building the factories of its industrial revolution, its population was growing at about 1.5 percent a year. It had wide open spaces of land, and those who wanted to emigrate had the world to choose from.

Today, large-scale emigration from the third world is simply not possible: Habitable places in the developed world are crowded too. Fertile land is at a premium.

Nonetheless, the United Nations Fund for Population Activities (UNFPA) in New York is able to point to some encouraging progress since the first World Population Conference in Bucharest, Romania, in 1974:

* Millions of women around the world have learned that they can control the size of their families and have done so.

* Some 85 countries in the developing world, containing 95 percent of its people, now provide some kind of public support to family planning programs.

* China, India, Indonesia, Thailand, Sri Lanka, Tunisia, Mexico, and some other countries have linked economic development, literacy, jobs, and status for women with family-planning programs.

''Yet the distance we have to go is much further than the distance we have come,'' says an experienced US official.

Excluding China, the third world is still expanding at the rate of 2.4 percent a year: ''And if the rate stays at 2.5 percent for the next 100 years, by the year 2100 the third world alone will contain 60 billion people,'' says Dr. Ansley Coale, Princeton economics professor and demographer.

Ridiculous? US officials point to growth rates already higher than 2.5 percent in key areas today - West and East Africa (above 3 percent a year), Central America (2.8 percent), Mexico (2.6 percent).

Dr. Coale, who is also chairman of the US National Academy of Sciences' Committee on Population and Demography, says that about 800 million people live in areas where family size has not fallen at all: ''Bangladesh, Pakistan, Africa , and a belt of Muslim countries.''

Some experts see the threat of widespread famine before the end of the century if urgent steps are not taken. Dr. Coale doesn't agree - ''but more people will live in poverty,'' he says, ''and there'll be a shameful lack of progress.''

Growth rates in the third world excluding China have actually edged up since 1975.

Raphael Salas of the Philippines, the veteran executive director of the UNFPA , hopes that world population might stabilize around the year 2100 at 10.2 billion.

Other, more pessimistic demographers say the world simply won't be able to sustain unchecked growth. Death rates would have to rise, they say, either through famine or wars for resources.

The average family in the Western world has two children. In the third world it has 4.4. Excluding China, it has 5.1. In Kenya, which is growing faster than any other country in the world, the average family has eight children.

''Population'' and ''family planning'' and ''development'' are abstract nouns that tend to sound remote and dull.

In uncrowded North America, Europe, and Australasia, populations are getting older, not younger. Growth rates are low (0.9 percent in the US, for example).

Yet what is at issue in the third world is far from abstract. The issue is individual lives and decisions.

How many children to have is hardly a dull question.

It involves what people think, and know, about their entire lives - beliefs, values, and traditions as well as health, jobs, and prospects.

Ultimate answers must tackle all these areas, including, but ranging wider than, contraception and health care.

Men and women need access to new information and opportunities before they can decide to have fewer children.

In Africa, tribal families have eight or more children per family because death rates are still high. Fathers want sons to work their land and care for parents in their old age: Female infants are still drowned (in parts of China) or starved (in parts of India). In too many countries, the status of women is still low.

Each country has to find its own ways to tell men and women about the benefits of smaller families, and to offer them a choice. There is no single panacea.

On one level, long-term answers must include faster economic development, more schools, more jobs, more housing, higher incomes.

Short-term solutions include expanded, more efficient, and more sustained health and voluntary family planning policies.

On a deeper level, the need is to encourage fundamental changes in thought - the kind of awakenings that lead from ignorance to shifts in the ways rich and poor people alike see their own identities. The task is finding ways to overcome fear - of illness, of old age, of a seeming lack of virility among men if children are few. In Africa, men fear that women who adopt contraception may be tempted into promiscuity.

Raphael Salas of the UNFPA put it this way in an interview in New York:

''The ultimate decision is in the human mind. We need the type of education that makes citizens of the third world think like people in the developed world. . . the opportunity to have fewer children per family and to treat them like human beings.''

''Unless the rate of world population growth slows markedly,'' says Lester R. Brown of the Worldwatch Institute in Washington, ''improving the human condition as a whole will be difficult.''

China, (pop.: 1.05 billion), has taken the most radical and controversial step to counter overpopulation so far: enforced limitation of most families in an entire nation to a single child.

''If you Americans had 1 billion people and were growing at the rate of 2 percent a year as we were, you'd do something too,'' said Mrs. He Li-Lian, a diplomat, defending such drastic measures in an interview in New York.

''Our policy is only for two decades ... and since 1982 we've got our growth rate down to 1.1 percent. Our people understand ... my daughter understands ... individual and national interests must be balanced,'' she said.

The Chinese method is an extreme example of government control over individual lives. It involves virtual coercion of women, enforced abortions, sometimes in advanced stages of pregnancy, some enforced sterilizations, and tempting incentives.

The Indian program, the oldest in the third world, has been voluntary, except for a brief period in the mid-1970s when overzealous doctors sterilized some 2, 000 men against their will.

The main method in India (pop.: 747 million) has been sterilization of women after they have had four or five children. This accounts for about 85 percent of all Indian contraception so far.

But only 26 percent of adults between the ages of 15 and 49 practice contraception. The official goal is 60 percent by the year 2000. (Now India is about to join with the US Agency for International Development to market contraceptives through a semiprivate organization.)

If the goal is to be reached, younger mothers need to use artificial birth control to space their children more widely before considering sterilization.

Saroj Bala, who lives in the hillside Delhi slum of Anand Parbat, is just the kind of woman the Indian government wants to reach - but so far, she is a tiny minority. She and her husband together made the decision to use contraception.

Mrs. Saroj says, ''I'm only 22, and I'm using an IUD. I want to be able to educate our children properly and feed them.''

Mrs. Neneng, in Jakarta's Tamansari quarter, is part of a successful Indonesian program to change thought about family size. The country now numbers 160 million people and is expected to grow to 204.5 million by 2000 (and to 255. 3 million only 25 years later).

Indonesia stresses literacy, out-in-the-village health care centers, nonmonetary incentives including public recognition and medals for long-term family planning users. It also has an innovative, creative government information program firmly supported by President Suharto himself and carefully designed to include and persuade local Muslim leaders.

Indonesia's growth rate is down to 1.8 percent a year, below the Southeast Asian average of 2.1.

Worrying the experts most is Africa, whose population is already 536.6 million.

''Africa has the fastest growth rate of any continent in history, and the fastest physical deterioration - deserts spreading, forests denuded, land overgrazed,'' says Lester Brown.

''Asia has had its green revolution in agriculture, but Africa is slowly losing the ability to feed itself.''

Overpopulation, tribal rivalries, and widespread polygamy, together with the current drought south of the Sahara, primitive farming methods, and thin soil, mean that 22 countries now face starvation, according to the UN Food and Agriculture Organization in Rome.

A powerful group of ''pro-life,'' anti-abortion lobbying organizations, however, in the US - the biggest donor of family-planning aid in the world - denies that there is a population crisis.

The group sees people as the globe's ultimate asset. It regards coercion and abortion as a sin. It looks to private industry to provide what it calls the best contraceptive of all - economic development based on capitalism.

These views are on the far right of US politics. The group has used its ties to the Reagan White House in an election year - the Mexico City conference is being held just before the Republican Party convention in Dallas - to shape a new US policy that reduces US funds to private groups which finance any programs abroad that include abortion.

The new White House position at Mexico City - stressing economic development over family planning - comes as a dramatic switch after a decade of pressing the third world to tell its people about family planning.

The group lobbied hard (and successfully) for the US delegation to Mexico City to be led by staunch Roman Catholic antiabortionist, James Buckley, former Republican senator from New York.

''It is a disgrace and the US will be a laughing stock,'' says Dr. Sharon L. Camp, vice-president of the private Population Crisis Committee in Washington.

''We are very pleased,'' says Judie Brown, president of the American Life Lobby in Virginia, which claims strong support from blue-collar Roman Catholics. ''It's a victory for us.''

The UNFPA is confident its own contributions from the US will be untouched, but a question mark now hangs over one-quarter of the $60 million annual budget of the International Planned Parenthood Federation in London. The 25 percent is provided by the US Agency for International Development.

Prominent in the Washington lobby group is Prof. Julian L. Simon of the University of Maryland at Baltimore.

''No, I don't see a population crisis at all,'' he says. ''The ultimate resource is the human spirit, which means people.'' In the short run, he adds, more people are a burden: They need to be fed and clothed. In the long run, however, they are a benefit.

The way to boost food production, Simon argues, is to give people land and information about the latest methods, then ''leave them alone.'' He blames inefficient third-world governments for mismanaging food supplies.

''Who are we to tell other countries what to do?'' he asks.

Judie Brown adds, ''We should be educating people how to better clothe and feed their families instead of using US taxpayers' money to eliminate their children....''

In its just-issued World Development Report 1984, the World Bank discerns ''important truths'' in the views of both population lobbies and Mr. Simon. But it also calls for intensified family-planning programs to ameliorate development problems which it sees arising from rapid population growth.

The Population Crisis Committee says the need for family planning is evident from even brief visits to villages and slums.

Monitor interviews with more than 50 men and women in slums and villages in eight countries indicate that many would have fewer children if they could.

In the vast Mexico City slum of Netzahualcoyotl, where 4 million people exist on unpaved streets amid uncollected garbage, Sofia Salinas Ugalde, mother of two sons, says: ''Yes, it bothers me to disagree with the Church. But to see children born unwanted, and growing up suffering, bothers me even more.''

Mrs. Ugalde runs a fly-ridden meat stall in a market with her husband. She practices contraception. ''Abortion is wrong,'' she said. ''I agree with that. So it's much better to prevent conception in the first place.''

Next: The supercities

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