Think, for a moment, about this: * Last year the normally quiet and undemonstrative people of Assam, in northern India, suddenly rose up against poor, newly arrived immigrants from teeming Bangladesh to the south. The newcomers came looking for jobs in Assam's oil fields and tea plantations. Five million had come before them but Assamese patience snapped. About 3,000 people were killed.
* A few years earlier, Hanoi ordered a mass eviction of Chinese from Vietnam, amid much bitterness and tension.
* Not long ago, the government of Africa's most populous country, Nigeria (92 million) summarily ordered out tens of thousands of Ghanaian workers. The rest of the world watched on television as the workers lined up at their own frontier for days before it opened. Mothers and children lugged suitcases and radios. Others tried to bribe their way out on ships.
* After an attempted coup in Indonesia in 1965, young Muslims started a rampage against suspected communists. No one knows how many died, but the United States Embassy in Jakarta estimated 300,000, ''plus or minus 200,000.''
* A 1969 World Cup soccer match between El Salvador and Honduras escalated into war.
All these events had one thing in common:
According to on-scene observers, a strong contributing factor was overcrowding - the pressure of too many people jammed together in miserable conditions.
In separate interviews, neither Robert S. McNamara, former World Bank chief and US secretary of defense, nor Dr. Sharon Camp, vice-president of the private Population Crisis Committee, argued that overcrowding was entirely to blame.
Both stress that the causes of violence and unrest are complex and varied.
But both - together with other sources - insist that there is a definite link between rapid population growth and tension and violence, both within and between families, communities, and countries.
''A very substantial contribution to unrest is made by imbalances between economic and political advances and resources on the one hand, and rates of population growth on the other,'' is how Mr. McNamara puts it.
''I don't say that over-population is the primary cause of instability, tension, and violence,'' says Dr. Camp. ''I do say that rapid population is an underlying, intensifying cause.''
Adds Marshall Green, former US ambassador to Indonesia and currently a population consultant to the State Department: ''Excessive population growth combines with other factors to cause instability in a number of ways. Since the US is usually associated with the status quo, the existing order, around the world, it has much to lose. It has major security interests in a string of countries with high growth rates: South Korea, the Philippines, Indonesia, Thailand, India, Pakistan, Egypt, Turkey, Mexico.
''And remember: the George Ball Commission included overcrowding in south Tehran as one of three major factors behind the overthrow of the Shah in 1979. The other two were over-rapid industrialization and corruption.''
In Mexico City, Western diplomats think this case is proved by a look at the five small, desperately poor countries in Central America which generate daily headlines about unrest and revolution.
In 1960, Central America held 11.2 million people, and the US virtually ignored it. Today it bulges with 22 million, is growing at the rapid rate of 3 percent a year - and Washington is preoccupied with it.
Central American families have an average of six children each. Death rates have fallen dramatically. Predictions point to 40 or 50 million people locked into tiny areas of arable land by the year 2010. Half the region is under 15 years of age.
Moreover, the most volatile group in the population - the one aged between 15 and 19 - is spiraling upward. From 886,000 in 1950 it has leaped to 2.3 million today and is estimated to hit 5 million in 2010 unless growth rates fall.
A quarter of a million young Central Americans look for new jobs each year. But those jobs are scarcer and scarcer as poverty spreads and conflict continues in Nicaragua and El Salvador.
The result, according to the Kissinger Commission, is ''a problem of awesome - and explosive - dimensions.''
The Futures Group in Washington, D.C., studies the impact of population growth on economic and social development. It sees as one result of overcrowding in Central America more and more refugees streaming northward into Mexico and the US. A potential flow of 100,000 a year is a ''distinct possibility,'' the group told the Kissinger Commission.
A veteran Western diplomat in Mexico City comments: ''Mexico adds 2 million people every year. Brazil adds 3 million. Both countries owe colossal debts. You can't tell me that doesn't add up to a security problem for the US.''
Brazil, the diplomat says, added 26 million people in the decade to 1980 - equivalent to another Argentina. The chief of staff of the Brazilian armed forces is reported to have named population growth as the biggest threat to Brazil's internal security.
Those who reject the very concept of a world population crisis also disagree with the McNamara, Camp, and Green views.
Prof. Julian L. Simon of the University of Maryland at Baltimore is a leader of the right-to-life lobby in the US and a senior fellow at the Heritage Foundation in Washington. He has written a book called ''The Ultimate Resource'' which says that the world needs more people, not less. He is also co-editor of a recent Hudson Institute study called ''The Resourceful Earth.''
''I absolutely reject the idea that overpopulation leads to war,'' he said in an interview. ''I don't think population is a factor.... It simply doesn't happen that one group of people has children and sets out to attack another group next door for more space.''
Mr. Simon offered no detailed analysis, but dismissed those who link overpopulation and tensions as ''doomsayers.''
Replies Mr. McNamara: ''People who take such (Simon) views cannot have been in Bangladesh, where 89 million people lived in 1980 in an area about the size of Wisconsin. Two-thirds of the country is either flooded or arid, depending on the time of year.
''By the year 2000, 157 million people will live there. By 2025, 40 years from now, the population will be 260 million. Life is hell for 60 to 70 percent of the people there now.''
Mr. McNamara illustrated his concern in an article in the current Foreign Affairs quarterly. He mailed out 15,000 copies and arranged to have more distributed at the UN Population Conference in Mexico City.
Among other implications that worry him:
* ''National programs of coercion'' such as the current Chinese drive to limit family size to a single child. Rising growth rates, he thinks, are a major contributor to more authoritarian government and to a movement away from ''democratic structures.''
* ''Brutal family practices'' - increasing rates of abortion and female infanticide which he says even the Chinese press is acknowledging. All this, he says, increases tensions in families and thus sows the seed of wider frictions in societies.
Other authorities who link rapid growth and instability make these points:
Such growth worsens unemployment. It swallows up economic gains. It makes the task of governing harder. It widens the gap between rich and poor within and between countries. It balloons illegal migration across borders: Guatemalans and Salvadoreans emigrating to southern Mexico, for instance, and millions of Mexicans finding jobs in the US.
Over-rapid growth also breeds crime and tension in third-world slums: ''The last conversation I had with (the late president) Anwar Sadat was about overcrowding in Cairo,'' says Ambassador Green.
''The last words he said to me were that the crowding was an 'absolute nightmare.' ''
The ambassador sees third-world cities as ''the forcing bed of extremism.'' One of Egypt's leading experts on extremism, Dr. Saad Eddin Ibrahim of the American University in Cairo, says that young Muslim fundamentalists tend to be recent arrivals in slums from rural areas. Rootless and adrift, they are susceptible to extremist ideas.
In Cairo, the police academy recently held a seminar on the links between overpopulation and security. ''Communists, Islamic extremists, and sabotage groups all find it easier to work where population density is high,'' says Dr. Maher Mahran, population adviser to President Hosni Mubarak. In two pockets in Cairo, density is said to be about 130,000 people per square kilometer.
In Jakarta, Emil Salim, an Indonesian Cabinet minister, says, ''The world will not be safe with such population densities. Look at our own Java: If its population reaches 120 million by the year 2000, people will be forced to leave: to (go) where? Sumatra or Borneo, where languages and religions are different?''
''When the rural poor come to the cities,'' says Dr. Pramila David of Hyderabad, India, a physician with years of experience in the population field, ''they see what is invisible in their villages: great wealth which is out of their reach. This is particularly true when they find work as domestic servants.''
To observers such as the Worldwatch Institute's Lester R. Brown, the litmus test is food supplies.
''Look at the food riots in recent years,'' Mr. Brown says. ''Poland. Tunisia. Supermarkets looted in the Dominican Republic, in Rio, and in Sao Paulo. Overpopulation is a large factor.''
Even before the current drought in sub-Saharan Africa, almost one African in every four was being fed by imported grain: 130 million out of a total population of 536 million.
Now, grain production in Africa has begun to plummet as the continent suffers the fastest population growth rates in history, along with a lack of investment in agriculture, and an overexploitation of poor soil.
Between 1970 and 1982, African grain production fell 12 percent. In 1983 alone output fell another 14 percent, and the UN Food and Agriculture Organization in Rome says 22 countries are facing famine.
Meanwhile, the third world as a whole by 1980 was spending more money importing arms ($19.5 billion) than on importing grain ($19.45 billion), the Worldwatch Institute reported.
Where are the answers?
The rich, ''have'' nations of North America, Europe, and Australasia need to be much more aware of the potential impact of too-rapid population growth in the ''have not'' nations of the third world, experts say.
The US State Department and Agency for International Development (AID) urge continued public and private support and funds for family-planning services linked to maternal and child health care clinics around the third world. Asia and Latin America show some progress at slowing down growth rates, although little progress is apparent in Africa.
On the far right of US politics, Sen. Jesse Helms (R) of North Carolina, Professor Simon, and some Catholic activists say the answer lies in economic development directed by private industry rather than governments.
Controversial White House policy for the Mexico City conference states that family-planning aid should be de-emphasized. Such policies ''cannot be a substitute for economic reforms,'' it says.
State Department and AID sources agree that economic development is vital. But they also insist that third-world families deserve the right to make the same choices on limiting family size that the developed nations have long taken for granted.
Ambassador Green advocates more family-planning incentives, such as bonuses of either one payment of 500 rupees ($46), or 50 rupees ($4.60) a month for life , offered by one Bombay factory to workers who limit their families to two children.
He sees this as not coercive but as an added inducement to a voluntary decision.
The right-to-life lobby in the US, however, sees such incentives as tantamount to coercion, and opening the way to enforced abortion and sterilization. The lobby is trying to stop any direct or indirect US funds going to China or any other country using such incentives.
The AID budget for family-planning projects this year is $240 million. Since 1974 no funds are permitted to go directly to any country where family-planning programs include abortion. The White House this year has added that no funds may go indirectly via international voluntary groups.
The Sri Lankan government offers the considerable incentive of 500 rupees ($ 23) for every man and woman who chooses sterilization. Presidential adviser Dr. Wickrema Weerasooria in Colombo denies this represents coercion. He calls it compensation for the time people lose in having the operation. The government, he says, has to match private tea plantations offering 700 rupees ($32) to each worker.
Incentives are part of China's campaign to limit its population to 1.2 billion by 2000. One-child families receive priority in housing, child and health care. In some communes one-child parents receive bonuses equal to one-third their annual wages, and an extra plot of land.
India is experimenting with similar incentives in some states.
Indonesia on the other hand has rejected monetary incentives as too open to abuse.
Meanwhile, many experts stress that the answers to overcrowding and unrest lie in a range of other fields such as more schools, better health care, and ways to enhance the status of women.
Next: Education -- facts for third-world women