Some new studies of world population raise new questions of public policy and illuminate how complicated the problem is. It's commonly held that explosive population growth threatens the world's resources, the environment, economic development, and social stability.
All of this remains true, and there is no doubt about the rapidity of growth. Just in the period from 1950 to 1995 - a minute in the history of people on Earth - total world population more than doubled from 2.5 billion to 5.7 billion. If fertility rates remain constant, says the UN, there will be a total of 57.2 billion people in the year 2100 and a mind-boggling 296.3 billion in 2150.
But that's not all. Tremendous shifts are taking place in the geographic distribution of people. Again assuming constant fertility rates, the UN study estimates that the population of the industrialized world will be the same in 2050 as in 1995 (1.1 billion) while the population of the rest of the world will triple - from 4.6 billion to 13.9 billion.
There is another aspect of this phenomenon. Low birth rates in industrialized countries and high birth rates in the third world produce distortions in the age distribution of the respective populations. The iudustrialized world has too few young people; the third world, too many. Not only are fewer people being born in the industrialized world, those already here are living longer.
This is the basis of the looming crisis in the US over Social Security and Medicare. The cost of social benefits is going up, and there are fewer people producing goods, services, and taxes to provide the benefits.
There are analogous problems in Japan and in every country of Western Europe.
In most of these, the social programs are more generous than in the US, and the problem of paying for them with a declining work force is therefore more acute. In countries like Italy and Japan, even the problem of maintaining a growing economy may itself become more acute. Italy already has more people over the age of 60 than under the age of 20. Germany, Greece, and Spain are not far behind. In Japan, birth rates are so low that the Ministry of Health estimates the population will start to decline in 2007. This will pose in extreme form the problem of how fewer young people will pay for more old people.
Put these changes together and what you get for the medium-term future is intensified pressure for emigration out of the third world, matched by greater need for immigration into the industrialized world. Some people in the industrial countries are in a state of denial over this, but the movement of people is happening anyway, and it will continue to happen.
We've seen it for some time in the US with respect to Mexico and Central America. It's becoming more obvious in Europe: France is feeling more pressure from Africans crossing the Mediterranean; Britain from citizens of Commonwealth countries; Germany from Turks who were welcomed when there was a labor shortage, but not now when there is high unemployment.
Everywhere, these new waves of immigration are generating political turmoil.
Even in the US, which has more experience than Europe in dealing with immigrants, there is debate between proponents of more and of less restrictive policies. There is even more debate over what to do about the immigrants once they're here. Can we, or should we, assimilate them while encouraging multiculturalism? Is bilingual education a good thing or not? Is it even possible when immigrant children speak scores of different languages? There will be growing competition between generations for public resources as children need schools and the elderly need nursing homes.
THE problem is more difficult in Europe, where immigrants will be superimposed on more homogeneous populations and higher unemployment than in the US. Anti-immigrant reactions are especially notable in France, where they have fueled the growth of a radical rightist political party, and in Germany, where neo-Nazi skinheads beat up immigrants or burn their houses.
Generally speaking, the immgrants, whether in Europe or the US, bring with them a proclivity to have more children than Americans or Europeans. This implies, over time, changing ethnicity of the general population - fewer white European Americans and more Hispanics.
A good many adjustments are going to be required, and many people will not like them.
* Pat M. Holt, writes on foreign affairs from Washington. His most recent book is "Secret Intelligence and Public Policy," Congressional Quarterly Press, 1995 .