Knock, knock. Who's there? Migrants. Millions upon millions of them, who would like to enter the United States in search of a better life.

No public opinion poll in the past 50 years has found a majority of Americans favoring increased immigration. Nonetheless, the total number of legal and illegal immigrants keeps growing, and the prospects of Congress acting to shrink the inflow are dim.

Is there a solution?

Over the long, long run, pressures to migrate to rich nations will ease as prosperity abroad spreads. Also, birth rates in poor nations - including Latin America - have been declining precipitously. Yet the world as a whole still adds 76 million people a year, mostly in the Far East and Africa, to today's 6.5 billion.

Since Mexico is the largest source of immigrants to the US, its population picture is the most relevant to citizens here. Each Mexican woman had an average of seven children in 1955, five children in 1980. Today, that number is 2.4. Within five years, as urbanization continues, Mexico's fertility rate should drop to 2.2. Soon thereafter, it could decline to 2.1, the "replacement rate," at which time Mexico's population will cease to grow - once the baby bulge has worked its way through the demographic situation.

By mid-century, though, Mexico will have about 139 million people, up from 107 million today. Unless there is a major improvement in the Mexican economy, the US will remain a big pot of gold to many Mexicans.

For decades, the supply of immigrants from many nations will be a large multiple of the US demand for new workers, notes Joseph Chamie, research director of the Center for Migration Studies, a New York think tank. The US must eventually put the brakes on illegal immigration, says Mr. Chamie, who was formerly head of the UN's Population Division.

If the status quo continues, the US population will soar from 300 million later this year to 400 million by 2050. The average age of Mexicans is 25, compared with 36 for Americans. So young Mexican immigrants on average will have far more children than American citizens will.

Without new immigrants, the US population will grow only to 320 million. But keeping the status quo in the US, including porous borders and not enforcing existing laws against hiring illegal aliens, is "politically advantageous," notes Chamie.

"It postpones making important - possibly painful - decisions, thereby avoiding alienating voters and important special-interest groups in an election year," he told the Population Association of America. Elected officials get "bulletproof" political cover by speaking of a "complex and difficult" issue, but not acting.

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