A record 175 million people now live outside the country of their birth, according to a new United Nations population survey. Migration from the world's developing countries continues to replenish the declining populations of Europe and North America.

According to the U.N. report, immigration from countries such as India, China and Nigeria is providing a much needed boost to the labor pool in industrialized countries.

Ian Kinniburgh, the author of the 2004 World Economic and Social Survey, says international migration serves as a powerful economic tool.

"It is very important to see migration as part of the development process, both from countries of origin and the countries of destination," he said.

For example, according to the survey, migrants send an estimated $79 billion to their home countries, exceeding levels of development aid. Likewise, migrants fill jobs in countries such as Italy, whose labor force has declined by 40 percent and whose population is expected to decline 15 percent by the middle of the century.

But Joseph Chamie, director of the U.N. population division, says security concerns since the September 11, 2001 terrorist attacks on the United States have made migration to developed countries increasingly difficult.

"We have competing interests. Employers are often very eager to have increased migration for their firms, for their labor force, governments are often very interested in bringing in people to meet the needs of the private sector as well as undertake activities that others may not want to do such as picking crops or grapes or fruits," he said. "On the other hand, the public may have a very different view because they may see people coming in who are different, their habits are different and these are challenges the governments will have to deal with."

Mr. Chamie added that migration has always been central to globalization.

"This migration flow is nothing new. It has been going on for thousands of years. People have been redistributing themselves all over the globe. From the very beginning of humanity, you have seen people move looking for ways to improve their livelihood and benefit themselves. We have seen them move across continents, across large bodies of water," said Mr. Chamie. "It is just recently, the last several hundred years, that we have had governments and borders formalizing this process. Now the challenge is how to make that a win-win situation for all concerned, rather than a lose-lose situation where everyone loses out, the migrants, the receiving countries, the sending countries, the transit countries."

According to the survey, Europe's population will continue to decline, and the U.S. population will only modestly increase by the middle of the century.

But between 2000 and 2050, the world population will increase by nearly three billion. Six countries, China, Indonesia, India, Pakistan, Nigeria and Bangladesh, will account for more than half of that growth.