By Steve Schifferes
Economics reporter, BBC News
Migration is putting growing strains on rich and poor countries alike, according to the Organisation for Economic Co-operation and Development (OECD).
The immigration issue has sparked protests across the US
The OECD boss, Angel Gurria, says that rich country governments need to do more to help immigrants integrate and make better use of their skills, which will improve economic prospects and reduce backlashes against immigrants.
"Much greater emphasis needs to be placed on helping recent immigrants learn the host-country language and become familiar with workplace practices," he said.
The OECD's new research shows that immigrants now make up 7.5% of the population of the rich countries as a whole, with the highest proportion in Luxembourg (32%), Australia (23%) and Switzerland (22.6%).
In absolute terms, the US (31 million) and Germany (8 million) have the most immigrants, making up nearly half the total in the OECD area.
Overqualified and underemployed
But immigrants - who are on average more highly-educated than native workers - tend to be over-qualified for the jobs they are actually doing.
The OECD says more integration and training of immigrants is needed
One quarter have higher (university-level) education, compared to one-fifth of all workers in OECD countries.
But in some countries in Europe, for example, Italy, Spain, and Sweden, the share of immigrants working at jobs below their qualifications is twice as high as for native workers.
"Addressing the problem of over-qualification is all the more urgent as a growing number of countries wish to attract more highly-skilled immigrants on the grounds that they will help boost economic growth and have fewer difficulties integrating into society," the OECD said.
The UK is one of the countries that is shifting its criteria for immigration to a points system that places more emphasis on skills and education.
Another problem is the underemployment of women immigrants.
There are an equal number of men and women immigrants in rich countries, but 79% of male immigrants between 15 and 64 are working in the labour market, as against 59% of women.
The high rate of immigration in recent years has had a mixed effect on the economies of developing countries.
On the positive side, the funds sent home by immigrants have given a significant boost to the economies of many poor countries, with Mexico, the Caribbean Basin and South Asian countries benefitting strongly.
According to the World Bank, returning migrants can also be a significant source of technology transfer to poor countries.
But high rates of migration among highly-educated workers, such as doctors, from small and poor countries can deprive them of key personnel who would help their development.
Migration is highest from areas of strife and poverty
More than half of trained doctors from the small islands and Caribbean states of Antigua, Grenada, Haiti, Jamaica, Trinidad and Tobago, and Guyana are working abroad.
And there are similarly high proportions in conflict-torn African states of Mozambique, Angola, Sierra Leone, Liberia and Congo, as well as the poor but English-speaking Tanzania.
These countries have also lost a considerable proportion of their highly educated population as a whole, with 67% of university-educated Haitians and 72% of university-educated Jamaicans living abroad, as well as 27% of Kenyans and 33% of Ghanaians.
But larger countries such as India or Nigeria have only a small proportion of their total graduates working abroad, although their numbers are larger in absolute terms.
In terms of the number of immigrants it accepts, the UK stands broadly in the middle of the OECD.
British ex-pats are moving to Spain in growing numbers
But it does lead in one area - it has the most expatriates living abroad among all the major OECD countries except Mexico (which is by far the largest with 8 million expatriates).
The UK has 3.2 million citizens who live abroad, slightly more than Germany, which has a substantially greater population, and substantially more than the US, with five times the population.
The largest country of destination for UK emigrants is Spain.
The duration of the stay of people who immigrate varies widely.
In the UK, France, Germany, Holland and Australia, more than 70% of immigrants have been there more than 10 years.
But groups of new immigrants - for example, from the conflict-torn areas of Yugoslavia or the Middle East - have arrived more recently.
And newly affluent countries, such as Spain and Ireland, have recently experienced a reversal of fortunes, with immigration from North Africa and Eastern Europe replacing emigration to Western Europe and North America.
The OECD member countries are: Australia, Austria, Belgium, Canada, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Japan, Korea, Luxembourg, Mexico, the Netherlands, New Zealand, Norway, Poland, Portugal, Slovak Republic, Spain, Sweden, Switzerland, Turkey, the United Kingdom, and the United States.