Mass migration: Who benefits and why?
It has often been called the brain drain - highly skilled workers in developing countries leaving their homeland to work in one of the rich nations.
It is perhaps best known in relation to healthcare professionals, but it also applies to many other groups, including computer software experts and a range of engineering specialisms.
The numbers are large. Some 214 million people are international migrants, living in a different country from the one in which they were born.
They are a very diverse group, with a very wide range of skill levels.
There are plenty with high level skills who end up working for at least part of their careers outside their home country.
Some take work they are overqualified for, because it still pays better than what is available at home. But others do use their skills.Lost skills
Highly skilled migrants are a minority, but an important one.
In the UK, 38% of immigrants have a degree level qualification, compared with 18% of the UK-born population, according to The Work Foundation.
Many of those highly qualified migrants are from other developed countries.
But there are also many who are not.
On the face of it, that looks like a bad thing for the developing nation they have come from, especially if it has paid for the skilled emigrant's education.
End Quote Offical Australia website for would-be immigrants
The points test is a transparent and objective method of selecting skilled migrants with the skills and attributes needed in Australia”
That nation loses skills that could have instead contributed to developing the local economy or health services and it appears that the investment in educating them has been wasted.Pay and benefits
Many rich countries actively encourage immigration by people with particular skills that are in short supply.
They are generally not shy about it.
This is what Australia says on an official website about the points system that it uses in assessing would-be immigrants: "The points test is a transparent and objective method of selecting skilled migrants with the skills and attributes needed in Australia."
There's a pass mark you need to achieve for certain visa types, and that pass mark "is subject to change in response to Australian labour market needs".
Australia's strategy gives a clue as to why countries are so keen to attract migrants with the skills that are needed.
If the set of skills required change, it is much harder to keep up if you want to rely on those who have been trained domestically.
It takes time - often years - to train people to take on jobs in highly skilled occupations.
The clickable guide gives some examples of what the developed countries have done to encourage migrants, and of the types of skills that are in high demand.
It also shows some well-trodden migration routes used by skilled workers.
For the skilled workers themselves, pay is obviously part of the motivation for uprooting themselves.
But in our case studies, some other factors come out very strongly.
They may not always be factors that encourage the move in the first place, but they can make it attractive to stay in the new country.
- Rebecca, the nurse from the Philippines, enjoyed the greater professional independence she found once she had started work in the United States.
- Thuy, the Vietnamese IT engineer, liked the fact that migrant staff in Norway were paid as much as the locals.
Often, what takes a migrant abroad is the availability of training. At times, they have to move to acquire skills in the first place, and some stay put once they have completed their training.Reverse flow
There is a more positive view of this type of migration.
Rather than brain drain, some prefer to call it the brain gain, and the Economist newspaper captures the main arguments here.
For one thing there are remittances - the money that migrants send home.
Of course you do not need the skills of a brain surgeon to be able to do that, but if you are that well qualified, the chances are you will be earning more, thus better able to send more back.
Remittances are now seen as an important source of funds for economic development.
These financial flows have grown rapidly. Last year, official figures showed that some $400bn (£263bn) of this money was being sent back to developing countries and there is a lot that is unrecorded.
Already this fast growing source of funds is far larger than official development aid, according to the World Bank.
Remittances are important tools that help reduce poverty, aid families who want to invest in education, or provide funds for small businesses, according to the Migration Policy Institute.
And of course, some of those skilled workers do ultimately return home.
Take the many Indian computing experts who went to work in California's Silicon Valley.
Initially, this drained India's resource of computing experts. But now, the country has its own thriving computer services industry and even a city, Bangalore, that has earned the nickname, the "Silicon Valley of India".