By Dominic Casciani
There are millions of Britons overseas - and until now we have not known a great deal about them.
How much do we know about Britons who live abroad?
The story of Britons going abroad is similar to any other story about why people move: people make their own decisions on their future - and whether their choice of a new life shows up in statistics is largely a matter of luck.
For instance it would be easy to count British academics in foreign universities - you just need to ask the national institutions. It's a lot harder to count people who marry a foreign national or set up their own business abroad.
So what are the numbers?
The Institute for Public Policy Research estimates at least 5.5m British citizens are living permanently overseas.
On top of that, the IPPR estimates a further 500,000 live abroad part of the year - either because they work in other countries or have second homes.
That means that almost one in 10 of British citizens are living abroad at the moment. The researchers reached this figure by looking at national census data and other sources, such as overseas passport applications.
How has emigration changed over the years?
The peak was 1966-67 when 468,000 people left over two years. There were lows in the mid-1970s and 1980s.
The numbers picked up again in the 1990s, reaching the most recent peak in 2004. In 2005 alone, the UK saw 198,000 Britons leave for a new life.
But only 91,000 others returned - a net loss of 107,000. Over the course of 40 years, those losses average out at 67,000 Britons every year.
Where do we think these people are?
Some 41 nations each have at least 10,000 British residents, according to the IPPR's research.
Australia and Spain, as most people would guess, count for the most expats. The big English-speaking economies follow along with some of our European neighbours.
There are also large communities of Brits developing in rising Middle Eastern and Asian economies. Even South America is pulling them in: there are more British expats living in Argentina than there are in the Falkland Islands.
So who emigrates?
Many of those going appear to be young and highly skilled; the government estimated that four in 10 of those leaving in 2004 were in managerial or professional occupations.
The second group, particularly in Europe, are the middle-aged, retired or semi-retired, who are investing in foreign property. But as countries like Spain becomes pricey, the Brits are moving eastwards.
The IPPR estimates there are 10,000 Brits who live part-time in Bulgaria.
So why do people go?
Clearly people don't go somewhere unless they have a really good reason for wanting to go. Of all the people the BBC has spoken to, most say that they seizing opportunities that may never have come their way again.
The contrary view is that people go because they think that the UK has gone to the dogs. Perhaps a useful way to assess these two arguments is to look at how migration figures relate to the economy.
The UK has been pulling in people as it has economically grown. This success may also encourage people to look overseas.
Does any of this matter?
The first key issue is "brain drain". One estimate suggests the UK loses one in six of its graduates. Some nations try to attract back those who leave. This issue may dovetail with wider migration policy.
Can demand for economic immigrants be linked to levels of emigration? In some cases it clearly is: Australia and New Zealand, for instance, recruit key public service workers - while we very often try to recruit theirs.
In other areas - such as the demand for low-skilled workers - the link is very difficult to make. Another key issue is whether the UK makes the most of the networks expats create - particularly in business.
There are potentially some issues for policy planning: What NHS services should expat Brits should be entitled to? Should pensions sent overseas be indexed to the local cost of living?
Is emigration likely to grow?
History shows migration statistics are very difficult to predict. The government's own official pensions advisers suggest the net loss of British citizens will continue at about 60,000 a year until 2008.
The IPPR suggests the figure could be higher - between 500,000 and one million over five years, if the economic remains strong.