In countries all over the developed world, people are living for longer and having fewer babies. So even in the current hard times, governments know they need migrant workers to keep their economies going and fund growing numbers of pensioners. Foreign workers are welcome. But then again, they are not. Many countries are struggling with a backlash against immigration: increasingly vociferous groups say newcomers from poorer nations are taking ever more precious jobs and are changing the social and cultural make-up of their countries.

So will leaders come under increasing pressure to select the religion and skin colour of migrants they invite to make up their workforce? Spain is, perhaps inadvertently, doing exactly that. Under the left-wing government of José Rodríguez Zapatero, Spain is facing up to the painful truths about its civil war (1936-1938) and the Franco era (1939-1975). The immigration laws have been changed so that anyone whose parents or grandparents fled Spain between 1936 and 1955, and lost their citizenship in the process, can claim it back.

“This redresses a great anomaly in our national history,” says Agustín Torres, director general of immigration. The government estimates that up to 500,000 people, out of the one million eligible, might take up the offer, and there has been a rush on embassies in Cuba and Argentina. Many of those applying may just be keeping their options open. They are born Mexican, Cuban or Argentine and don’t particularly want to leave. But if their home economies fail, it helps to have a second passport to whisk them away.

The legislation, which came into effect at the end of December, is designed to correct the wrongs of the past. But the decision comes as Spain battles immigration problems, and it has recently started offering to pay economic migrants to return home. Latin American immigrants, however, are easier to integrate than people from North Africa and the Middle East (Spain already has over two million Latin American migrants and about 800,000 from Arab-speaking and Muslim countries). Latin Americans speak Spanish, have a Catholic heritage, and their histories are closely linked with Spain. “We can’t just open the doors unlimited to everyone,” says Torres. “It makes sense that we do everything for those who have a right to be Spanish.” In the long term, if significant numbers of descendents of political exiles claim their right to Spanish citizenship, the new legislation could add significantly to the numbers of people with a similar culture who could claim their right to choose to live in Spain in the future.

“All migration policy is, in a way, social engineering,” says Demetrios Papademetriou, president of the US-based Migration Policy Institute. He says that Spain (which has one of the world’s lowest birth rates) has been “immigration happy” in the past decade, in particular recruiting immigrants from former colonies. Since 2000, its foreign-born population has grown faster than that of any other country. Today, migrants make up nearly 10 per cent of the population (five million, compared to half a million just over a decade ago). And Spain now faces a problem supporting those immigrants, many of whom are losing their jobs: by January 2009, more than five million people were unemployed compared to 1.8 million a year earlier.

Other countries face similar problems: Italy, Greece, Japan and Ukraine all have plummeting birth rates. The UN ­predicts that Ukraine’s population will drop from 46 million today to 37 million by 2050. While they may not have Spain’s historical errors to correct, all have large communities overseas.

As these countries look at expanding their workforces, will they encourage their own people to return? Perhaps the global economy will do the job for them.

During Spain’s civil war and the Franco era, hundreds of thousands fled the country. Some were ordinary citizens who returned when the violence subsided, others were political exiles who could not return while Franco remained in power. Above, we chart where the largest groups took refuge.

(Source: Asociación de Descendientes del Exilio Español. Figures for Cuba are not included.)

We picked 10 nations with plummeting populations but large numbers of citizens living somewhere else. The figures show the largest groups of people born in each of the 10 countries who are now living elsewhere.

(Source: Development Research Centre on Migration, Globalisation and Poverty, University of Sussex, UK.)

Australians in the UK

Most Australians have some British or Irish blood if they trace their family lines far enough back. So it’s not surprising so many chose the UK as their base (if only temporarily).

Spaniards in France

Thousands of Spaniards fled to France during the Spanish Civil War and an estimated half a million when Franco took power in 1939.

Japanese in the US
Japanese mass immigration to the US began in the 1880s, most settling in Hawaii or on the Pacific Coast.

Italians in Germany

Italian emigration to Germany took off in the 1930s. They are the second largest immigrant community there today, after the Turks.

Poles in the US
Chicago is home to the largest ethnic Polish community outside Poland. There are 100 Polish language newspapers in the US.

Ukrainians in Russia
Ukrainians are the third largest ethnic group in Russia.

Koreans in the US
The number of Koreans in the US grew from 38,711 in 1970 to one million in 2007. The largest communities live in California, New York, New Jersey and Illinois.

Germans in the UK
It goes a long way back. First German traders in the Middle Ages, then German protestants after the Reformation moved to the UK. Today, many of the clusters of German-born people in the UK live in wealthy London neighbourhoods.

Singaporeans in Malaysia
Tiny Singapore was part of its larger neighbour Malaysia for a few years before it became a sovereign state in 1965.

Lithuanians in Russia
Lithuania was ruled by Russia for over 120 years until it declared independence in 1918. But Russia grabbed it back again in 1940. Since the breakup of the Soviet Union, Russia has been a major destination for Lithuanian emigrants.