AT THE height of the Dutch golden age, merchants exported their goods and their families to colonies on four continents. Four centuries later their descendants are less impressed by such adventuring. A new law proposed by the Dutch government aims not only to limit dual nationality among immigrants (in 2011 around 20,000 people gained Dutch nationality through naturalisation) but also to make it easier for the authorities to strip members of the 850,000-plus Dutch diaspora of their nationality, should they secure a second citizenship abroad.

Guus Bosman, a Dutchman living in Washington, DC, calls the proposal “mean-spirited”. Eelco Keij, a Dutch citizen in New York and one of the loudest critics of his government's proposals, thinks that these days dual nationality is no more than “a harmless side-effect of globalisation”.

By seeking to toughen its nationality laws, the Netherlands is bucking a global trend. Other governments have increasingly abandoned such policies. In 2008 the Migration Policy Institute, a think-tank, found that almost half the world's countries tolerate dual nationality in some form. Armenia, Ghana, the Philippines, Kenya, Uganda and South Korea are all recent reformers. Haiti and Tanzania have new laws in preparation. Even Denmark, which places strict restrictions on citizenship, is mulling a change.

The idea that it is possible, let alone desirable, to allow multiple citizenship is relatively recent. In 1849 George Bancroft, an American historian and diplomat, said that for a man to have two countries was as intolerable as for him to have two wives. In 1930 the League of Nations proclaimed that “every person should have a nationality and should have one nationality only”. A treaty in Europe required countries to limit dual citizenship, until it lapsed in the 1990s. Immigrants have commonly had to renounce their old citizenship when taking on a new one; the countries that they left have often disowned emigrants naturalised abroad. These practices were intended in part to preserve the sanctity of citizenship, but they have also been aimed at closing loopholes that might allow migrants to escape taxes or conscription.

One reason for more liberalisation is practicality: dual nationality has become harder to control. Increased migration and rising numbers of cross-border marriages mean that ever more children are born to multinational families. The number of Dutch citizens holding a second nationality, for instance, almost tripled to 1.2m between 1995 and 2010, with newborns accounting for a significant share of the growth. Governments could once force women to take only their husband's nationality, says Maarten Vink of Maastricht University. In an era of sexual equality such policies are untenable.

Governments that take in many immigrants also see benefits from allowing them to keep their old passports. Research suggests that immigrants who do not fear losing their existing nationality are more likely to pursue naturalisation in their adopted countries—and subsequently more likely to integrate than those who maintain long-term residence as aliens. (Whether they go on to make better or worse citizens is harder to prove.)

Tides and dykes

Conversely, countries that send migrants abroad want to make sure that expatriates do not become ex-patriots. For poor countries, diasporas are a source of remittances, of political clout and of reflected glory. The first Costa Rican in space was a naturalised American citizen, Franklin Chang-Diaz. Some want to atone for past mistakes. Spain's Law of Historic Memory tried to heal the wounds of the civil war by offering dual citizenship to the descendants of those who had fled Franco's regime. Long queues formed outside Spanish consulates in Havana and Buenos Aires on December 27th, the final day of this three-year scheme.

Few tolerant states now want to reverse their dual-citizenship reforms, although some seem keen to stop further liberalisation. In November politicians in Germany, which generally offers dual nationality only to applicants from Europe, turned down a proposal that would have allowed Germans born to foreigners to retain their parents' nationalities in adulthood. From January 1st new citizens in France are required to sign a charter accepting that they “will no longer be able to claim allegiance to another country while on French soil”, even though dual nationality remains tolerated. Marine le Pen, leader of France's far-right National Front, wants to end dual citizenship altogether.

New world, new passport

Other countries have embraced reform reluctantly. America's citizenship ceremony continues to demand that candidates “renounce and abjure all allegiance and fidelity to any foreign prince” despite its government's largely liberal approach to the issue. In 1967 it took a Supreme Court ruling to confirm that dual citizens voting overseas should not lose their American nationality. America's unusual requirement that its passport-holders pay it tax no matter where they live gives many qualifying residents good reason not to apply.

In large parts of the world, especially in poorer and more dictatorial countries, dual nationals remain anathema. Less than half the countries in Africa condone dual citizenship. Asian holdouts include Japan and Singapore. China insists that its sizeable diaspora may hold only one passport, but makes it easy for ethnic kin naturalised abroad to return home when they wish. India now issues “overseas citizenship” to emigrants forced to renounce their birth nationality by the country's exclusive laws. This gives them many of the privileges enjoyed by their fellow Indians, but not the right to vote. Christian Kälin of Henley & Partners, a Swiss-based law firm specialising in what it terms “citizenship planning”, says a more formal tolerance of dual nationality there is likely.

For many ordinary citizens, dual passports still seem dodgy: a convenience for the cosmopolitan few or a sop to the menacing many, rather than a natural feature of a migratory world. A poll in May showed that over 60% of Dutch adults, much concerned by tides of immigrants from Morocco and Turkey, find dual citizenship undesirable. The tighter rules are part of the coalition agreement behind the minority government that is backed from outside by Geert Wilders's nationalist Freedom Party. The golden age of multiple nationality may be dawning. But it is not here yet.