“THIS is a remarkably rewarding industry,” Timothy Harris, the prime minister of St Kitts & Nevis, told the leaders of three similarly small island nations—Antigua, Dominica and Grenada—last month. Selling passports is indeed a boon to those four mini-states. Clients do well too: their documents give them visa-free entry to many countries, including Britain and the 26 European nations of the Schengen area. (In May, Grenada and Dominica joined the other two in enjoying Schengen access.) Apart from money, the islands don’t ask much. You have to spend five days on Antigua; the others do not demand even that.

In St Kitts, this year’s budget statement called the sales a “mainstay”. The country’s income topped $100m in 2013: over 13% of GDP, and more than one-third of state revenue. It entered the business in 1984 but sales surged after the terms were revised in 2007. A cheap option is a $250,000 gift to the Sugar Industry Diversification Foundation, a state body that has published no accounts since 2011.

Every so often, there is trouble. In 2013 an Iranian flew to Canada on a St Kitts passport; he falsely claimed to be meeting the prime minister. Canada reacted by imposing a visa rule. Last year America’s Treasury said Iranians had used St Kitts passports for dodgy financial deals. The island says it has tightened up, stopping Iranians and Afghans from buying its documents and insisting that country of birth, and name-changes, be shown.

Antigua decided in 2013 to enter the trade; it says this year’s takings will be one-quarter of state revenue. Up to last month, it had sold 510 passports: two-fifths went to China and a third to the Middle East.