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America's Berlin Wall
...Under the proposed legislation, expatriates surrendering their citizenship with a net worth of $2m or more, or a high income, will have to act as if they have sold all their worldwide assets at a fair market price. If the unrealised gains on these assets exceed $600,000, capital-gains tax will apply. A study by the Congressional Budget Office guesses that the new law will progressively net the government up to $286m over five years. It is unclear, however, why people would suffer the consequences if they did not expect to save money in the long run by escaping American taxes.

That expats want to leave at all is evidence of America's odd tax system. Along with citizens of North Korea and a few other countries, Americans are taxed based on their citizenship, rather than where they live. So they usually pay twice—to their host country and the Internal Revenue Service. As this makes citizenship less palatable, Congress has erected large barriers to stop them jumping ship. In 1996 it forced people who renounced citizenship to continue paying income taxes for an extra ten years. Theoretically, the new law allows for a cleaner break.

But even as the law tries to prevent people from renouncing their citizenship, it may have the opposite effect. Under the new structure, it would make financial sense for any young American working overseas with a promising career to renounce his citizenship as early as possible, before his assets accumulate. For everyone else, plunging stock and property prices mean now may be as good a time as any to hand back the passport, says Kurt Rademacher, a partner at Withers, a global tax-planning firm....