The jump in renunciations reflects evolving views about national identity, said Nancy L. Green, an American professor at the L'Ecole des Hautes Etudes en Sciences Sociales in Paris. When the U.S. got its start, citizenship was defined by "perpetual allegiance" — the British notion of nationality as a birthright that could never be changed.
American colonists rejected that to justify becoming citizens of a newly independent country. But changeable citizenship wasn't widely embraced until the mass immigration of the late 1800s, says Green, a historian of migration and expatriation.
Even then, U.S. artists and writers who moved to Europe in the 1920s were criticized, suspected of trying to avoid taxes. Until the 1960s, U.S. citizenship remained a privilege the government could take away on certain grounds. It's only since then that U.S. citizenship has come to be viewed as belonging to an individual, who could keep — or surrender it — by choice.