How do you become a dual citizen?
Let's say I want to work in the UK for a long time. Is it possible to obtain dual citizenship as an American and a British citizen?
Each country uses its own laws to define who its own citizens are. Countries traditionally recognize natural-born citizens using standards based on at least one of two concepts: jus soli, legal Latin for "right of the soil," meaning that a citizen is someone born within the country's territory, or jus sanguinis, "right of blood," meaning a citizen is someone born to one or more parents who are citizens.
We can see immediately, then, that it's possible for a person to have equally legitimate claims to citizenship in two countries at once: imagine a child born in the United States to, say, Italian parents. The U.S. is a jus soli nation (though not exclusively – jus sanguinis operates as well, within certain restrictions), and automatically recognizes as a citizen anyone born inside its borders; Italy is a jus sanguinis nation and recognizes as a citizen anyone born to Italian citizens. Someone who, like this hypothetical child, can claim citizenship in two nations is said to be a dual citizen.
If you're not born a citizen of a given country, the process of becoming a citizen there is called naturalization, and the possibilities of dual citizenship get a bit more confusing when the second citizenship is attained this way. In many countries, newly naturalized citizens must renounce their prior citizenship claims. And some countries, by law, automatically revoke the citizenship of one of their own who accepts naturalization in another country. Even those born with dual citizenship may be caught in a legal vise in some cases, as certain countries place the onus on citizens to renounce any other citizenship claims when they reach the age of majority.
The specifics of renunciation and acceptance, as you might imagine, vary widely, especially since each country has tended to write its own citizenship law without a great deal of deference to the laws of other nations. A renunciation may or not be viewed as legally effective by the renounced nation, which thus may continue to view the renouncer as its citizen.
Since you ask about the United States and the United Kingdom, though, let's take a closer look at that particular example. A citizen of the UK must renounce his UK citizenship before being naturalized as a U.S. citizen. This renunciation isn't, however, enforced in any substantive way by U.S. law; it essentially consists of the first clause of the oath taken by all naturalized citizens: "I hereby declare, on oath, that I absolutely and entirely renounce and abjure all allegiance and fidelity to any foreign prince, potentate, state, or sovereignty of whom or which I have heretofore been a subject or citizen." So since the UK does not treat this as a true renunciation, the individual taking the oath will effectively become a U.S. citizen under U.S. law while remaining a UK citizen under UK law.
What about going the other way? U.S. statutory and case law provides that anyone who obtains naturalization in a foreign state loses U.S. citizenship, but only if this was done "with the intention of relinquishing United States nationality." So it's possible to pull this off. In fact, in most cases it's easy, because except under certain specific and somewhat extreme circumstances the State Department presumes you didn't intend to relinquish your citizenship. Even if the State Department doesn't so presume, it's got the burden of proving that this was in fact your intent. Of course, there are risks. For one thing, this presumption could be changed administratively by the State Department. And if you were to bring into play any of the conditions under which the presumption doesn't apply – say you accept a policy-level position in the British government, or commit an act of treason against the U.S. – you could soon find yourself sitting in the UK, still a plain old single citizen.
8 USC § 1481: <http://www.law.cornell.edu/uscode/8/1481.html>
Vance v. Terrazas, 444 U.S. 252 (1980)
Afroyim v. Rusk, 387 U.S. 253 (1967)
United States v. Wong Kim Ark, 169 U.S. 649 (1898)
"Advice About Possible Loss of U.S. Citizenship and Dual Nationality," U.S. Department of State Web site. <http://www.travel.state.gov/law/citizenship/citizenship_778.html>
Thanks to Randy Seltzer for setting us straight on some important details.