Last week, I registered to vote. Yaser Esam Hamdi made me do it.

Yes, the alleged Taliban fighter -- who has been lingering in solitary confinement in a Norfolk naval brig for the past nine months -- prompted me to take one more step down the road to becoming a full-fledged American. That's because I've been struck throughout the arcane debate about the legality of Hamdi's detention that the 22-year-old is trying to reap the benefits of American citizenship without ever having shouldered any of its responsibilities. And while his legal claim may be correct, it distorts the notion of what citizenship once meant. Citizen Hamdi has done none of the things that most Americans do: He hasn't lived in this country since he was a toddler; he hasn't gone to school here; he hasn't worked, shopped, volunteered or carried a passport, let alone paid taxes, served on a jury or voted, to list just a few of the social, economic and political ties that bind most members of a participatory democracy such as the United States.

Instead, he's been living the life of a Saudi. Which shouldn't be a surprise, because by any normal measure, that's what he is. He was born to Saudi parents in Baton Rouge, La., while his father, a chemical engineer, had a temporary visa to work on a project for Exxon. The family left for their homeland when Hamdi Jr. was 3. He didn't return to the United States (and there's no evidence that he pined for it) until he was brought, under guard, to Norfolk.

Paper citizenship like Hamdi's makes origami of the whole notion of belonging. In the name of economic and political gain, many governments have promoted the idea that you can be a citizen of more than one state. And we modern, global nomads have been quick to take advantage of that change. We make of our citizenship what we will. Or tear it up and start over with a second. The whole process is undermining a centuries-old system that was intended to tie people closely to their state and to their fellow citizens. Once an exclusive privilege that fostered allegiance and inspired civic attachment, citizenship is losing its very essence. It's more about rights and less about responsibilities.

Like Hamdi, I am a dual citizen, though I came by that status in a different way. I am British by birth and married to an American; I was naturalized 11 years ago at the Edward A. Garmatz Federal Courthouse in downtown Baltimore. We're part of a burgeoning population, Hamdi and I, though there's no way of knowing exactly how many dual citizens there are in this country. According to the Washington-based Center for Immigration Studies, 90 percent of the more than 1 million people who immigrate to the United States each year come from countries that allow some form of multiple citizenship. The center's Web site gives a list of almost 100 such countries. Israel and Ireland are prime examples, of course. Mexico passed laws in 1998 permitting dual citizenship; Australia followed suit last year; and India's prime minister, Atal Bihari Vajpayee, announced earlier this month that his country plans to grant the status to many of the 20 million people of Indian origin living overseas. (Saudi Arabia, incidentally, does not permit dual citizenship.) Stanley Renshon, a political science professor at the City University of New York who is working on a book titled "The 50 Percent American," estimates that as many as 40 million Americans are eligible to claim the citizenship of another country.

Think about it: That's about a seventh of the population who could, potentially, vote elsewhere, serve in another country's armed forces, run for public office -- or simply pick up and leave if their American dream failed to become a reality.

Some observers defend this trend as a legitimate reflection of our increased mobility and complex cultural identities; they see Hamdi as an aberration, and I like to think they see me as a sign that we are at last creating a truly global community. Others decry it as a dangerous erosion of America's national identity and of the sense of common purpose it engenders. They see Hamdi as an example of the indiscriminate manner in which citizenship is handed out, and I know some view me as a bigamist, cheating on two sides of the Atlantic. Ambivalent as I have often felt about my own double life, I can understand both views. But I think one underestimates the importance of belonging while the other oversimplifies the realities of modern life.

The reality, everyone agrees, is that we are mobile as never before. More and more people live in this country, as Hamdi did, for a few years or less.

Hamdi's claim to American citizenship stems from the Fourteenth Amendment, ratified in 1868, soon after the Civil War. It states in part that, "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside." The amendment was written to prevent individual states, which then set their own citizenship policies, from denying fundamental rights to any particular group -- especially to newly freed black Americans whose citizenship rights had been denied in many states of the Union.

There's some debate today among legal scholars about what the clause "subject to the jurisdiction thereof" actually means. It has long been used to exclude the children of foreign diplomats from immediate rights of citizenship, and for a while it also excluded Native Americans on the grounds that they owed their allegiance to their tribes rather than to the federal government. That was put right in the 1920s. A similar argument could be made to exclude the children of temporary workers, tourists, foreign graduate students and illegal aliens, according to Craig Nelsen, director of Friends of Immigration Law Enforcement, a group of about 75 volunteer lawyers, law enforcement officers, academics and immigration experts. Hamdi, says Nelsen, is a "shining example of the absurdity of calling people American because this is where their feet first touched the ground." Most Americans would agree, he says, that an accident of birth (or job assignment) isn't what citizenship is all about.

But few birthright Americans are in Hamdi's shoes. The bigger problem is more profound: It's not really clear what citizenship is all about these days. On June 6, 1776, even before the Declaration of Independence proclaimed the right of the people to establish a new government, the Continental Congress offered a definition of citizenship that held the concept of loyalty as central: "All persons abiding within any of the United Colonies, and deriving protection from the laws of the same owe allegiance to the said laws." A century ago, most U.S. diplomats agreed that multiple citizenship was "an aberration to be stamped out as much as possible," according to David Martin, a law professor at the University of Virginia. Their motive, Martin explains, was to protect immigrants from demands (such as conscription) that their countries of birth might continue to impose on them. Each individual, the thinking went, should have a single set of citizenship obligations.

That view was dominant throughout the first half of the 20th century -- and codified in a Hague treaty in the 1930s. During this period, the State Department sometimes tried to notify dual citizens living overseas when they reached the age of majority that they had elected another nationality and therefore lost U.S. citizenship. But the department lacked statutory authority for its actions. Over the past 50 years, the United States has gradually allowed its citizens greater leeway to linger in the ambiguous borderland of belonging. A 1967 Supreme Court ruling (Afroyim v. Rusk) denies Congress the right to "rob a citizen of his citizenship" for voting in another country's elections. The same ruling has been interpreted to allow Americans to serve in another country's military and run for office.

The State Department, which long resisted dual citizenship, has adopted a "don't ask, don't tell" policy. The department even states on its Web site that "U.S. law does not mention dual nationality or require a person to choose one citizenship or another." And the U.S. government takes no steps to enforce the unequivocal wording of the naturalization oath (which I swore, as every new American must): to "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."

In any case, it isn't obvious how to "renounce and abjure all allegiance." I could, of course, have torn up my British passport in a symbolic gesture of my new affiliation. But many countries don't recognize the validity of the U.S. oath. One British diplomat even quipped, when I inquired what would happen if I took American citizenship, "Once British, always British. You can't walk away from us that easily." And although I've since learned that it is possible (for a small fee) to give up British citizenship, that provision is used only by people in special circumstances such as accepting a job here that requires security clearance. Perhaps the renunciation oath, written in the 1790s, is simply out of date, as several law professors suggest. Perhaps, as some have argued, we should be thinking in terms of "ampersand" instead of "hyphenated" Americans.

Perhaps. But the anomalies that citizenship policies create give a sense of how great a change we face. Countries such as Mexico or El Salvador, acting to advance their interests as nation states, encourage dual citizenship to capitalize on the political and economic clout of their citizens overseas. Valdas Adamkus became president of Lithuania in 1998 when he was a U.S. citizen; Vaclav Havel once suggested that Madeleine Albright reclaim her Czech citizenship and succeed him as the country's leader. And I can think of green card holders who have contributed more to this country than either Hamdi or I but who aren't "American."

So choose your metaphor. Call me a global citizen or a bigamist. Or accept that there is, as I suspect, a gradual evolution in one's sense of belonging that the dual citizen experiences much like any other immigrant. In order to encourage that process of identification -- and the sense of community it brings -- there probably are rights that dual citizens should have to give up if we want to remain dual, such as fighting in the armed forces of another country. (There goes Hamdi.) And it would make sense to me to be allowed to vote only in my country of residency. The reason I didn't register to vote the moment I walked out of the courthouse as a newly branded American was because I was trying to make sense of what it meant to have those rights in two countries. But I've sent off the Maryland voter registration form now. And I haven't tried to cast a ballot in Britain for several years. That, I suppose, is what assimilation is all about. The privilege of retaining my old citizenship has made it a more haphazard process, but that is no excuse for being an American of convenience, a paper citizen. Frances Stead Sellers, Outlook's deputy editor from 1999 to 2002, is on leave from The Post as an Alicia Patterson Foundation fellow researching the evolving nature of national identity.