Some countries consider multiple citizenship undesirable and take measures to prevent it; this may take the form of an automatic loss of a citizenship if another citizenship is acquired voluntarily (China, Japan or Singapore) or criminal penalties for exercising another citizenship (e.g. carrying a foreign passport in Saudi Arabia). Others may allow a citizen to have any number of nationalities. However, since each country decides for itself who its citizens are, based solely on its own laws and generally without regard for the laws of other countries, it is quite possible for a given individual to be considered a citizen by two or more countries even if some or all of these countries forbid dual or multiple citizenship.
Many countries, even those which "permit" dual or multiple citizenship, do not "recognize" dual or multiple citizenship under their laws: individuals are treated either as citizens of that country or not, and their citizenship with respect to other countries is considered to have no bearing. This can mean, for example, that consular officials abroad may not have access to their citizens if they also hold local citizenship (eg. Iran, Mexico, many Arab countries, former Soviet republics.)
Citizenship of multiple countries
Each country has different requirements for citizenship, as well as different policies regarding dual citizenship. An Australian study estimated that 4-5 million Australians (up to 25% of the Australian population - by far the largest group at 1.6 million of these, from the UK) had dual citizenship in 2000. An estimated sixty percent of Swiss nationals living abroad in 1998 were dual citizens. Approximately 89 countries in the world officially allow some form of dual or multiple citizenship. In the United States it is estimated that millions of Americans are also citizens of other countries. Although Germany has a very restrictive nationality law, it does allow dual citizenship under certain circumstances and the number of dual-citizens was estimated at 1.2 million in 1994 .
Under the U.S. Constitution, 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.
Switzerland has a three tier system of citizenship - Confederation, canton and commune (municipality).
The Hong Kong and Macao Special Administrative Regions of the Peoples Republic of China make a distinction in their immigration laws between Chinese citizens with the right to reside in the territory and those without. Chinese citizens with right of abode in these territories can hold a different type of Chinese passport which gives more favourable travel rights internationally.
In European Union law there is the concept of EU citizenship which flows from citizenship of a member state.
The Commonwealth of Nations has a Commonwealth citizenship for the citizens of its members. Some member states (such as the UK) allow non-nationals who are Commonwealth citizens to vote and stand for election while resident there. However, other member states make little or no distinction between citizens of other Commonwealth nations and citizens of non-Commonwealth nations.
Being a citizen of more than one country can have many advantages as it allows to draw various citizenship benefits from multiple sources. This includes the rights to establish residence, to work, and to acquire property, educational opportunities, eligibility for various government subsidies, including healthcare and retirement, etc. However, it is prudent to realize that each citizenship carries also responsibilities and obligations and that being a citizen of another country may be a liability. There are a number of categories where potential problems call for caution or even for obtaining professional legal counsel.
In a some cases, multiple citizenship can create additional tax liability. Countries that impose tax will generally use a combination of three factors when determining if a person is subject to taxation: Residency - they tax anyone who lives there, regardless of citizenship; source - they tax income earned there; or citizenship - they tax their citizens. Most countries use residency and/or source when determining if a person should be subject to taxation. A few countries, such as the Philippines and the United States, do use citizenship, as one of the determining factors for tax liabiliity.
A person with multiple citizenship may have a tax liability to his country of residence and also to one or more of his countries of citizenship; or worse, if he was unaware that one of his citizenships created a tax liability, then that country may consider him to be a tax evader. Many countries and territories have contracted tax treaties or agreements for avoiding double taxation. Still, there are cases, where a person with multiple citizenship will owe tax solely on the basis of holding one of those citizenships.
Example: A person who holds both Australian and United States citizenship, lives and works in Australia. He would be subject to Australian taxation, because Australia taxes its residents, and he would be subject to US taxation because he holds US citizenship. In general, he would be allowed to subtract the Australian income tax he paid from the US tax that would be due. Plus, the US will allow some parts of foreign income to be exempt from taxation; for instance, in 2006 the foreign earned income exclusion allowed up to US$ 82,400 of foreign salaried income to be exempt from income tax. This exemption, plus the credit for foreign taxes paid mentioned above, often results in no US taxes being owed, although a US tax return would still have to be filed. In instances where the Australian tax was less than the US tax, and if there was income that could not be exempted from US tax, the US would expect any tax due to be paid.
There may be also problems with conscription, travel restrictions, embargoes and sets of laws issued by multiple governments governing one's behaviour domestically and while travelling abroad. In extreme cases, such as when the countries of citizenship are at war with each other, a dual citizen's international status can be very complicated, and historically, often led to incarceration of the 'enemy' citizen, notwithstanding his actual loyalties. If someone faces problems resulting from multiple citizenship, they may choose to resolve them by renouncing the undesirable citizenship(s).
The number of multiple citizens is large and increasing. Millions of people in the world are now citizens of more than one country.
After the events of September 11, 2001, the security issue was raised of persons with multiple citizenship travelling under different names - having passports under their old and new names from different countries - and using one kind of passport to exit a country, while travelling on another passport in a different name abroad, and not disclosing this travel upon return. Legislation is being prepared in Canada to end this practice, and to identify persons travelling abroad under different names and passports, and identify security threats from such individuals.