US State Department Services Dual Nationality.

The concept of dual nationality means that a person is a citizen of two countries at the same time. Each country has its own citizenship laws based on its own policy.Persons may have dual nationality by automatic operation of different laws rather than by choice. For example, a child born in a foreign country to U.S. citizen parents may be both a U.S. citizen and a citizen of the country of birth.

For more information about “dual citizenship” see:

http://travel.state.gov/travel/cis_pa_tw/cis/cis_1753.html