The 14th Amendment of the U.S. Constitution guarantees citizenship at birth to almost all individuals born in the United States or in U.S. jurisdictions, according to the principle of jus soli. Certain individuals born in the United States, such as children of foreign heads of state or children of foreign diplomats, do not obtain U.S. citizenship under jus soli.
Certain individuals born outside of the United States are born citizens because of their parents, according to the principle of jus sanguinis (which holds that the country of citizenship of a child is the same as that of his / her parents). The U.S. Congress is responsible for enacting laws that determine how citizenship is conveyed by a U.S. citizen parent or parents according to the principle of jus sanguinis. These laws are contained in the Immigration and Nationality Act.
In addition, Each year, many people adopt children from outside the U.S. The Child Citizenship Act of 2000 (CCA) grants those children the ability to automatically become U.S. citizens when they immigrate to the United States.