Subject to the jurisdiction means that a legal entity, such as a court or the government of a country, has the right to exert physical control over as well as apply and enforce its laws against a person. It is a stipulation contained in the Fourteenth Amendment to the U.S. Constitution that defines who is eligible for citizenship. The precise meaning of the phrase has been the subject of debate by scholars and lawmakers, and has been defined in particular situations by the U.S. Supreme Court.
The citizenship clause of the U.S. Constitution confers automatic citizenship on anyone who is "born or naturalized within the United States, and subject to the jurisdiction thereof." Establishing whether or not a person was born or naturalized in the U.S. has historically been relatively easy. The question of whether a person born or naturalized in the U.S. is also subject to its jurisdiction has been less clear when applied to certain populations.
In court cases, the U.S. Supreme Court has held that it does not apply to children of diplomats, ministers, consuls, or embassy staff. As foreign nationals in the U.S. on the business of their governments, the parents and children owe their allegiance to their home country. They are not subject to the jurisdiction of the U.S., and are immune from most laws and from prosecution.
A child born in the U.S. to parents who are not on assignment by a foreign government is subject to the jurisdiction of the U.S. and is considered a citizen. The parents could be traveling in the U.S. on vacation or in the process of legal or illegal immigration. In legal terms, the distinguishing point is not whether the parents are citizens of another country, but whether the parents are active agents of a foreign government, immune from the laws of the U.S. If the parents are in the U.S. of their own free will, the child born in the U.S. is considered a U.S. citizen.
This interpretation of the subject to the jurisdiction part of the citizenship clause is hotly debated. It grants automatic U.S. citizenship to children who are born in the U.S. to parents who are citizens of another country. The loophole effectively allows illegal immigrants to have children in the country who automatically become citizens, making it much more complicated to deport families with mixed legal status. Opponents of this interpretation argue that the illegal parents are no more subject to the jurisdiction of the U.S. than the diplomat, since the U.S. would deport them back to their own country rather than exert legal jurisdiction over them, and the children should have the same status as the parents despite their birth on U.S. soil.