Although the U. S. Constitution does not define natural born citizen, the Congressional Research Service has found numerous historical and legal writings that indicate that a natural born citizen is anyone who legally and Constitutionally qualifies as a U. S. citizen at the time of birth, including those who are born abroad to U. S. citizens and those born in the U. S. to foreign parents. Therefore, someone born on any ship is a U. S. citizen if his/her parents are. If the parents are not U. S. citizens, he/she is a citizen if the ship is within U. S. territorial waters at the time of birth. However, a person born on a flagged U. S. ship in international or foreign waters is not a U. S. citizen if neither of his/her parents is a U. S. citizen.