From US v. Wong Kim Ark
'Allegiance and protection are, in this connection (that is, in relation to citizenship) reciprocal obligations. The one is a compensation for the other; allegiance for protection, and protection for allegiance.' 'At common law, with the nomenclature of which the framers of the constitution were familiar, it was never doubted that all children born in a country, of parents who were its citizens, became themselves, upon their birth, citizens also. These were natives or natural-born citizens, as distinguished from aliens or foreigners. Some authorities go further, and include as citizens children born within the jurisdiction, without reference to the citizenship of their parents. As to this class there have been doubts, but never as to the first. For the purposes of this case, it is not necessary to solve these doubts. It is sufficient, for everything we have now to consider, that all children, born of citizen parents within the jurisdiction, are themselves citizens.'
Helllooo the Supreme Court justice who wrote the opinion said that natural born citizen is not defined in the Constitution.
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