From the Harvard Law Review:
https://harvardlawreview.org/2015/03/on-the-meaning-of-natural-born-citizen/
The Constitution directly addresses the minimum qualifications necessary to serve as President. In addition to requiring thirty-five years of age and fourteen years of residency, the Constitution limits the presidency to “a natural born Citizen.”
All the sources routinely used to interpret the Constitution confirm that the phrase “natural born Citizen” has a specific meaning: namely, someone who was a U.S. citizen at birth with no need to go through a naturalization proceeding at some later time. And Congress has made equally clear from the time of the framing of the Constitution to the current day that, subject to certain residency requirements on the parents, someone born to a U.S. citizen parent generally becomes a U.S. citizen without regard to whether the birth takes place in Canada, the Canal Zone, or the continental United States.
While some constitutional issues are truly difficult, with framing-era sources either nonexistent or contradictory, here, the relevant materials clearly indicate that a “natural born Citizen” means a citizen from birth with no need to go through naturalization proceedings. The Supreme Court has long recognized that two particularly useful sources in understanding constitutional terms are British common law
Both confirm that the original meaning of the phrase “natural born Citizen” includes persons born abroad who are citizens from birth based on the citizenship of a parent.
As to the British practice, laws in force in the 1700s recognized that children born outside of the British Empire to subjects of the Crown were subjects themselves and explicitly used “natural born” to encompass such children.
These statutes provided that children born abroad to subjects of the British Empire were “natural-born Subjects . . . to all Intents, Constructions, and Purposes whatsoever.”
The Framers, of course, would have been intimately familiar with these statutes and the way they used terms like “natural born,” since the statutes were binding law in the colonies before the Revolutionary War. They were also well documented in Blackstone’s Commentaries,
a text widely circulated and read by the Framers and routinely invoked in interpreting the Constitution.
No doubt informed by this longstanding tradition, just three years after the drafting of the Constitution, the First Congress established that children born abroad to U.S. citizens were U.S. citizens at birth, and explicitly recognized that such children were “natural born Citizens.”
The Naturalization Act of 1790 provided that “the children of citizens of the United States, that may be born beyond sea, or out of the limits of the United States, shall be considered as natural born citizens: Provided, That the right of citizenship shall not descend to persons whose fathers have never been resident in the United States . . . .”
The actions and understandings of the First Congress are particularly persuasive because so many of the Framers of the Constitution were also members of the First Congress. That is particularly true in this instance, as eight of the eleven members of the committee that proposed the natural born eligibility requirement to the Convention served in the First Congress and none objected to a definition of “natural born Citizen” that included persons born abroad to citizen parents.
The proviso in the Naturalization Act of 1790 underscores that while the concept of “natural born Citizen” has remained constant and plainly includes someone who is a citizen from birth by descent without the need to undergo naturalization proceedings, the details of which individuals born abroad to a citizen parent qualify as citizens from birth have changed. The pre-Revolution British statutes sometimes focused on paternity such that only children of citizen fathers were granted citizenship at birth.
The Naturalization Act of 1790 expanded the class of citizens at birth to include children born abroad of citizen mothers as long as the father had at least been resident in the United States at some point. But Congress eliminated that differential treatment of citizen mothers and fathers before any of the potential candidates in the current presidential election were born. Thus, in the relevant time period, and subject to certain residency requirements, children born abroad of a citizen parent were citizens from the moment of birth, and thus are “natural born Citizens.”