Both parents must be the citizens of, and the birth must take place in the concerned country, assuming that the citizenship inherited by this child and the loyalty are never changed ever after.
This definition is in Vattel's "Law of Nations." Since that was just cited by a birther about Obama, it would apply to Marco Rubio as well, correct?
Rubio was born in 1971. His parents became citizens in 1975. Therefore, Marco Rubio could not be a Natural Born Citizen according to this definition as his parents were not US citizens at the time of his birth.
So is it a BS citation for both, or does it mean that Marco Rubio is not eligible to be President? My opinion, for the record, is that it's an invalid citation and Rubio could become President, as Obama did.