It may vary a lot, depending - besides the general conventions on born at sea - on the Law of the countries the parents of the newborn and/or the ship/aircraft

It may vary a lot, depending - besides the general conventions on born at sea - on the Law of the countries the parents of the newborn and/or the ship/aircraft belong to. The world is divided between jus soli and jus sanguinis: https://en.wikipedia.org/wiki/Jus_soli https://en.wikipedia.org/wiki/Jus_soli https://en.wikipedia.org/wiki/Jus_sanguinis https://en.wikipedia.org/wiki/Jus_sanguinis The US jus soli applies to births (on US ships/aircrafts) only if within the territorial waters: https://en.wikipedia.org/wiki/Birth_aboard_aircraft_and_ships https://en.wikipedia.org/wiki/Birth_aboard_aircraft_and_ship... So, in practice, it is almost a case by case decision, to which you add the complication of different Laws about double citizenship.