[malandrew]

What makes someone a citizen? In the US, being a citizen is a birthrate or comes via a long, tedious bureaucratic naturalization process. Other countries have different definitions of citizen.

IMHO citizenship by birthright is a pretty absurd notion, but its largely the default for historical reasons tied to war and conscription. Originally governments only gave you the right to citizenship in exchange for the ability to send you to war to protect often economic interests.

The world would be for more efficient and interesting is the "market" for citizenships were far more liquid and people could easily choose their government the way they can already do internationally by moving from state to state and city to city.