The term citizen originally designated the inhabitant of a town. In ancient Greece property owners in the city-states were citizens and, as such, might vote and were subject to taxation and military service. Citizenship in the Roman Empire was at first limited to the residents of the city of Rome and was then extended in A.D. 212 to all free inhabitants of the empire. Under feudalism in Europe the concept of national citizenship disappeared. In time, however, city dwellers purchased the immunity of their cities from feudal dues, thereby achieving a privileged position and a power in local government; these rights were akin to those of citizenship and supplied much of the content of later legislation respecting citizenship.
Modern concepts of national citizenship were first developed during the American and French revolutions. Today each country determines what class of persons are its citizens. In some countries citizenship is determined according to the jus sanguinis [Lat.,=law of blood], whereby a legitimate child takes its citizenship from its father and an illegitimate child from its mother. In some countries the jus soli [Lat.,=law of the soil] governs, and citizenship is determined by place of birth. These divergent systems may lead to conflicts that often result in dual nationality or loss of citizenship (statelessness).
Although the Constitution of the United States, as written in 1787, uses the word citizen and empowers Congress to enact uniform naturalization laws, the term was not defined until the adoption (1868) of the Fourteenth Amendment, which gave citizenship to former black slaves. As this amendment indicates, the United States generally follows the jus soli. However, Congress has also recognized, subject to strict rules, the principle of jus sanguinis so that children born of American parents abroad are citizens during their minority and can retain this citizenship at majority if they meet certain conditions. In addition, in 2000, Congress granted automatic citizenship to most minor children of American parents who were adopted from abroad; previously such adopted children needed to be naturalized. Until the 1940s the United States recognized several classes of nationals who were not citizens, e.g., Filipinos and Puerto Ricans. Today, however, all U.S. nationals are citizens. The United States recognizes the right of voluntary extradition, and in 1967 the Supreme Court ruled that citizenship can be lost only if freely and expressly renounced; Congress does not have the power to take it away.