The United States is one of only 30 or so countries to have jus soli—Latin for “right of soil”—better known as birthright citizenship, in which anyone born in the territory automatically has a right to citizenship or nationality.
Countries in dark blue offer unconditional or near-unconditional birthright citizenship; the clear majority are in the Western Hemisphere, with Pakistan, Tanzania, and a few Pacific island nations being the few “Old World” countries to have it. Those in medium blue (Australia, France, South Africa, etc.) offer jus soli with some restrictions, such as requiring at least one parent to be a citizen or resident. Countries in teal (only India and Malta) have abolished jus soli citizenship.
In the U.S., birthright citizenship is enshrined in “Citizenship Clause” of the Fourteenth Amendment to the U.S. Constitution, which reads:
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.
The Naturalization Act of 1790 was the first law to allude to birthright citizenship, specifying that the right of citizenship did “not descend to persons whose fathers have never been resident in the United States”.
Between this Act and the ratification of the Fourteenth Amendment in 1868, there were several more acts of Congress that variably limited or expanded immigration or naturalization. The 1952 Immigration and Nationality Act (INA) reaffirmed birthright citizenship, and in 1965 was amended and expanded removed remaining racial and ethnic quotas that effectively limited immigration to only Europe.
Most of the rest of the world has jus sanguinis (“right of blood”), in which citizenship or nationality is granted only to those whose parents are already citizens or nationals. (Though as with jus soli, some countries make exceptions for certain groups based on legal status or origin.)
Jus soli originates in English common law, namely the 1608 case called “Calvin’s Case”, which was the first to articulate a form of birthright citizenship, and which shaped American laws and court decisions. By contrast, jus sanguinis derived from Roman law, which influenced the civil law systems of continental Europe and most of the world.