America started doing so at the end of the civil war. The constitution was amended then to overturn the Dred Scott decision of 1857, which held that black people were not Americans. The 14th Amendment ensured that freed slaves and their children would henceforth be citizens.

More than 30 countries grant unrestricted birthright citizenship based on the ‘jus soli’ principle – and nearly all of them are in the Western Hemisphere. The reason is more complicated than you might think.