Populated for centuries by aboriginal peoples, the island was claimed by the Spanish Crown in 1493 following Christopher COLUMBUS' second voyage to the Americas. In 1898, after 400 years of colonial rule that saw the indigenous population nearly exterminated and African slave labor introduced, Puerto Rico was ceded to the US as a result of the Spanish-American War. Puerto Ricans were granted US citizenship in 1917. Popularly elected governors have served since 1948. In 1952, a constitution was enacted providing for internal self-government. In plebiscites held in 1967, 1993, and 1998, voters chose not to alter the existing political status with the US, but the results of a 2012 vote left open the possibility of American statehood. Economic recession on the island has led to a net population loss since about 2005, as large numbers of residents moved to the US mainland. The trend has accelerated since 2010; in 2014, Puerto Rico experienced a net population loss to the mainland of 64,000, more than double the net loss of 26,000 in 2010.