In 1855, birthright citizenship was extended to children with citizen fathers and noncitizen mothers.10 Stat. 604 In 1934, it was extended to children with citizen mothers and noncitizen fathers.48 Stat. 797 From 1940 until 1978, a child born abroad who acquired U.S. citizenship at birth but had only one U.S. citizen parent had to fulfill a "retention requirement" of residing, or being physically present, in the United States or its outlying possessions for a certain number of years before reaching a specified age. Otherwise the child would not retain the U.S. citizenship (hence the name "retention requirement"). The retention requirement was changed several times, eliminated in 1978, and subsequently eliminated with retroactive effect in 1994.Henry J. Chang: U.S. Citizenship Acquired by Birth Abroad
