If you are born in Guam or Puerto Rico, you are automatically granted citizenship. If you are born in American Samoa, that isn't the case. Despite American Samoa's status as a U.S. territory, the people who are born there aren't technically U.S. citizens. They're called U.S. nationals, a status that means they pay American taxes but cannot vote, run for office, or serve on a jury. They also have special passports that declare them nationals, but not U.S. citizens.
