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U.S. CITIZENSHIP ACQUIRED BY BIRTH ABROAD

The Immigration and Nationality Act of 1952
In the case of a child born to one U.S. citizen parent and one alien parent, the U.S. citizen parent now had only to be physically present in the United States or its outlying possessions prior to the child’s birth for 10 years, at least 5 of which were after the age of 14. “Physical presence” was different from the concept of “residence” which had applied under the previous statute. The physical presence requirement could be satisfied by mere presence in the United States even if the person had not established a legal residence there.

You can read more here:
http://www.americanlaw.com/citabrd.html