http://en.wikipedia.org/wiki/United_States_nationality_lawFor persons born between December 24, 1952 and November 14, 1986, a person is a U.S. citizen if all of the following are true [7]:
One of the person's parents was a U.S. citizen when the person in question was born
The citizen parent lived at least ten years in the United States before the child's birth;
A minimum of 5 of these 10 years in the United States were after the citizen parent's 14th birthday.