There is also a continuing clause in Common-Law that someone born abroad to American citizen (s?)is NBC.
Whether or not it requires BOTH parents for such NBC to be USA citizens is another question. I have seen that stated elsewhere, but did not have forethought to save or bookmark it. If I remember correctly, only one citizen parent is required.
For citizenship perhaps, but not NBC.
Study on this: http://www.freerepublic.com/focus/f-bloggers/2830605/posts
Only as a result of an act passed in Congress in 1934. Prior to this act, the Father had to be a citizen. It was regarded as discriminatory, hence the passage of the Citizenship act of 1934.