story
https://en.wikipedia.org/wiki/Project_MKULTRA
http://www.cracked.com/blog/five-fun-facts-about-the-cia-and...
1. Whether you're likely to voluntarily leak any secure information.
2. Whether someone who dug up some dirt on you could blackmail you into leaking secure information.
Or as the saying goes, it's fine to have a mistress, but having a mistress that your wife doesn't know about is a problem...
I believed they polygraphed you about drug use, and I recall that they had a threshold number of "experimental" sessions with marijuana that were ok, as long as you disclosed them during the background check and polygraph.
Generally, the key things were, "are you a crook? are you liable to be bribed/coerced?".
E.g. one chap was a transvestite, but the appeals court ruled that since his wife and minister knew, it wasn't something that could be leveraged against him.