I think it is an effect, not a cause.
It is extremely difficult to have a simple conspiracy that remains consistent with both external evidence and itself. And the moment you try to confront something like, "But this science over here disagrees with your theory" you find it necessary to add an additional theory (to wit, that the science in question is wrong or a coverup or whatever). Each time an outsider asks, "Why wouldn't this person notice/do something?" the conspiracy has to be expanded again. Eventually, it encompasses everyone but the theorist.