Hi Millie!
Your raise a really good question, one that has been examined and studied, but--at least as far as I have been able to determine--has resisted a definitive answer.
That being said, it's nonetheless very clear that the two factors create a powerful negative synergy. And while some delusions maybe somewhat benign (see GreatTeacher's post above), many are clearly malignant, destructive and very dangerous.
Still, a delusion is a delusion. Why would anyone want to believe in things that aren't true? Reality isn't always pretty, but it's always real.
That's what I'm all about: keeping it real.