Anecdotally I am intelligent and reasonable, but when I was younger around when 9/11 happened I couldn't wrap my head around why someone would do what they did, it was to me incomprehensible, irrational, my world view couldn't account for what happened. In that null space conspiracy theories created plausible explanations for my young and impressionable mind to latch onto.
I soon began questioning authority in general, the nationalized narratives provided were clearly propagandized, and in that wake of dissonance real conspiracies, like the war on drugs, started to add credibility to other outlandish ones. It is intelligent to question, and even entertain that which is irrational from time to time, if not just to test the waters, so to speak.