To me this seems kind of obvious. Intelligent people have the capacity to question "truths" the society around them tells them to believe, because they can think and examine the validity of such proposed ideas, and rules, and authority.

Whereas less intelligent people do not have the capacity to question ideas and come up with plausible alternatives, then examine those critically. Therefore they just accept what authoritites, like say their father, or their Dear Leader tells them to believe.