Religious people are less intelligent than atheists

[drfuzzy]

This topic is problematic.  I think the term of "intelligence" is wrong here.  We need to discuss indoctrination and liberal education.

I was raised funditard, and yes, I use that completely not politically-correct term deliberately.  A lot of the people in these groups were actually quite intelligent.  
   My Mother was my best example.  My Father was a Summa Cum Laude college graduate.  My Mother did not attend college.  But she could solve problems a lot faster than my Father could.  She was VERY intelligent.  But she was completely hobbled by her beliefs.  She took us out of public school when she learned that we would be taught about the Devil's Theory of Evolution.  She had a fit when the 3rd grade teacher played some rock music.  The world was evil, we had the only way to heaven, the Bible was literally true, and the Rapture was happening tomorrow.  For education, the Bible and preachers were all that was needed, and she followed the preachers like a cult member.  (She would practically worship a preacher until he was caught having affairs or raping children or both, then she would move on to another until he was caught, then another . . . )
   These people have their own "Bible College" -- still. All secular education is evil.  All scientists are evil.  
   
I'm so grateful that I ended up going to a regular university.

It is the anti-education, indoctrinated mindset that kills the use of natural intelligence.  It is a worldview based upon fables and emotional responses to an imaginary friend and group-think delusions of superiority that kills the use of natural intelligence.  But to say that these people are unintelligent is not precisely correct. (And now I need to go find that study -- to accurately respond to the OP.)