Define intelligence, are we talking IQ? GPA? In general I would say people are more likely to be atheist if they find out that the god everyone talks about can't be seen, and that usually comes about by learning about their world. That being said, you have brilliant and nice atheists, and you have dumb and despicable atheists, same as religious folks.

Don't understand how personal beliefs make you smart or less smart. Religion or lack of religion is fine, it's the person that describes it.