Does religion make people dumber?
No, I don't think so.
Or does it just attract dumber people?
Intelligence isn't a just some wonderful superiority that atheists can idly boast about. Intelligence is something that that has to be demonstrated. It isn't something that people
are, it's something that they have to actually
do.
Numerous studies have correlated atheism with higher IQs.
I think that as a group, atheists probably do have higher average IQs than the general population. That sounds wonderful for the atheists, right? Maybe, but...
It doesn't mean that any particular atheist is going to be smarter than any particular religious person. The distributions have way too much overlap to conclude that.
And there's the problem that the concept of "religion" takes in a lot of territory. I think that while atheists might have slightly higher IQs than the mostly religious general population as a whole, there are probably religious denominations and traditions that will have higher average IQs than atheists.
A lot of those kind of differences have to do with economic class and amount of education received. And there are certainly religious groupings that have higher average educational attainments than religious non-adherents. (Episcopalians, religious Jews and Buddhists, for example.) On the other hand, groups like the Baptists and Pentecostals tend to be poorer and have less education, thus pulling down the "religious" average as a whole. Roman Catholics as a whole score a little below the national average in educational attainment, but if you removed the large numbers of poorly educated recent Latin American immigrants from their ranks, you might find that Irish and Italian Catholics are better educated than religious non-adherents too.
In other words, if you broke down the results a bit, things might not favor the atheists quite as unambiguously at it might seem at first.
My own impression is that smart people are like cats. They don't take to herding as well as less-intelligent people. Smart people are more apt to have ideas that are uniquely their own. But that's no guarantee that their ideas are going to be true or correct. They're just going to be less conventional.