I think the only people who required a study to notice something so obvious, were religious ones
The only kind of intelligent religious person I can imagine is that of someone who is fully aware of how delusional they are, but they are in it for the comforting aspect of having a "father figure" that watches over us, so they stick to it anyway
Fortunately atheism is becoming more and more widespread nowadays, thanks to the "information age"
There is a HUGE amount of people who are/were religious because they simply dont know better... Religion tends to make people AVOID questioning things, so they just go with whatever they are told since kids
The idea of there not being a god etc doesnt even occur to them, but once they start researching, and see/read some Dawkins, Krauss, etc they quickly realize how stupid they were, and in many cases they wind up becoming some of the most vocal atheists out there (a good example being Seth Andrews with his Thinking Atheist podcast)
The thing I find really baffling though is how religious as a whole the USA is... The way religion is weaved into politics is disturbing to say the least... I love how every presidential speech ends with "God bless America" or such
Meanwhile, look at some of the top european countries, like Sweden, Holland, Denmark, with atheism rates of 90%+
Good thing we dont have rednecks over here