I know it's just an anecdote, but in my personal experience, at the higher levels of football (college and up: I played in college and have known several people that have moved on to the NFL) it takes a certain amount of intelligence to succeed, simply because the plays and the calls get more complicated. And while the stereotype of offensive linemen is that they are big and dumb, from what I have seen is that they actually tend to be smarter than other players. On my college team at least 2-3 out of our teams academic top 10 every year were offensive linemen, and a surprising number of our offensive linemen went on to graduate school, whether at our school or others. Of course, I may be slightly biased as I was an offensive lineman, I was one of those that went to grad school, and I was on my team's academic top 10 all 4 years.
Another interesting observation I have made is that certain personalities or characteristics seem to congregate to certain positions. For example, if you were to walk through a college or NFL locker room, more often than not you can tell if a player is offense or defense based solely on the state of their locker: offensive players tend to have cleaner, more organized lockers while defensive players tend to have messier, jumbled lockers.