Of all of the major sports (baseball, football, basketball, hockey), which athletes are the dumbest on average?
I have to say football. Every sport has penalties and fouls, but no game has as many "bonehead" penalties as football. They occur so often that we've come to expect these penalties on a regular basis every time we watch a football game.
Knowing the severity of being hit with a penalty in football, even during crucial points of the game, football players can usually be expected to do any of the following:
- Jump before the ball is snapped
- Grab an opponent's face mask
- Commit a late hit out of bounds
- Commit a late hit on the quarterback or punter
- Carry a handgun onto the field and fire it into the air after a successful play (This has not yet happened to my knowledge but it seems inevitable)
Now perhaps the comparison is unfair because fewer equivalent penalties exist in other sports. I suppose one could point to a basketball player commiting an "and 1 foul" needlessly surrendering a free throw on a made basket, but the ones which occur with a touch foul on a layup or dunk are relatively rare. Some other similar basketball penalties might include the double dribble, or entering the lane too early on a free throw. I'd guess if basketball players had the intelligence of the average football player you'd see more fouls for things like tripping, grabbing jerseys, flagrant fouls and traveling. You'd have turnovers in the 30's every game because guys would be throwing elbows on picks and passing into the backcourt.
In baseball? I don't know, I guess you'd see more batters beaned or players tagging too early on a sac fly. Plus the occasional player running from home to third on a base hit.
Hockey players have a convincing argument, though. They may actually be the dumbest as a group because in football the stupidity seems to vary by position. I admit I don't know about hockey, but what I do know is this is a game where there is very little scoring and yet teams will regularly put themselves at a disadvantage by commiting penalties (like high sticking or fighting) that give them one less player on the ice vs. the opponent. This sort of thing happens several times in every game. Seems like any team that could manage to keep all of its players on the ice by eliminating certain penalties would win a lot of games.
Special shout out to soccer which because of its simplicity is the sport of choice for children and the dumbest fans of any sport in the world.

It's hard to get a read on the average intelligence of soccer players because all they do is run around a big field and kick a ball.