It's not exactly a problem. It's just a reality. You can run down the whole list of mental attributes, you can not be any of them for real, you can only fake doing it. Just take being brave and fearless. It's easy to write that on a character sheet, and if in safe easy button game they can "act" that way: as long as they know the DM will never kill their character or do anything else negative ever as the DM follows a restrictive "gentleman's agreement" and "house rules". It's easy to "act brave and fearless" when your in a safe place. Of course, some games like mine are not 'safe'. A player can loose their character to death easily in my game. And well as have their character suffer from unimaginable horrors. And yes, that includes things that "ruin" the character and/or make them "impossible to play". In such a game you would find the player is "suddenly" not role playing brave and fearless like it says on the character sheet.But basically, that's my point. If you can't roleplay a character who is smarter (or wiser, or more charismatic) than yourself, then the whole premise of roleplaying games, or at least how we approach mental stats in them is...problematic.
Generally a smart person is also aware enough that you can just ask them to role play their character. You don't "need" to do anything.And maybe the best approach is to ditch mental stats and we need to at least figure out a way to reward smart Rob for handicapping his game by playing a dumb character if he chooses to do so.
But this is the same problem as above. A player can "act dumb" when it's fun or funny.....but amazingly....they will suddenly not be dumb when they don't want to be.
You could give bonus things, experience, inspiration, fate points, or whatever. But you'd have to give a big bonus....and be willing to give a huge penalty when they "suddenly" don't role play their character.Disregarding the numbers on my sheet when I go to roleplay feels like cheating. If I make my sheet do the heavy lifting for my combat stats and "dump" my mental ones, I feel like I've gotten something for nothing. But if I deliberately "play dumb," the game may not be as fun for me.