Superhuman Ability Scores

Something I noticed in the MouseGuard rpg rules recently struck a chord with me. In MouseGuard one of the attributes frequently used is called Nature, every creature has a Nature score. Nature represents the strength of the creatures innate abilities and is different for every type of creature.  For example an Owl's Natutre encompasses being silent, having excellent hearing, flying, and being a predator; a Mouses Nature encompasses escaping, climbing, hiding and foraging. Nature is ranked from 0-7, a high rank means the creature is very faithful to its nature, a low score means the creature acts in a way untypical for its kind.  There are ways to both gain and loose nature in MouseGuard but that isn't my current interest. What really got my thinking was that in MouseGuard your character has to stay in that 1-6 range, if your Nature drops to 0 or exceeds 6 there are possibly dire consequences for your character, Nature 0 means your character become completely unmouselike, unable to view the world in the way that his fellow mice do, you might even lose your character to the GM. Nature 7 means your character has become too focused on acting mouselike and starts losing his heroic traits.

Anyway, how does this relate to D&D? well a trend I've noticed in my own games is the eagerness of characters to improve their ability scores which is something I'm not particularly keen about. Although the methods of improving ability scores are rare and difficult I would like to take some of the emphasis off better scores and focus it more into story and character goals. To that end I propose a cap on the limits of character ability.

If a character's ability score exceeds 20 then something begins to break in their mind, eventually taking a self-destructive turn.

Strength: You are stronger than any mere mortal and you know it, you become obsessed with pushing your physical limit and proving your strength. 

Dexterity: You find it impossible to focus, you always need to be moving. Additionaly you develop a paranoia and flinch constantly without provocation.


Constitution: You begin to over-indulge in many things and refuse to take care of yourself.  You become overconfident in your bodies ability to endure the ravages you inflict upon it.

Intelligence: Your mind withdraws into itself and into a realm of pure intellect, eventually leaving the physical world behind.  Your body becomes an empty shell.

Wisdom: You increasingly become certain of radical and dangerous convictions. You are willing to die and kill to prove your beliefs. 

Charisma: You either develop multiple personalities, become a raving megalomaniac, or both.

If a characters score exceeds 20 they being to suffer from the above problems. Eventually they will either die or become NPC's as the problems worsen. The better a player acts out the characters spiral into madness the longer I will let them keep control of the character. If the player doesn't acknowledge the affliction in game then the character will become an NPC much faster.

I haven't used it yet but I really like this idea in theory. I can have exceptional villains and also explain why they are so crazy and evil.  Any thoughts?