Mental Ability and Alignment

[Kalirren]

I'm not sure the whole thing about which alignment gets the stupid people is really on-topic.

At least in 3E, if you're not aligned (or some sorts of extraplanar, or powered by some random energy type, or undead/deathless or the like,) then you have to have Int 3 to have an alignment at all.

As an extension from this, I'd say that mental stats influence how strong of a stance a character can possibly take on alignment, whatever that alignment may be. A player who plays a retarded orc barbarian, Int 6 - Wis 8 - Cha 6, could probably get away with saying that his character just wants people to be nice to him and his friends and his favorite worg, even while he understands that a lot of smacking-down goes on in the world, and he himself does a fairbit of it. That's a perfectly acceptable stance for an 6-8-6 CN character.

On the other hand, I wouldn't let a player who plays a 16-18-16 CN character get away with anything less than a strongly individualistic personal philosophy.

I don't think alignment should affect anything at all - it's a characteristic derived from personality and outlook, not the other way around.