Emotional capacity is another axis that matters a lot.
But we do deprive people of rights if they are really lacking, mentally. We put crazies in madhouses (are bad at doing so accurately). We deny minors most rights of self-determination (more than strictly based on intelligence, but the idea’s the same).
Heck, to sign your will, you have to represent that your noggin isn’t fried.
Edit (response to pyre): I never said we treat unintelligent people the same way we do animals. Psychologically we don’t even like to disrespect dead people (intelligence: 0). I’m just saying that intelligence does affect the rights we afford other humans.
I absolutely agree that emotional processing is an important component of intelligence. But I'm still not convinced that it's a good way of ascribing rights.
Regarding your examples: we certainly do act in the interests of the insane, mentally disabled and children. But that is not because they have fewer rights or have less personhood. We act for them because and in defense of their rights. We do this because they are unable to protect themselves and need guardians to defend them from abuses to their dignity, bodily integrity, etc.
So, if you are deemed unfit to make decisions for yourself, can we send you to the slaughterhouse to end up in a shrink-wrapped package at a supermarket?
Taking away your ability to sign a will is a lot different than the way we treat animals/non-humans.
But we do deprive people of rights if they are really lacking, mentally. We put crazies in madhouses (are bad at doing so accurately). We deny minors most rights of self-determination (more than strictly based on intelligence, but the idea’s the same).
Heck, to sign your will, you have to represent that your noggin isn’t fried.
Edit (response to pyre): I never said we treat unintelligent people the same way we do animals. Psychologically we don’t even like to disrespect dead people (intelligence: 0). I’m just saying that intelligence does affect the rights we afford other humans.