Beavis. Butt-Head What's their problem? Psychological Dopers

Their mothers never appear, but they do apparently have a mother each, and aren’t brothers. Look under “Beavis’s Parents

The old disclaimer:

“Beavis and Butt-Head are not real. They are stupid cartoon people completely made up by this Texas guy whom we hardly even know. Beavis and Butt-Head are dumb, crude, thoughtless, ugly, sexist, self-destructive fools. But for some reason, the little weinerheads make us laugh.”

In the episode It’s a Miserable Life, where Butt-head’s guardian angel shows Butt-head what life would be like without him ever being born, we see Beavis volunteering at a soup kitchen with Stuart bossing him around.

My guess is mild mental retardation.

Beavis - ODD
Butt-Head - Moderate FAS

I agree that Butthead is run-of-the-mill stupid, while Beavis actually needs some sort of treatment. I think it was the episode “The Beaverly Buttbillies” where the boys chop into a sewer line, and Butthead explains to Beavis that crap goes down the sewer. Beavis says no, his crap goes into jars that he keeps in the basement. He also likes to listen through the ladies’ room door.