Although you can't make a specific disability with the Sims3, you can use various traits to make them appear more disabled, especially when you have more expansion packs. There are a few options which could be used to make AS/ADD etc. These are my personal favourites:
Absent minded
Bookworm
Childish
Clumsy
Couch potato
Coward
Daredevil
Dislikes children
Dramatic
Easily impressed
Eccentric
Evil (my personal fave!)
Exciteable
Genius
Grumpy
Hates the outdoors
Hot-Headed
Hydrophobic
Innappropriate
Insane
Kleptomaniac (you can get some pretty cool stuff with this trait!)
Mean spirited
Mooch
Neurotic
Never nude
No sense of humour
Over-emotional
Perfectionist
Rebellious
Shy
Slob
Unflirty
Unlucky
Obviously you can only choose a few, but if you pick the right combo you can get a pretty messed up Sim!
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I have HFA, ADHD, OCD & Tourette syndrome. I love animals, especially my bunnies and hamster. I skate in a roller derby team (but I'll try not to bite

)