Well, alot of mental illnesses cause lower brain functions, and most people seem to be RP their characters this way. I just like to pretend they have brain diseases, because any doos insane person should be sly, or inteligent. It works pretending they have mental illnesses IC.
And there also several mental illnesses that do not affect brain function or enhance it. Insanity should really be reserved for the sickest, most manipulative and most evil characters, not people who just want to act as morons and kill people.