Answered on Quora:
Indeed, repeated studies have shown that schizophrenia is associated with a somewhat lower IQ – possibly 5–10 points. These studies showed that schizophrenics had lower IQ even before the schizophrenia developed.
Why schizophrenia is associated with lower IQ is not known, but schizophrenia is a disease of the brain, and it seems that whatever is causing the schizophrenia is also lowering the IQ.
