Although of course schizophrenics vary in IQ, the research shows that lower IQ is associated with schizophrenia. Typically the lower IQ was present before the schizophrenia hit. Whether these people already had pre-schizophrenia and the low IQ was a signal of that or whether lower IQ is an independent risk factor is not known. I suggest the former.
I am not sure if schizophrenia itself, once it hits, causes an IQ decline, but it would not surprise me. The process of developing schizophrenia involves considerable damage to the brain. It makes sense that this brain damage, in addition to causing the disorder, also lowered your IQ.
In summary, people with schizophrenia tend to have lower IQ’s than normals on average, but the difference may not be large. I think it was only ~3 points.
