Schizophrenia is usually associated with intellectual problems. I don't think that cognitive deficits are only associated with vulnerability to schizophrenia, though there is the much cited
Russell et al (1997) recently proposed that intellectual decline in schizophrenia was a nothing more than a ‘myth’. They followed up patients who had received an intelligence test as a child, who then developed schizophrenia, and had their intelligence re-tested some 19 years later. There were no differences between the child and adult IQs. However, others have criticised this study on the grounds that the sample was unrepresentative in that the individuals had presented to child psychiatry units and were far more intellectually impaired than subjects in other studies of children who subsequently developed schizophrenia.
From my research prior to this question, it seems that there is a loss of ability, but likely not chronic decline over the course of the illness, one before or during the transition to psychosis. Another well known meta review claims this, that there's
no support for hypothesizing a degenerative process in schizophrenia once positive symptoms appear... a static encephalopathy rather than a degenerative process
I've seen it said that it is centered on mathematical ability.
What do we know about mathematical ability pre and post psychosis, in schizophrenia? e.g., was John Nash a worse mathematician after he went mad?