Is a drop in intellect necessary for one to be diagnosed with psychosis? Some psychiatrists claim that when someone has psychosis, there is usually a drop in intellect. Does a drop in intellect always happen, or are the boundaries for psychosis not as discrete? Why are they sometimes confused in the absence of a drop in intellect?
This question pertains to the perception of psychosis by psychiatrists and not to up-to-date interpretation of research data on psychosis classification. So I'll focus on how pure psychosis is roughly conceptualised by the average psychiatrist.

First of all, some diagnosis such as bipolar disorder or schizo-affective disorder, while usually being considered psychotic, have the additional component of problematic affective or mood states attached to it. I'll therefore leave that out to focus on the "pure psychosis" part of psychiatric diagnoses.

The notion of pychosis is usually presented as "being out of touch with reality". A more operational description of it would be to say that a psychiatrist perceives that (1) there is something about reality or your perception thereof or your lack of perception thereof that has a damaging impact on your life and (2) he does not objectify that item.

I purposely rephrased that definition since people are sometimes "persecuted" or "deluded" (or whatever else there may be, such as hypocondria, erotomania, et ceterae) in a way that is real though not objectifiable. Or at least not objectifiable at first glance.

That being said, clinical observations of psychosis have led, broadly, pure psychosis to be conceptualised as lying qualitatively on a linear spectrum extending from psychosis typical of the schizophrenia spectrum to psychosis typical of persistent delusional disorder.

Simply put, persistent delusional disorder psychosis pertains to a very well logically structured system of thought, where the patient adamantly holds some claim that seems self evident for him, and where evidence presented by him or to him is perceived discordantly by psychiatrist and patient.

On the other end of the psychosis spectrum lies schizophrenia-like psychosis. Which broadly speaking is a pattern of thought that is perceived by the psychiatrist as inconsistent, loosely organised, wishful thinking or otherwise unsupported "bullshit". This can range from "my parents are assholes, screw school" to "yeah, I've had a revelation, I can help save the world but I do not know how or why and it bothers me".

Persistent delusional disorder psychosis is a fixed set of thought pattern met by patients around 40 and that, aside from co-occuring depression or other form of distress, does not impact per se the ability to meet family obligations or job expectations. Intellect does not seem to be affected, and it is indeed used vigorously to fuel the persistent pattern of thought.

Schizophrenia-like psychosis on the other hand is considered a disorganised pattern of thought that will lead the patient into hardships family-wise or job-wise. To diagnose schizophrenia, a usual criterion is that this pattern of thought happens at the same time as a job loss, family dysfunction, or academic or school underachievement. It is conceptualised as an impairment of intellect that will get worse the more the patient undergoes further or future life hardships. Also known as "persecutions".

That is why schizophrenia-like psychosis is supposed to be diagnosed with the crucial symptom of functional impairment with respect to so-called pre-morbid intellectual or social baseline functioning. Usually, in practice, determining this premorbid baseline depends on what parents or relatives say to the psychiatrist. IQ tests are not currently used for that.

In a nutshell, if a psychiatrist meets a patient who is not on the delusional disorder side of the psychosis spectrum, with no intellectual impairment compared to its premorbid baseline functioning, he should be confused as to why the patient is now in front of him in the first place. Every medical intervention is indeed triggered one way or another by someone asking for something... He might or should be confused as to what that motive really is or was.