My colleagues, other psychotherapists who frequently discuss Trump's mental condition, wonder whether any of us could diagnose someone exactly like him without prior information about his position and behavior. It would be a cold interview, so-to-speak. Outpatient mental health therapists conduct thousands of them in a career.
We've concluded that his glib persona would raise our suspicions that he was trying to hide something in order to look good. Unless there were neurological symptoms of dementia, our diagnosis would be inconclusive. We might have this hypothetical client undergo some standard psychological and hard to fake tests. (Related: “Faking It: Can Job Applicants ‘Outsmart’ Personality Tests?”) If we had to come up with a diagnosis, say for the court, we'd have him return for one or more interviews, and if he had the authority, we would require a battery of psychological tests. Forensic psychologists do this all the time.
We still might only be able to come up with "inconclusive" as our conclusion. We might use descriptive behavioral observation words like “evasive” and “guarded” as we noted that the client claimed that he didn’t experience any bothersome symptoms.
Basically, though, we'd be unlikely to have the luxury of more than a 90-minute session. We already know from the most recent sit-down with a non-Fox interviewer, Lester Holt which did not go well for the president, that he can more-or-less present as sane for an hour.
Mueller and his investigators wouldn't - and absolutely shouldn't - have such a time limit. They must interview him in person. They often interview subjects for hours on end.
I have my doubts whether Trump could maintain his composure and attention for that length of time. I think he would contradict himself. I think he would demonstrate short-term memory impairment. I think he would be hard-pressed to treat his questioners as anyone worthy of his precious time and respect and therefore might lower his guard.
Trump's lawyers must know this. They will do everything possible to put off an in-person interview. Trump may or may not accept that he is guilty of criminal activity. Trump, because of his mental state, will implicate himself.
This is what Mueller is looking for. However, in the process of determining this, he will do what therapists have been clamoring for by assessing his mental state and competence. (See Psychiatrists “Deeply Concerned” by Trump’s Instability Call for Urgent Mental Health Evaluation and Why doesn’t the president get a thorough mental health exam? CNN)
If they already don't have ample information to have made this determination, they would after their interview. Unfortunately, we wouldn't learn about this conclusion for months or years unless it is leaked, and this probably wouldn't happen. (Related article: Trump, the "Very Stable Genius," Is Falling Apart as Mueller Seeks Interview: New revelations raise the question: Is he a crook or is he nuts?" AlterNet)