I get the premise, but clearly psychiatry is one of, if not the least, "hard science" of all the medical specialties. I mean, it's not like an orthopedic doc who can present an x-ray or an oncologist who can present a biopsy result. Psychiatrists give opinions more than present provable facts. I'd wager they are also the least trusted medical doctors among the public in general, if true an important point considering the general public makes up juries.
I do understand the big picture. I mean, if you accept the possession defense, what's next, the alien abduction or "I fell through a portal defense"?