In terms of culpable mental state, and evaluations of intent to kill how does a forensic psychologist assess a sleepwalking murder case, or a dissociative amnesia case, particularly when other factors are involved such as in the case I saw recently about the boyfriend who "had been up all night drinking" yet claims he was sleepwalking because he hadn't slept in two days or something like this and fell asleep for a few minutes before stabbing his wife multiple times. While the timeline didn't add up because of how long it would take to reach REM sleep, some sleep specialists still claimed that it is quite possible given he had been "up all night drinking for days." Is it the sleepwalking then or the alcoholism? Why was the focus on the sleepwalking and not the alcohol abuse? Where do we draw the line in terms of these complex assessments, given human beings are made up of much more than altered states of consciousness. What does having some form of altered state of consciousness mean for individuals in the legal system?
Another story that comes to mind is the one about the kid who had a tumor in a specific region of the brain that caused severe aggression and violence. This article sums up some great questions related to this broader question about altered states of consciousness:
"If their actions were caused by brain damage and a disrupted neural network, were they acting under their own free will? Should they be held morally responsible for their actions and found guilty in a court of law? Should we see them as patients or perpetrators—or both?"
https://www.scientificamerican.com/article/how-responsible-are-killers-with-brain-damage/
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Do you know why they did not consider the alcohol for that case, @Eve Fritz? I don't understand that at all.
Wow...it sounds like it get's complicated given the two fields are different in their way of approaching assessments of individuals. I can imagine DID, or multiple personalities, or any dissociative states brings on a lot of skepticism given it was used a lot back then in the 80's as an excuse for "not remembering." I wonder if the laws or ways of doing things have changed since then because of this, and what else has changed given new information from psychology....
It makes sense that each case is obviously unique and has many variables to consider across the board before coming up with any psychological conclusions.
Great question, @Eve Fritz! You have struck upon a complex issue which many psychologists, lawyers, and the courts struggle with. As a quick aside, I especially like the point that @Daniel Sumner made about a binary system. The legal system in general is very dichotomous in its thinking whereas psychology sees nearly everything as a possibility on a continuum. This difference in thinking and conceptualization at times results in some friction between the fields.
To answer your main question about how psychologists make such assessments, my fairly disappointing answer is "it depends." It depends on quite a few things. One thing is the specific wording or definition of culpable mental state and related terms used by a particular jurisdiction (e.g., the state the crime occurred in). Also unique are the specifics of that case. Different cases may call for different assessment approaches. A third point is the psychologist's own training and experience with such work.
For sleepwalking and dissociative amnesia, usually the other two major competing theories are (1) substance use as you mentioned and (2) they are simply making it up. In fact, a very high number of people who commit serious violent crimes report they cannot remember anything about the time of the alleged offenses. A big part of the evaluation for the psychologist is sifting through the information available on the offenses and the person's mental health (often from police records, interrogation videos, video footage, collateral interviews, clinical interview, psychological testing, etc.). The majority of individuals who claim amnesia do, in fact, remember the offense. As for the substance intoxication, in most states this could undermine any mental health defense given voluntary intoxication is not a defense. If the person does not remember what happened, could the intoxication by the most likely explanation? If the person acted out of character or showed poor impulse control or poor judgment, couldn't think be the intoxication? Likely yes. I have thus far only had one sleepwalking murder case I did an evaluation for and the person was not genuinely sleepwalking based on the evaluation. Since you mentioned altered mental states for the legal system, there are a number of avenues to consider. If insanity as a defense is unavailable, there is culpable mental state. If that is unavailable, there are mitigating circumstances. Thus, even if the mental state is inappropriate for or irrelevant to a particular legal issue, there are often others for which it applies.
An excellent post which reflects the problem of establishing a nuanced view of criminal behaviour in the light of neurological disorder.
Unfortunately the law is based upon a binary system 'Guilty' 'Not Guilty', whereas the complexity of human psychology does not allow itself to such binary thinking.
The appeal of the law is that is fits with our notions of the world that an individual is either culpable or not for heinous acts, it's doesn't often sit well with us to 'explain' criminal behaviour in the light of socioeconomic, mental health or taking into consideration organic disease like FTD for example.
What is clear is that the law will have to evolve as the ways in which our minds work becomes increasingly clear.
An interesting example is a pheochromocytoma a type of tumour which can affect hormone release for instance adrenaline. This may lead to aggression see: https://www.sciencedirect.com/science/article/abs/pii/S1530891X20430769 for an interesting case history.