Reason Magazine has an excellent article on why our knowledge about the psychology and neuroscience of mental illness doesn’t really help when trying argue for or against the insanity defence in court.
The insanity defence concerns whether a person accused of a crime should be considered legally responsible.
Some of the first legal criteria for judging someone ‘not guilt by reason of insanity’ are the M’Naghten Rules created after Daniel M’Naghten tried to assassinate the British Prime Minister Robert Peel in 1843.
He ended up killing Peel’s secretary, but when caught was found to be suffering from paranoid delusions and it was judged that his crime was motivated by his unsound mind and he didn’t understand the ‘nature and quality’ of what he did.
Most Commonwealth law in this area is still based on these criteria, and most US law was too, until shortly after John Hinckley shot US President Ronald Reagan and was found not guilt by reason of insanity.
This caused a backlash against the insanity defence and many US states have variously abolished it or made it much more difficult to prove (near impossible in some cases).
The Reason Magazine article examines why, when it does arise, the evidence is largely based on descriptions of the person’s mental state and why recent advances in understanding mental illness don’t really help very much.
One of the main reasons is that studies that find differences between people with mental illness and those without, do so on the group level. The same differences might not be present when comparing any two individuals.
In other words, on average, there are mind and brain differences between people affected by mental disorders and unaffected people, but the individual variation is so great that you couldn’t reliably say it would be present in one particular person.
As these criminal trials are focused on the actions of one individual much of the objective science goes out the window because it can’t reliably indicate an diagnosis, state of mind or reasoning abilities on the individual level.
This means that the most relevant evidence is usually the testimony of a psychiatrist or psychologist who is giving his or her clinical, descriptive judgement of the person’s state of mind.
The Reason Magazine article examines what sort of dilemmas this causes, and considers how developments in psychology and neuroscience are likely to impact on the legal judgement of insanity.
It’s an excellent guide to some of the key issues and the difficulties of making legal judgements on subjective states of mind.
Link to article ‘You Can’t See Why on an fMRI’.
Mind Hacks 
Ultimately, the problem is that insanity is a legal concept, not a medical one. Hanging a diagnosis like schizophrenia on someone isn’t terribly relevant in a trial. In the end, it’s the call of the judge or jury whether someone is insane. It’s not as if anyone can truly know what the defendant’s “state of mind” was at the time that the offense occurred, all that a psychiatrist or psychologist can testify on is how the defendant presented at the time of examination.
Interestingly, the radio show All In The Mind did a really great piece covering this same topic
http://abc.net.au/rn/allinthemind/stories/2007/1955379.htm
A recent CBS News report about mentally ill criminals [1] suggests that the San Jose, CA legal system strikes a good balance in trying to handle each person as an individual. Some defendants are simply claiming temporary insanity, while others really can’t function in courts and jails. The San Jose judge is trying to get the “truly ill” out of the penal system and into group homes with frequent follow-ups. By the way, the report also says that the largest public mental health facility in the US is the LA County Jail mental ward.
– Michael from The US Desk at TheNewsRoom.com
[1] http://www.thenewsroom.com/details/437613/US?c_id=wom-bc-mdm