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We all know that the symptoms of mental illness can influence the way a person thinks and acts. Most of the time, these changes are minor or temporary; a friend who has been a car accident suddenly develops a driving , or a family member gets during a . But in rare situations, someone with a severe mental disorder commits a . In most of these cases, the crime has nothing to do with their mental illness; one study found that, among crimes committed by people with serious mental disorders, only 7.5 percent were directly related to symptoms of their mental illness. Because of the uncommon link between the two, mental illness alone is not a valid defense; even a person who suffers from severe psychosis is considered guilty if he commits a crime intentionally.

Even rarer, someone with a mental disorder develops severe symptoms that actually lead to criminal behavior. For example, a individual runs a stranger off the road when voices tell him that the driver is a terrorist on his way to blow up the local convention center. It’s for these individuals that the insanity defense is designed.

Who Is Insane?

The purpose of the insanity plea is to distinguish between those who break the law because of mental illness and those who do so with sound judgement, much in the same way we would distinguish between a person who kills someone in self-defense from one who hunts humans. In order to convict someone of a crime, our U.S. court system requires more than the harmful act itself; we must believe that the person had a “guilty mind” when s/he committed the crime. Otherwise, running over a squirrel who darted out in front of your car would merit the same as abusing a pet.

Oddly, who is legally insane in the U. S. varies by state. In four states (Kansas, Montana, Idaho and Utah), in fact, there are no legally insane defendants because those states abolished the insanity plea. Of the remaining 46, a successful insanity defense requires a defendant who, because of his severe mental impairment, meets some variation of at least two out of three criteria; he either fails to understand the nature of the crime of which he is accused, fails to appreciate the moral wrong of his actions, or, in a minority of states, can’t his actions to requirements under the law.

Three Myths About the Insanity Defense

Convincing a jury that you were so mentally ill that you shouldn’t be held responsible for your actions is a tough road to hoe. Contrary to the widespread, but inaccurate, belief that the insanity plea is an often-used strategy to get out of jail free, juries are generally unsympathetic to mental illness as part of a criminal defense. Attorneys know this and rarely use it; only one percent of felony defendants nationwide raise the insanity defense. Of these, about ¼ are successful.

Another common misconception is that there is no downside to an NGRI (Not Guilty by Reason of Insanity) plea so it’s worth a shot. In reality, it’s a gamble. Defendants who unsuccessfully plead insanity are unable to bargain for a reduced charge because pleading NGRI is essentially admitting to the crime. As a result, an unsuccessful plea is likely to result in the defendant getting a larger prison sentence than if s/he had simply pled guilty.

And, finally is the often-hyped view that experts are easily fooled by a clever con artist who’s read a few psychology books. While we’re not infallible, mental health doctors are between ninety-two and ninety-five percent successful in determining whether a defendant is faking mental illness, making abuse of the insanity defense unlikely in reality.

The Investigation of Insanity

So how do we determine if a defendant is trying to pull the wool over our eyes or legally insane? For a , it is a complicated dance involving multiple partners in a set of specific moves. When I conduct an insanity evaluation, I am gathering data in an attempt to answer 3 questions:

Did the defendant have a mental disorder at the time he committed the crime?

If so, was there a relationship between the mental disorder and the criminal behavior?

If so, did the relationship meet the criteria for insanity as defined by the state this defendant is living in?

I generally start an insanity evaluation by having the defendant him tell me everything he could remember starting a week before the crime until after he was arrested. To make sure I don’t influence his (or unintentionally skew his recall in a certain direction), I would want him to tell me this in story form. What was he thinking? How was he feeling? What was he doing? What happened next?

After he tells his story, I dig into specific questions about his symptoms at the time of his offense. When did they start? How often did he have them? How did they influence his behavior? What, if anything, did they have to do with his crime? Who else observed them? What else was going on in his life? How was his overall functioning around this time; was he still going to work? Was he getting along with the people around him? Was he drinking or doing drugs? As I am asking these questions, I have to constantly evaluate whether his report of symptoms is consistent over time, whether they are consistent with well-documented pathologies, and whether they are consistent with what others observed during the same time frame.

Three other areas are important to explore – his psychiatric history, his criminal history and his social history. There’s a reason why over 90% of successful insanity pleas have a documented psychiatric disorder before the crime; mental illness doesn’t develop overnight, so it’s important to know if he’s been hospitalized or treated for similar symptoms. I also have to constantly whether this person’s history supports his insanity plea or whether alternative explanations are more likely. For instance, a defendant says he killed his wife because God told him to sacrifice her to prevent earthquakes from destroying San Francisco; discovering that he has been previously arrested for and his wife recently filed for divorce would certainly lead me away from mental illness as a primary motive for his actions.

During an insanity evaluation, a forensic psychologist must take nothing for granted. I always look for information from multiple sources that can either corroborate, or contradict, what the defendant is saying. Witness statements, arrest reports, audiotaped or videotaped interrogations, observations from post-arrest jail officials, interviews with friends and family members who were around the defendant at the time of the crime, psychiatric and medical records – all of these would be valuable resources for the forensic expert to start to tease out the psychological factors that either support, or discourage, an insanity plea.

The Bottom Line

Evaluating an insanity plea is a complicated process during which a forensic expert retroactively evaluates the defendant’s state of mind at the time of an offense. Not only is insanity a tough legal standard to meet, it requires a very specific set of psychological circumstances – a severely mentally ill defendant whose symptoms directly impaired his inability to understand and appreciate the moral wrongness of the act; that's why it's there. When that perfect storm does occur, refuge (and treatment) in a secure psychiatric hospital is often a fair aftermath.