Most people are held accountable for their actions. If diagnosed with a "mental disorder" however, they are often forgiven/treated more leniently and exempted from punishment.

The same thing couldn't be said about a bad personality trait however. No one is exempted from murder for say "not being very compassionate" or "being an asshole".

Mental disorders however only seem to be a term of describe a set of bad personality traits that are displayed concurrently though. So why the special treatment?

What's the difference between a mental disorder and a bad personality trait?


2 Answers 2


I don't want to address the issue of defining mental disorders in general here, because just about any malfunctioning system in the brain can be considered a disorder. However, some disorders affect the set of processes in the brain which ultimately determine whether or not someone takes an action (the neural basis of volition, or "free will", if you will...). If these decision processes are broken, people shouldn't be held responsible for what they do, because they really have no control. So, a "mental disorder" which exempts one from responsibility can be considered one which consists of damage to the following processes:

Each of these have identifiable neural correlates, see article in link.

Of course, there is the philosphical objection that mechanistic explanation (deterministic or random, doesn't matter) means nobody has free will - but the issue in society is really about free will as the ability to learn rules and follow them. A bad personality trait usually doesn't affect the above processes, and therefore doesn't impede someone's ability to make their own decisions.


I Upvoted @StrangeLoop's excellent answer. I did want to point out that the law is applied unevenly across the United States.

Are people ever exempted from prosecution for being a$$holes? Absolutely. The California Highway Patrolman who beat the homeless woman trying to cross the highway was supported by his commander (I haven't heard the latest; I know she's suing.) The video is here. The CHP beat her up to "protect her from injuring herself". Read about police killing mentally retarded, unarmed teenagers, little old ladies, etc.

Most people in prison have a mental illness called Conduct Disorder. The key deciding factors in prosecution of people with mental illness are 1. does the person know right from wrong, and 2) can they control their behavior. In Conduct Disorder, the answer is yes to both.

There was a fascinating case in the news of an epileptic who had a part of his brain removed to control his seizures. After the surgery, he began to buy pornography, then child pornography. Neurosurgeons testified that the part of the brain they removed was indeed responsible for this predilection, but because he knew better than to put the porn on his work computer, he was judges to be in charge of at least some of his behavior. He got half the usual sentence and drugs have rendered him back into the nice guy he was before (and he really was. You can listen to his story on Radiolab.)

An entire field called neurolaw has cropped up to deal with how the law should treat criminals with neurological conditions.

To answer your original question, many people with bad personality traits have mental disorders. Borderline Personalities drive people crazy. So do kids with Oppositional Defiant Disorder. These people, as long as they are "rational" aren't spared from the law (Jodi Arias, for example, probably suffers from Borderline Personality Disorder. Still, she knew killing someone wasn't really a reasonable option.)

Andrea Yates, though, suffering from severe post-partum depression with psychosis, really believed that the only way to "save" her children from hell was to drown them. The most responsible person in that case - her husband - got off scott-free. (He was warned after her PPD/P with her fourth child that she should never get pregnant again. He wanted a basketball team of children (he actually said this). He stopped her psychiatric visits and meds, got her pregnant again, and in the midst of her psychosis, even though she admitted she felt the need to kill her children (which his family and the psychiatrist she was seeing knew; he stopped those visits, too), he left her alone with the kids.

Th point is the law (courts) is not consistent or even consistently necessarily rational. Often, though, it does the right thing.

  • 1
    $\begingroup$ Interesting cases to consider! It makes one think: even if all the neural processes (such as inhibition after the forward check) in Haggard's model are perfectly functional, should one really be held responsible if an unstoppably strong drive from other processes/areas make inhibition of the unacceptable actions effectively impossible? Mental illness is incredibly complex to define, it's suprising that courts manage to keep up in any sensible way at all. $\endgroup$
    – user6682
    Sep 16, 2014 at 19:49
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    $\begingroup$ @StrangeLoop - it is really a burgeoning field in crimes by people with certain brain tumors. Strange, but reasonable. If inhibitory centers are destroyed/impaired, what is left of the ability to choose. $\endgroup$ Sep 16, 2014 at 19:52
  • $\begingroup$ Didn't mean to necro-post, however I want to point-out about Phineas Gage's case, who had an accident which destroyed part of his frontal-cortex associated with social-inhibition functions. Following the recovery, he was reported to have changed from someone decent into someone “fitful, irreverent, indulging at times in the grossest profanity…capricious and vacillating” $\endgroup$
    – Nono
    Aug 11, 2016 at 10:49

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