What is Diminished Capacity?

Mary McMahon
Mary McMahon

Diminished capacity is a defense people may use in criminal cases, arguing that they lacked the full capacity to understand what they were doing in a situation. It is not an attempt to be absolved of guilt in the case, but rather a technique for pleading down to a lower charge, thus reducing the sentence. The acceptance of diminished capacity cases varies worldwide and it should not be confused with an insanity plea, where people argue that they are not guilty by reason of not being able to understand their actions and should be allowed to go free or released into a mental health facility for treatment.

When arguing diminished capacity, a defendant must be able to prove that they lacked the capacity to understand their actions.
When arguing diminished capacity, a defendant must be able to prove that they lacked the capacity to understand their actions.

Generally, in criminal cases, the prosecution needs to prove that the defendant committed the crime while acting with the intent to commit harm. In a diminished capacity plea, the defense admits commission of the crime, but argues that the defendant's state of mind was such that intent cannot be proved. This plea may be used when people were intoxicated at the time of a crime or when drugs were involved, including prescription medications known to impair judgment or cause an altered mental state.

It is difficult to win a case in court when using a diminished capacity argument, which could reference the defendant's intoxication, as a defense.
It is difficult to win a case in court when using a diminished capacity argument, which could reference the defendant's intoxication, as a defense.

Diminished capacity pleas can backfire and must be undertaken with care. Someone who has been drinking and driving, for example, will not receive very much sympathy from the court because this is deemed a grave offense. A person arguing that alcohol or drug intoxication made comprehension of a situation difficult may be viewed prejudicially as a result of attitudes about intoxication. The defense will need to be able to show how the client failed to meet the intent standard for a harsher conviction.

Experts can be called upon to support a diminished capacity plea. These can include neurologists, psychologists, and other people familiar with altered mental states and capacity. The opposing side may call in its own expert witnesses to discredit testimony, providing evidence that the defendant in fact had mental capacity and could have chosen more wisely in the situation under discussion.

If a diminished capacity plea is successful, people will be convicted of a lesser charge than the original one and will face a less severe sentence. People can still be held liable in civil suits, though, and the victim or the victim's family may opt to pursue the matter in civil courts to recover damages. In these settings, the diminished capacity plea can be used against the defendant to prove negligence and liability.

Someone with a diminished capacity might struggle in social situations.
Someone with a diminished capacity might struggle in social situations.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


Can someone please assist me with the meaning of Reuisite legal capacity and an explanation of it?


@David09 - What if someone has mental disabilities and they commit a crime? These people might be able to skirt past the diminished capacity threshold to earn a lighter conviction.

But even then it’s not a slam dunk the way I see it. Most murders are premeditated, and require that the killer is in his right mind. This argues against the possibility that someone who is mentally handicapped could carry out the crime. So I agree, it’s not an easy plea.


I think that diminished capacity is effective at reducing a murder conviction to a manslaughter conviction. But what exactly is diminished capacity – and how can you prove it?

I think it’s a tough sell, with today’s skeptical juries. I can imagine that if you were on a prescription medication that made you psychotic and caused you to kill someone without realizing it, that would be easier to sell than just regular drugs and alcohol.

But again, you’ll be cross examined. No doubt there will be other physicians – including possibly attorneys for your doctor – that will argue that your medication is not sufficient to make you go crazy.


The diminished capacity plea doesn't always work. A lady in my neighborhood took some strong cough medicine and got behind the wheel, and she ended up serving time in prison.

She took the medicine at night, and then got an urgent call from her son. His wife had been rushed to the hospital, and he needed her to come take care of their baby.

She thought she could make it to their house, but the drowsiness was too much for her. She hit another car head on and paralyzed a man for life. She was found guilty, even though her lawyer pointed out her altered state.


I heard about one case involving a kid who had been drugged by a group of teenagers. The drug they gave him drove him crazy, and he thought everyone around him was an alien.

He grabbed a gun from one of the teenager's hands and started shooting. He killed three of them before one wrestled him to the ground.

The kid's lawyer used the diminished capacity angle, and it worked. The two living teenagers were in big trouble for drugging him, though. They were found to be the ones guilty of the murder in an indirect way.


@OeKc05 – I am fascinated by bizarre legal cases, and I actually have heard of a couple involving murder while sleepwalking. I don't mean to scare you, but it is possible, and several people have entered successful diminished capacity pleas because of it.

One man killed his toddler by hurtling him against a wall while sleepwalking. He told the authorities that he had been having a nightmare, and he mistook his son for a monster. He thought he was protecting himself.

This guy was found not guilty because of diminished capacity. In his case, though, his doctor had proof that he had been having horrible nightmares for some time.

You might want to consider getting your husband some help. There are drugs he could take to prevent sleepwalking, or he could possibly benefit from some therapy.


Has anyone ever heard of someone getting off easier because they were sleepwalking while they committed a crime? I am curious about this, because my husband sleepwalks a lot.

His behavior can be pretty bizarre while he is in this state. I once saw him swinging a baseball bat around the room, and he appeared to be fighting something invisible. When I asked him about it the next day, he had no memory of it.

Sometimes, I get afraid that he might accidentally hurt one of our kids while sleepwalking. I have started locking the bedroom door, and so far, he has been giving up after trying to turn it and finding it locked. I just hope he doesn't learn how to unlock the door in his sleep!

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