not guilty by reason of insanity

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“Not guilty by reason of insanity” is a plea entered by a defendant in a criminal trial, where the defendant claims that they were so mentally disturbed or incapacitated at the time of the offense that they did not have the required intention to commit the crime, and are therefore not guilty.

The Bouvier Law Dictionary explains that not guilty by reason of insanity is a plea “essentially admitting the defendant committed the act of the offense yet denying responsibility because the defendant lacked the capacity to act with criminal intent at the time.”

It can also be a verdict entered by a jury in a criminal case, stating that the defendant cannot be held guilty because of the defendant’s insanity (however, such a verdict may require the defendant to be admitted into a mental institution).

The defense of “not guilty by reason of insanity” goes to the concept of mens rea. Most offenses have two components: actus reus and mens rea. Actus reus is the requirement that the individual must have committed the criminal act and mens rea is the requirement that the individual have the required mental state, such as negligence, recklessness, etc. when committing the crime. Under this framework, a defendant can plead that they did not have the required mens rea for the crime because they were insane at the time they committed the crime, i.e. their insanity negated their culpability in the crime. For example, the crime of intentional murder requires that an individual have killed the victim intentionally. However, if the individual makes a showing that they were insane at the time they committed the murder and therefore could not have committed it “intentionally,” the individual may be deemed as not-guilty. Insanity is classified as an excuse defense. Principle behind the defense is that the behavior of a defendant who is not acting as a free moral agent is not worthy of social condemnation through the institution of punishment. To prove legal insanity, defendant’s usually put forth expert testimony on their psychological evaluation. Various jurisdictions use various test to judge such pleas (for example, cognitive test, irresistible impulse test, substantial capacity test, etc.).

Compare with “guilty but mentally ill.”

[Last updated in July of 2020 by the Wex Definitions Team]