Insanity Defense

In criminal trials, the insanity defense is where the defendant claims they are not responsible for their actions due to mental health problems (psychiatric illness or mental handicap). Exemption of the insane from full criminal punishment dates back to at least the Code of Hammurabi. There are different definitions of legal insanity, such as the M'Naghten Rules, the Durham Rule, the American Legal Institute definition, and various miscellaneous provisions (e.g., relating to lack of mens rea).

In the United Kingdom, Ireland, and the United States, use of the defense is rare; however, since the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991, insanity pleas have steadily increased in the UK. Mitigating factors, including things not eligible for the insanity defense like intoxication (or, more frequently, diminished capacity), may lead to reduced charges or reduced sentences.

The insanity defense is based on evaluations by forensic mental health professionals with the appropriate test according to the jurisdiction. Their testimony guides the jury, but they are not allowed to testify to the accused's criminal responsibility, as this is a matter for the jury to decide. Similarly, mental health practitioners are restrained from making a judgment on the issue of whether the defendant is or is not insane or what is known as the "ultimate issue".

Some jurisdictions require the evaluation to address the defendant's ability to control their behavior at the time of the offense (the volitional limb). A defendant claiming insanity is pleading "not guilty by reason of insanity" (NGRI) or "guilty but insane/mentally ill" in some jurisdictions which, if successful, may result in the defendant being committed to a psychiatric facility for an indeterminate period.