The insanity defense is one of the areas in which law and psychology intersect, as the expertise of a psychiatrist is required in cases that involve the principle. The origin of the “insanity” defense can be traced back to 1843 in Britain. M’Naughten, a mentally unstable woodcutter, had devised a plan to kill the Prime Minister because he believed that the government was planning to kill him. The plan failed, and instead, he killed the secretary. After conducting thorough investigations, the high court concluded that he was insane, and as a result, he was hospitalized to undergo treatment. In the process of determining the case, the court established what is today known as the M’Naughten Standard.
It recognizes that an individual who is accused of committing a crime had a prevailing mental illness or defect that interfered with his ability to judge the nature, quality, and the consequences of the act, hence his/her involvement in the specific activity. In the legal system, one of the major requirements for a sanction against an individual for involvement in a crime is mens rea. Mens rea refers to an individual’s resolve to commit a crime and attain an anticipated outcome. The insanity defense is largely based on the notion that certain mental illnesses or disorders can alter a person’s ability to form mens rea as outlined in the law for a criminal sanction.
The insanity defense standard is not fixed but has undergone an evolution over the years. Regardless of the changes within the greater cultural context, the defense is rarely used in contemporary society. Several studies have been conducted to evaluate the standard’s frequency of application and its rate of success. The majority of the studies found that the insanity defense is applied in less than 1 percent of felony cases, and the rate of success is infinitesimal. Successful cases are those that involve a not guilty by reason of insanity plea agreement between the prosecutor and the defense.
The insanity defense is rarely used because of the difficulty encountered in providing proof to support legal insanity. Many criminal defendants can provide evidence in the form of testimonies from psychiatrists to prove that they are mentally ill. However, there is a stark difference between medical insanity and legal insanity. The rationale for a different definition of legal insanity is that the primary goal of criminal prosecution is to punish the defendant through incapacitation and deterrence.