Daniel McNaughtan

Under the U.S. Constitution, a person is presumed innocent until proven guilty when charged with a crime. It is the duty of the prosecution to prove the accused is guilty beyond a reasonable doubt in criminal trials. Criminal prosecution in America allows for "justifications" or "excuses" for the defendant's crime.

A "justification" defense allows for the possibility that some situations require a person to choose between the lesser of two evils in deciding on an action. Self-defense is an example of justification. The degree and type of punishment is considered when there is a justification defense. Legal excuses, such as insanity, cover the other major type of criminal defense. If the accused is determined to be irresponsible, a successful defense in a court of law will receive a verdict of not guilty by reason of insanity. The person will not be punished but will be committed to a secure mental health facility until it is determined he or she is no longer mentally ill or dangerous to society.

Most states use a form of the McNaughtan Rules to determine a person's legal responsibility for a crime. Some states have added additional elements and others have abolished the insanity defense entirely. John Hinckley (1955–) was acquitted of murder by reason of insanity when he made an assassination attempt on U.S. president Ronald Reagan (1911–2004; served 1981–89) in 1981. As a result, Congress enacted a stricter insanity test that is now used in all federal criminal prosecutions.