Publisher Summary The purpose of criminal law is to establish and define standards of human conduct. A general legal principle is that all persons have the power to choose between right and wrong and to do or refrain from doing that which the law commands. The courts have recognized that there are situations in which persons who commit acts that would ordinarily be crimes may introduce evidence to show they do not have the capacity to form the criminal intent that is required as an element of some crimes, or to demonstrate other legitimate defenses. An infant is exempt from criminal responsibility for his or her act if he or she lacks sufficient mental capacity to entertain the criminal act. Also, most states provide a method of treating juvenile offenders that is distinct from that of treating adult offenders. Under the common law, and by statutes and court decisions in all states, mental incapacity or insanity may be alleged by one accused of crimes at several points in criminal procedure. Several tests have been used by the courts over the years to determine whether the mental disorder or impairment is sufficient to eliminate the ability of the defendant to form any level of mens rea that would create criminal culpability.