A fundamental principle from English common law is that a crime generally requires both a wrongful act (actus reus) and a guilty mental state (mens rea). The actus reus is the physical act committed by a person, while mens rea refers to that person’s intent when committing the act. Under common law, a crime required a marriage of both factors. In other words, a person could only be convicted of a crime if he committed an unlawful act and if he knew that the act was illegal or if the conduct was of such a nature that a person should recognize it as wrongful.[*] The result of this approach was to penalize only those who voluntarily choose to commit an unlawful act.[3]

The mens rea requirement can be traced back through centuries of legal tradition. William Blackstone, an 18th century English jurist, wrote: “[A]n unwarrantable act without a vicious will is no crime at all. So that to constitute a crime against human laws, there must be, first, a vicious will; and, secondly, an unlawful act consequent upon such vicious will.”[4]

This principle was carried over the Atlantic and incorporated into the American legal system. As Oliver Wendell Holmes Jr. observed, “Even a dog distinguishes between being stumbled over and being kicked.”[5] U.S. Supreme Court Justice Robert Jackson described how American courts relied on various devises to infuse blameworthiness into criminal codes:  

Crime, as a compound concept, generally constituted only from concurrence of an evil-meaning mind with an evil-doing hand, was congenial to an intense individualism and took deep and early root in American soil. As the state codified the common law of crimes, even if their enactments were silent on the subject, their courts assumed that the omission did not signify disapproval of the principle but merely recognized that intent was so inherent in the idea of the offense that it required no statutory affirmation. Courts, with little hesitation or division, found an implication of the requirement as to offenses that were taken over from the common law. The unanimity with which they have adhered to the central thought that wrongdoing must be conscious to be criminal is emphasized by the variety, disparity and confusion of their definitions of the requisite but elusive mental element. However, courts of various jurisdictions, and for the purposes of different offenses, have devised working formulae, if not scientific ones, for the instruction of juries around such terms as ‘felonious intent,’ ‘criminal intent,’ ‘malice aforethought,’ ‘guilty knowledge,’ ‘fraudulent intent,’ ‘wilfulness,’ ‘scienter,’ to denote guilty knowledge, or ‘mens rea,’ to signify an evil purpose or mental culpability. By use or combination of these various tokens, they have sought to protect those who were not blameworthy in mind from conviction of infamous common-law crimes.[6]

The issue of mens rea was central to the resolution of Joseph Morissette’s case. His conviction was upheld by a federal appeals court, but the case eventually came before the U.S. Supreme Court. Justice Jackson (referenced above), who wrote the opinion of the court, acknowledged that the matter would have been a “profoundly insignificant case” had it not raised “fundamental and far-reaching” questions.[7]

Justice Jackson observed that the concept of criminal intent “is no provincial or transient notion,” but is rather “as universal and persistent in mature systems of law as belief in freedom of the human will and a consequent ability and duty of the normal individual< to choose between good and evil.”[8] Recognizing the relation between one’s mental state, a criminal act, and the concomitant punishment, Jackson wrote, was essential in replacing the vengeance-based, Old World criminal justice system with an enlightened one based on “deterrence and reformation.”[9]

Justice Jackson noted the profound consequences of holding Joseph Morissette liable for theft of the government’s abandoned bomb casings:

The Government asks us by a feat of construction radically to change the weights and balances in the scales of justice. The purpose and obvious effect of doing away with the requirement of a guilty intent is to ease the prosecution’s path to conviction, to strip the defendant of such benefit as he derived at common law from innocence of evil purpose, and to circumscribe the freedom heretofore allowed juries. Such a manifest impairment of the immunities of the individual should not be extended to common-law crimes on judicial initiative.[10]

The Supreme Court signaled its approval of requiring intent as an inherent element of crimes where a federal statute codifies a common-law crime, even when Congress does not specify a threshold for harmful intent.[†] Thus, as noted in the Morissette case, the “mere omission … of any mention of intent will not be construed as eliminating that element from the crimes denounced.”[11] Subsequently, the Supreme Court held that Morissette could not be held liable unless the jury was afforded an opportunity to evaluate his intent, and the court accordingly reversed his conviction.

[*]  This could include conduct commonly recognized as wrong, such as gross indifference to the safety of others.

[†]  The Supreme Court acknowledged that certain exceptions to the rule of culpability have been recognized over time, such as sex offenses and offenses of negligence where a crime occurs because of an omission of duty. Morissette v. United States, U.S. 342 at 251 n.8.