Mens Rea

The law did not always require mens rea for liability. Early Germanic tribes, it is suggested, imposed liability upon the causing of an injury, without regard to culpability. But this was during a period before tort law and criminal law divided. It seems likely that as the distinction between tort and crime appeared—that is, as the function of compensating victims became distinguished from the function of imposing punishment—the requirement of mens rea took on increasing importance.

The phrase mens rea appears in the Leges Henrici description of perjury—reum non facit nisi mens rea—which was taken from a sermon by St. Augustine concerning that crime. The sermon is also thought to be the source of the similar maxim in Coke's Third Institutes, the first major study of English criminal law: "actus non facit reum nisi mens sit rea" (the act is not guilty unless the mind is guilty). The Church had much influence on the development of this part of English law for several reasons. First, it preached the importance of spiritual values and mental states to a wide audience. Physical misconduct was significant only because it manifested spiritual failure; it was the inner weakness that was the essence of moral wrong. For example, "Whoever looketh on a woman to lust after her hath committed adultery with her already in his heart" (Matthew 5:27–28). Second, clerics were influential in the administration of government and governmental policy, both because they were among the few who could read and write and because of the Church's own political power. And third, the Church had its own courts, for trying clergy. In these courts new offenses were developed that put the new ideas of the importance of mental state into criminal law form.

While Christian thought on mens rea had a dominant influence over its development in English law, similar concepts are found in nearly all criminal laws, often without a history of Christian influence. The cross-cultural presence of concepts like mens rea provides some evidence that the notion of moral blameworthiness expressed by the broad conception of mens rea arises from shared human intuitions of justice and would have developed in English law through some other means, if not through the spread of Christian thought.

Once adopted as a basic principle of criminal law, the legal meaning of mens rea continued to evolve. The early stages of its development are illustrated by the decision in Regina v. Prince (13 Cox's Criminal Cases 138 (1875)). The defendant took an underage girl "out of the possession" of her father, reasonably believing she was over the age of consent. That the defendant's conduct was generally immoral was sufficient for Lord Bramwell to find that the defendant had the mens rea necessary for criminal liability. Lord Brett, on the other hand, would require that Prince at least have intended to do something that was criminal, not just immoral.

A somewhat more demanding requirement is expressed in Regina v. Faulkner (13 Cox's Criminal Cases 550 (1877)). In the process of stealing rum from the hold of a ship, a sailor named Faulkner accidentally set the ship afire, destroying it. Building upon Lord Brett's conception of a more specific and demanding mens rea, Lords Fitzgerald and Palles concluded that the mens rea requirement meant that Faulkner must have at least intended to do something criminal that might reasonably have been expected to have led to the actual harm for which he was charged. Thus, Faulkner ought not be liable for the offense of burning a ship when he intended only to steal rum from it; stealing in the normal course of things, does not lead one to reasonably foresee that a ship will be destroyed.

This last shift in the notion of mens rea marked not only a dramatic increase in the demand of the requirement, but also a significant qualitative change. No longer did there exist a single mens rea requirement for all offenses—the intention to do something immoral or, later, something criminal. Now each offense had a different mens rea requirement—the mens rea required for the offense of burning a ship was different from the mens rea required for the offense of theft. Liability now required that a person intend to do something that might reasonably be expected to lead to the harm of the particular offense charged. As some have expressed it, there is no longer a mens rea for criminal liability but rather mentes reae.

Common Law often grouped offenses according to whether an offense required a specific intent or a general intent. The categorization had practical significance. For a specific intent offense, a reasonable mistake often was a defense, while for a general intent offense only a reasonable mistake was a defense. Voluntary intoxication could provide a defense to a specific intent offense but not a general intent offense. The distinction has been largely abandoned, however, because it rested upon no coherent conception, which made it difficult to determine reliably into which category an offense fell. Further, it became apparent that the distinction assumed that each offense had a single kind of mens rea—a general intent or a specific intent—when in fact the law's practical operation showed increasingly that no such generalization could be made. Courts increasingly found that their desired mens rea formulations applied one kind of mens rea to one element of an offense and a different kind to other elements.

The Model Penal Code carried this insight to its logical conclusion. Section 2.02(1) requires the proof of culpability "with respect to each material element of the offense." In what might be termed a shift from offense analysis to element analysis, the Code expressly allows offense definitions in which a different level of culpability is required as to different elements of the same offense.

This element analysis approach—defining required culpability as to each offense element rather than as to each offense—provided, for the first time, a comprehensive statement of the culpability required for an offense. The early conceptions of mens rea were not simply undemanding, they were hopelessly vague and incomplete. They failed to tell courts enough about the required culpability for an offense to enable them to resolve the cases that commonly arose. For example, a prior case might tell a court that intentionally destroying a person's house was arson. But what results if the person intended the destruction but mistakenly believed she was destroying her own house? The previously announced intention requirement did not speak to what culpable state of mind was required as to the ownership of the building. A prior case might say intentionally killing a viable fetus was a crime. Was the defendant liable even if she reasonably (but mistakenly) believed the fetus was not viable? What culpable state of mind was required as to the viability of the fetus? When the mens rea requirement is unspecified or vague, it is left to the courts to decide ad hoc, and necessarily ex post facto, the precise culpability required for the offense. Element analysis permited legislatures to reclaim from the courts the authority to define the conditions of criminal liability and, for the first time, to provide a comprehensive statement of the culpability required for an offense.

The shift to element analysis, then, was not so much an attempt to change the traditional offense requirements, as it was to make them complete. Common law lawyers and judges were wrong to think that their offense-analysis view of culpability requirements was adequate to describe the required culpability. Their misconception stemmed in part from their conceptualization of an independent "law of mistake," which they saw as supplementing the culpability requirements of an offense definition. Thus, a person might satisfy the requirements of theft by intentionally taking someone else's property, yet have a defense if the law of mistake allowed a defense in the situation, such as when the defendant reasonably believed the property was his. To the common law mind, offense culpability requirements and the "law of mistake" that governed when a mistake provided a defense could be separate and independent doctrines.

The Model Penal Code drafters, in contrast, recognized that a mistake defense and an offense culpability requirement are one and the same. To say that negligence is required as to the victim's age in statutory rape is the same as saying that only a reasonable mistake as to age will provide a mistake defense. To say that recklessness is required as to "another person's property" in theft is the same as saying that only a reasonable or a negligent mistake will provide a mistake defense. This interchangeability between mistake defenses and culpability requirements informs Model Penal Code section 2.04(l)(a), which provides simply that mistake is a defense if it negates an offense culpability requirement. This is sometimes called the rule of logical relevance because it makes a person's mistake relevant to the determination of criminal liability only if the mistake is inconsistent with the existence of an offense culpability requirement.