Mens Rea

Common law doctrine traditionally paired mens rea with actus reus. Liability required both a guilty mind and a bad act. It is unclear, however, whether this most basic organizing distinction is coherent and useful to our understanding of offense requirements.

The actus reus of an offense typically is described as including the conduct constituting the offense, as well as any required circumstances or results of the conduct. The conduct must include a voluntary act. Where a result is an offense element, proof of the actus reus requires proof that the person's conduct and the result stand in a certain relation, as defined by the doctrine of causation: the conduct must have caused the result. Not every offense is defined in terms of conduct, however. In the absence of an act, liability may be based upon an omission to perform a legal duty of which the person is physically capable, or upon a person's knowing possession of contraband for a period of time sufficient to terminate the possession; these elements are part of the actus reus of the offenses. Thus, the actus reus of an offense commonly is said to include the doctrines of causation, voluntary act, omission, possession, and the conduct, circumstance, and result elements of the offense definition.

Undoubtedly, the actus reus–mens rea distinction is an extension of the obvious difference between a person's conduct, which we can directly observe, and the person's intention, which we cannot. In the simple case—the person shoots another person intending to injure him—both the person's conduct and intention are prerequisites to liability. The concepts of actus reus and mens rea adequately capture these two facts and note the empirical difference between them. It is natural to broaden the mens rea requirement beyond an intention to injure, to include recklessness or negligence as to injuring another person (as when a person target shoots in the woods without paying adequate attention to the possibility of campers in the overshoot zone). Similarly, it is natural to expand the actus reus requirement beyond an affirmative act of shooting another to include cases of injuring another by failing to perform a legal duty (as in failing to feed one's child) and cases of possession of contraband (such as illegal drugs), even though these may occur without an affirmative act.

While such an evolution is understandable, even logical, it does not follow that the resulting distinction is one around which criminal law is properly conceptualized, for the resulting concepts of actus reus and mens rea have limited usefulness.

First, there is no unifying internal characteristic among either the actus reus doctrines or the mens rea doctrines. Aspects of the actus reus requirements are not all "acts" or even all objective in nature. For example, a circumstance element of an offense may be entirely abstract, such as "being married" in bigamy or "without license" in trespass. Indeed, actus reus elements may include purely subjective states of mind, such as the requirement of causing "fear" in robbery or the necessary absence of "consent" in rape. Nor are the mens rea doctrines all state of "mind" requirements, or even subjective in nature. The mens rea element of negligence, for example, is neither subjective nor a state of mind, but rather a failure to meet an objective standard of attentiveness. Mens rea elements seem no more common in form than actus reus elements.

Further, the mens rea requirements and actus reus requirements do not serve functions distinct from one another. Most mens rea elements go to assess whether a violation is blameworthy, but so do many aspects of the actus reus, such as the voluntariness portion of the voluntary act requirement in commission offenses, the physical capacity requirement in omission offenses, and the possession offense requirement that the person have possession for a period sufficient to terminate possession. Similarly, while many aspects of the actus reus define the conduct that is criminal—specifically, the conduct and circumstance elements of the offense definition—some aspects of mens rea, such as the culpability requirements in inchoate offenses, serve the same function of defining the conduct that is prohibited. (That is, the conduct that will constitute an inchoate offense cannot be defined without reference to the offense's mens rea requirement—an intention to commit the completed offense. Conduct that constitutes an attempt is not a violation of the rules of conduct in the absence of the defendant's intention to commit an offense.)

In large part because of these difficulties, modern usage tends to avoid the mens rea–actus reus distinction. The closest substitute is the more modest distinction between "culpability" requirements and "objective" requirements of an offense definition. The former include those elements that require the defendant have a particular state of mind or negligence; the latter refer to all other offense requirements, commonly grouped into conduct, circumstances, and results.