Mens Rea and Actus Reus

To obtain a criminal conviction, the prosecution must establish the presence of two elements at the time of the crime—namely, actus reus (“guilty act”) and mens rea (“guilty mind”). A failure to show the presence of these elements will lead to an unconditional acquittal of the charged crime. Because both must be proven with evidence beyond a reasonable doubt by the prosecution at trial, the argument that there was no actus reus or mens rea is not a defense per se. They are unlike various affirmative defenses, such as the insanity defense or self-defense, which are viewed as excuses or justifications for otherwise disfavored conduct.

Actus Reus

Actus reus is the conduct requirement for a crime. The actus reus requirement excludes from criminal liability mere thoughts, a person’s condition or status (e.g., being an alcoholic as opposed to criminal behavior committed while intoxicated), and involuntary acts. Voluntary acts that satisfy this requirement include positive conduct, omissions of required or reasonably expected conduct, and possessions of criminally proscribed objects. The definition of a voluntary act is construed broadly to include any exercise of will; for example, an individual acting under threats or pressure is still considered to be acting voluntarily (although this may constitute an affirmative defense of duress).

Acts that might be considered involuntary can be divided into two categories: involuntary conduct and impaired consciousness. The first category includes physically coerced movements (e.g., someone pushes an individual into a third individual, causing harm to the third individual), reflex movements (e.g., the reaction of a person suddenly stung by a swarm of bees), muscular contractions caused by disease, and unconscious acts. Medical conditions that may cause involuntary conduct include strokes, epilepsy, and narcolepsy. These behaviors are all characterized by a break in the mind-body connection that leaves the person’s actions undirected by a conscious mental process. Behaviors that fall within this category are more widely accepted as lacking the actus reus component than those in the second category.

The second category, impaired consciousness, includes behaviors where there has been a sufficient diminishment of the link between mind and body so that the person is not consciously aware of the actions being taken but can engage in goal-directed conduct based on prior learned responses. Behavior during such periods may be referred to as “automatic” and the individual may be described as an “automaton.” Temporary brain damage from a concussion and sleep disorders such as night terrors are two common examples of impaired consciousness. Speculation has centered on whether symptoms resulting from hypoglycemia should be included within this category.

Evidence of a mental disorder is almost never permitted in conjunction with an assertion that the defendant lacked actus reus. Instead, this evidence can be used to support an insanity defense in those states that allow volitional arguments (e.g., the defendant’s behavior was the result of an irresistible impulse that resulted from a mental disorder). Under the insanity defense, if the individual’s inability to conform his or her behavior with the law is the result of a mental disorder, the defendant can be acquitted and subsequently committed for treatment, a result that cannot be imposed on those acquitted due to a lack of actus reus.

Mens Rea

Mens rea, or guilty mind, is the requirement that a defendant possess a particular state of mind at the time the crime is committed. The mens rea requirement for a crime is usually represented in the relevant criminal statute by one of the following terms: intent, purpose, knowledge, recklessness, or negligence. Most criminal statutes impose only a general or objective mens rea requirement, where the inquiry focuses on whether a reasonable person would have known that the act would cause harm. To be convicted of a so-called general-intent crime, the defendant must have known that he or she was acting but not that any particular criminal consequences would result from his or her act; this is usually captured by the rubrics of “recklessness” or “negligence.” To meet this requirement, the prosecution does not have to explore the mental state of the defendant. Other crimes require a showing of specific or subjective mens rea, in which the prosecution must establish that the defendant did actually know or intend that a particular harm would result from his or her act. Because this can be difficult to prove, specific or subjective mens rea is usually reserved for more serious crimes with more severe punishments.

Evidence of a mental disorder is rarely allowed in cases involving general or objective mens rea, because an individual’s mental state is irrelevant to this inquiry. In contrast, some states permit the introduction of evidence of mental disorder whenever it is logically relevant to rebut the specific or subjective mens rea requirement (i.e., the state of mind associated with a specific-intent crime). If this argument (sometimes called diminished capacity) is successful, the usual result is that the defendant will only face conviction of a lesser included offense that merely requires a showing of general or objective intent. Prosecutors who charge a defendant with a specific-intent crime will frequently also charge the defendant with a general-intent crime as a means to enable them to impose some criminal sanction on the defendant should they be unable to convince the judge or jury that the requisite specific intent was present at the time of the crime. The use of evidence of a mental disorder discounting the presence of specific intent differs from the use of evidence of mental disorder for an insanity defense in that the former can result in an unconditional acquittal that does not lead to the civil commitment and treatment typically associated with the latter. This result is the same as that which results from a lack of actus reus because, as noted above, both mens rea and actus reus are elements of the crime that must be proven beyond a reasonable doubt, and if they cannot be proven the defendant will be exonerated of the charged crime. The insanity defense, in contrast, can excuse a defendant from punishment but does not exonerate him or her.

Courts and legislatures have often restricted the use of expert testimony concerning mental disorders. For example, some jurisdictions do not permit testimony regarding any evidence of a mental disorder except when it is being used to support an insanity defense. This position was upheld in Clark v. Arizona (2006), in which the Supreme Court ruled that Arizona’s decision to restrict evidence of a mental disorder to insanity claims only, thus not admitting such evidence when it could be used to address mens rea, does not violate due process. Nevertheless, even in those states where expert testimony relating to mens rea cannot be admitted in court, the information about a defendant’s mental state could still be used in plea negotiations and at sentencing.


  1. Clark v. Arizona, 126 S. Ct. 2709 (2006).
  2. Morse, S. J. (2003). Diminished rationality, diminished responsibility. Ohio State Journal of Criminal Law, 1, 289-308.
  3. (1994). Act and crime. University of Pennsylvania Law Review, 142, 1443-1890.

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