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The standard common law test of criminal liability is expressed in the Latin phrase actus reus non facit reum nisi mens sit rea, i.e. "the act is not culpable unless the mind is guilty". In jurisdictions with due process, there must be both actus reus ("guilty act") and mens rea for a defendant to be guilty of a crime (see concurrence). As a general rule, someone who acted without mental fault is not liable in criminal law. Exceptions are known as strict liability crimes.
In civil law, it is usually not necessary to prove a subjective mental element to establish liability for breach of contract or tort, for example. But if a tort is intentionally committed or a contract is intentionally breached, such intent may increase the scope of liability and the damages payable to the plaintiff.
In some jurisdictions, the terms mens rea and actus reus have been replaced by alternative terminology. In Australia, mens rea is now called "fault elements" or "mental elements" and actus reus is now called "physical elements" or "external elements". The point of the changes was to replace obscure Latin with simpler words.
Levels of mens rea
Under the traditional common law, the guilt or innocence of a person relied upon whether he had committed the crime (actus reus), and whether he intended to commit the crime (mens rea). However, many modern penal codes have created levels of mens rea called modes of culpability, which depend on the surrounding elements of the crime: the conduct, the circumstances, and the result, or what the Model Penal Code calls CAR (conduct, attendant circumstances, result). The definition of a crime is thus constructed using only these elements rather than the colorful language of mens rea:
Murder is the unlawful killing of a human being with malice aforethought.— 18 U.S.C. §1111 (traditional common law)
A person commits an offense if he:
(1) intentionally or knowingly causes the death of an
individual— portion of Texas Penal Code ch. 19 §19.02 (modern offense element)
The traditional common law definitions and the modern definitions approach the crime from different angles.
In the traditional common law approach, the definition includes:
- actus reus: unlawful killing of a human being;
- mens rea: malice aforethought.
Modern law approaches the analysis somewhat differently. Homicide is a "results" crime in that it forbids any "intentional" or "knowing" conduct that results in the death of another human being. "Intentional" in this sense means the actor possessed a "purpose" or "desire" that his or her objective (i.e. death of another human being) be achieved. "Knowing" means that the actor was aware or practically certain that the death would result. Thus, the actus reus and mens rea of homicide in a modern criminal statute can be considered as follows:
- actus reus: any conduct resulting in the death of another individual;
- mens rea: intent or knowledge that the conduct would result in the death.
In the modern approach, the attendant circumstances tend to replace the traditional mens rea, indicating the level of culpability as well as other circumstances. For example, the crime of theft of government property would include as an attendant circumstance that the property belong to the government.
Modes of culpability
The levels of mens rea and the distinction between them vary between jurisdictions. Although common law originated from England, the common law of each jurisdiction with regard to culpability varies as precedents and statutes vary.
England and Wales
- Direct intention: the actor has a clear foresight of the consequences of his actions, and desires those consequences to occur. It's his aim or purpose to achieve this consequence (death).
- Oblique intention: the result is a virtually certain consequence or a 'virtual certainty' of the defendant's actions, and that the defendant appreciates that such was the case.
- Knowingly: the actor knows, or should know, that the results of his conduct are reasonably certain to occur.
- Recklessness: the actor foresees that particular consequences may occur and proceeds with the given conduct, not caring whether those consequences actually occur or not.
- Criminal negligence: the actor did not actually foresee that the particular consequences would flow from his actions, but a reasonable person, in the same circumstances, would have foreseen those consequences.
- Intention: the accused willingly committed a criminal act entirely aware of his actions and their consequences. Necessary for murder and for assault.
- Recklessness: the accused was aware the criminal act could be potentially dangerous but did not give a second thought to its consequences, for example involuntary culpable homicide.
- Negligence: the accused unintentionally committed the criminal act by accident for one reason or another. However this tends not to be a valid excuse.
State criminal law
The vast majority of criminal prosecutions in the United States are carried out by the component states in accordance with the laws of the state in question. Historically, the states (with the partial exception of civil-law Louisiana) applied common-law rules of mens rea similar to those extant in England, but over time American understandings of common-law mens rea terms diverged from those of English law and from each other. By the late 1950s to early 1960s, the common law of mens rea was widely acknowledged to be a slippery, vague, confused mess. This was one of several factors that led to the development of the Model Penal Code.
Model Penal Code
Since its publication in 1957, the formulation of mens rea set forth in the Model Penal Code has been highly influential throughout North America in clarifying the discussion of the different modes of culpability.
- Strict liability: the actor engaged in conduct and his mental state is irrelevant. Under Model Penal Code Section 2.05, this mens rea may only be applied where the forbidden conduct is a mere violation, i.e. a civil infraction.
- Negligently: a "reasonable person" would be aware of a "substantial and unjustifiable risk" that his conduct is of a prohibited nature, will lead to a prohibited result, and/or is under prohibited attendant circumstances, and the human-individual was not so aware but should have been.
- Recklessly: the human-individual consciously disregards a "substantial and unjustifiable risk" that his conduct is of a prohibited nature, will lead to a prohibited result, and/or is of a prohibited nature.
- Knowingly: the human-individual is practically certain that his conduct will lead to the result, or is aware to a high probability that his conduct is of a prohibited nature, or is aware to a high probability that the attendant circumstances exist.
- Purposefully: the human-individual has the "conscious object" of engaging in conduct and believes or hopes that the attendant circumstances exist.
Except for strict liability, these classes of mens rea are defined in Section 2.02(2) of the MPC.
Federal criminal law
As the federal government of the United States does not have a generalized police power like that of the states, the scope of its criminal statutes is necessarily circumscribed; ordinary prosecutions are the province of the states, and only crimes of special federal import are pursued by the federal government. Consequently, Title 18 of the United States Code does not have a culpability scheme but relies on more traditional definitions of crimes taken from common law. For example, malice aforethought is used as a requirement for committing capital murder.
The Supreme Court of Canada has found that the Canadian Charter of Rights and Freedoms guarantees a minimum requirement for the mental state of various crimes. For example, the crime of murder must include a mental requirement of at least subjective foresight of death. For crimes where imprisonment is a sanction, there is a requirement of at least a defence of due diligence.
Mens rea needs to be proved by prosecution from offence to offence. If it is a common law offence, mens rea is found out by relevant precedent (DPP v Morgan  AC 182). Where the offence is in legislation, the requisite mens rea is found by interpreting the intention of the legislation (He Kaw Teh). They must intend to commit the full offence.
Ignorance of the law and mens rea
The general rule under common law is that "ignorance of the law or a mistake of law is no defense to criminal prosecution." In some cases, courts have held if knowledge of a law, or the intent to break a law, is a material element of an offense a defendant may use ignorance as a defense to willfulness if his misunderstanding is in good faith:
The proliferation of statutes and regulations has sometimes made it difficult for the average citizen to know and comprehend the extent of the duties and obligations imposed by the tax laws. . . . [T]he Court almost 60 years ago interpreted the statutory term "willfully" as used in federal criminal tax statutes as carving out an exception to the traditional rule."
Crimes like tax evasion are specific intent crimes and require intent to violate the law as an element of the offense. In R. v. Klundert, for example, the Ontario Court of Appeal found as follows:
- " Section 239(1)(d) is part of an Act which is necessarily and notoriously complex. It is subject to ongoing revision. No lay person is expected to know all the complexities of the tax laws. It is accepted that people will act on the advice of professionals and that the advice will often turn on the meanings to be given to provisions in the Act that are open to various interpretations. Furthermore, it is accepted that one may legitimately structure one’s affairs so as to minimize tax liability. Considered in this legislative context, I have no difficulty in holding that a mistake or ignorance as to one’s liability to pay tax under the Act may negate the fault requirement in the provision, regardless of whether it is a factual mistake, a legal mistake, or a combination of both."
Not all offenses require specific intent, and a misreading, even in good faith, may not excuse the criminal conduct. A good-faith belief that a law is unjust or unconstitutional is no excuse, but "reasonable compliance upon an official statement of law, afterward determined to be invalid or erroneous" does not constitute a criminal act.
Subjective and objective tests
The test for the existence of mens rea may be:
- (a) subjective, where the court must be satisfied that the accused actually had the requisite mental element present in his or her mind at the relevant time (for purposely, knowingly, recklessly etc) (see concurrence);
- (b) objective, where the requisite mens rea element is imputed to the accused, on the basis that a reasonable person would have had the mental element in the same circumstances (for negligence); or
- (c) hybrid, where the test is both subjective and objective.
The court will have little difficulty in establishing mens rea if there is actual evidence – for instance, if the accused made an admissible admission. This would satisfy a subjective test. But a significant proportion of those accused of crimes make no such admissions. Hence, some degree of objectivity must be brought to bear as the basis upon which to impute the necessary component(s). It is always reasonable to assume that people of ordinary intelligence are aware of their physical surroundings and of the ordinary laws of cause and effect (see causation). Thus, when a person plans what to do and what not to do, he will understand the range of likely outcomes from given behaviour on a sliding scale from "inevitable" to "probable" to "possible" to "improbable". The more an outcome shades towards the "inevitable" end of the scale, the more likely it is that the accused both foresaw and desired it, and, therefore, the safer it is to impute intention. If there is clear subjective evidence that the accused did not have foresight, but a reasonable person would have, the hybrid test may find criminal negligence. In terms of the burden of proof, the requirement is that a jury must have a high degree of certainty before convicting, defined as "beyond a reasonable doubt" in the United States and "sure" in the United Kingdom. It is this reasoning that justifies the defenses of infancy, and of lack of mental capacity under the M'Naghten Rules, an alternate common law rule (e.g., Durham test), and one of various statutes defining mental illness as an excuse. Moreover, if there is an irrebuttable presumption of doli incapax - that is, that the accused did not have sufficient understanding of the nature and quality of his actions – then the requisite mens rea is absent no matter what degree of probability might otherwise have been present. For these purposes, therefore, where the relevant statutes are silent and it is for the common law to form the basis of potential liability, the reasonable person must be endowed with the same intellectual and physical qualities as the accused, and the test must be whether an accused with these specific attributes would have had the requisite foresight and desire.
In English law, s8 Criminal Justice Act 1967 provides a statutory framework within which mens rea is assessed. It states:
- A court or jury, in determining whether a person has committed an offense,
- (a) shall not be bound in law to infer that he intended or foresaw a result of his actions by reasons only of its being a natural and probable consequence of those actions; but
- (b) shall decide whether he did intend or foresee that result by reference to all the evidence, drawing such inferences from the evidence as appear proper in the circumstances.
Under s8(b) therefore, the jury is allowed a wide latitude in applying a hybrid test to impute intention or foresight (for the purposes of recklessness) on the basis of all the evidence.
Relevance of motive
One of the mental components often raised in issue is that of motive. If the accused admits to having a motive consistent with the elements of foresight and desire, this will add to the level of probability that the actual outcome was intended (it makes the prosecution case more credible). But if there is clear evidence that the accused had a different motive, this may decrease the probability that he or she desired the actual outcome. In such a situation, the motive may become subjective evidence that the accused did not intend, but was reckless or willfully blind.
Motive cannot be a defense. If, for example, a person breaks into a laboratory used for the testing of pharmaceuticals on animals, the question of guilt is determined by the presence of an actus reus, i.e. entry without consent and damage to property, and a mens rea, i.e. intention to enter and cause the damage. That the person might have had a clearly articulated political motive to protest such testing does not affect liability. If motive has any relevance, this may be addressed in the sentencing part of the trial, when the court considers what punishment, if any, is appropriate.
Recklessness (United States: "willful blindness")
In such cases, there is clear subjective evidence that the accused foresaw but did not desire the particular outcome. When the accused failed to stop the given behavior, he took the risk of causing the given loss or damage. There is always some degree of intention subsumed within recklessness. During the course of the conduct, the accused foresees that he may be putting another at risk of injury: A choice must be made at that point in time. By deciding to proceed, the accused actually intends the other to be exposed to the risk of that injury. The greater the probability of that risk maturing into the foreseen injury, the greater the degree of recklessness and, subsequently, sentence rendered. For example, at common law, an unlawful homicide committed recklessly would ordinarily constitute the crime of voluntary manslaughter. One committed with "extreme" or "gross" recklessness as to human life would constitute murder, sometimes defined as "depraved heart" or "abandoned and malignant heart" murder.
Here, the test is both subjective and objective. There is credible subjective evidence that the particular accused neither foresaw nor desired the particular outcome, thus potentially excluding both intention and recklessness. But a reasonable person with the same abilities and skills as the accused would have foreseen and taken precautions to prevent the loss and damage being sustained. Only a small percentage of offences are defined with this mens rea requirement. Most legislatures prefer to base liability on either intention or recklessness and, faced with the need to establish recklessness as the default mens rea for guilt, those practising in most legal systems rely heavily on objective tests to establish the minimum requirement of foresight for recklessness.
- Actus reus
- Animus nocendi
- Command responsibility
- Henry de Bracton
- Morissette v. United States (1952)
- Flores-Figueroa v. United States (2009)
- Voluntas necandi
- Elizabeth A. Martin, ed. (2003). Oxford Dictionary of Law. Oxford: Oxford University Press. ISBN 0-19-860756-3.
- Black's Law Dictionary, p. 889 (5th ed. 1979).
- Barron's Law Dictionary, p. 289 (2d ed. 1984).
- Brent Fisse, "Howard's Criminal Law" (1990) 12-13.
- Meyers v R  HCA 43.
- Dubber p. 46
- R v Nedrick  1 WLR 1025
- R v Woollin 
- R v Matthews & Alleyne 
- R v Cunningham 
- R v G & R 
- R v Caldwell 
- Dubber pp. 60-80
- Dubber p. 55
- He Kaw Teh (1985) 157 CLR 523 AustLII; see also CTM (2008) 236 CLR 440 AustLII
- Knight v R  HCA 56
- Cheek v. United States, 498 U.S. 192 (1991)
- Ostrowski v Palmer  HCA 30
- Cheek, at 199-200
- Cheek, at 194
- People v. Marrero, 507 N.E.2d 1068 (N.Y. 1987)
- Cheek, at 205-7
- State v. Godwin, 31 S.E. 221 (N.C. 1898)
- In re Winship, 397 U.S. 358 (1970)
- R v Majid  EWCA Crim 2563
- Crown Bench Book: Directing the Jury. Judicial Studies Board. 2010.
- In re Devon T., 584 A.2d 1287 (Md. Court App. 1991)
- Criminal Justice Act 1967 (c.80)
- California Penal Code §§ 188-199
- Dubber, Markus D. (2002). Criminal Law: Model Penal Code. Foundation Press.
- Criminal Responsibility and Intent
- Mens Rea: The Need for a Meaningful Intent Requirement in Federal Criminal Law: Hearing before the Over-Criminalization Task Force of 2013 of the Committee on the Judiciary, House of Representatives, One Hundred Thirteenth Congress, First Session, July 19, 2013.