In criminal law, mens rea the Latin term for "guilty mind is usually one of the necessary elements of a crime. The standard common law test of criminal liability is usually expressed in the Latin phrase, actus non facit reum nisi mens sit rea, which means that "the act does not make a person guilty unless the mind is also guilty". Thus, in jurisdictions with due process, there must be an actus reus accompanied by some level of mens rea to constitute the crime with which the defendant is charged (see the technical requirement of concurrence). The exception is strict liability crimes (in the civil law, it is not usually necessary to prove a subjective mental element to establish liability, say for breach of contract or a tort, although if intentionally committed, this may increase the measure of damages payable to compensate the plaintiff).
Quite simply therefore mens rea refers to the mental element of the offence that accompanies the actus reus. In some jurisdictions the terms mens rea and actus reus have been superseded by alternative terminology. In Australia for example the elements of all federal offences are now designated as "fault elements" (mens rea) and "physical element" (actus reus). This terminology was adopted in order to replace the obscurity of the Latin terms with simple and accurate phrasing.
Under the traditional common law, the guilt or innocence of a person turned on whether they had committed the crime, actus reus, and whether they intended to commit the crime, mens rea. However, many modern penal codes have created levels of mens rea called modes of culpability which vary depending on the offense 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 in traditional common law:
The traditional common law and the modern element definitions approach the crime from different angles.
In the traditional common law approach, the definition includes:
In the modern element approach, the definition includes:
In the modern offense element approach, the attendant circumstances tend to take over for the traditional mens rea, indicating the level of culpability as well as other circumstances, i.e. the crime of theft of government property would include as an attendant circumstance that the property belong to the government.
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.
The general rule under U.S. law is that "ignorance of the law or a mistake of law is no defense to criminal prosecution." See Cheek v. United States, 498 U.S. 192 (1991). There are exceptions to this rule which are sometimes referred to as crimes of "specific intent", For example, in the case of tax evasion under the defendant must be shown to have a specific intent to violate an actually known legal duty. See Tax avoidance and tax evasion.
The test for the existence of mens rea may be:
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. It is this reasoning that justifies the defences of infancy, and of lack of mental capacity under the M'Naghten Rules, and the various statutes defining mental illness as an excuse. Self-evidently, 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:
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 defence. 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.
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.
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.