Mens rea (; Law Latin for "guilty mind") is the mental element of a person's intention to commit a crime; or knowledge that one's action or lack of action would cause a crime to be committed. It is a necessary element of many crimes.
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".:113 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. Moreover, when a person intends a harm, but because of bad aim or other cause, the intent is transferred from an intended victim to an unintended victim, the case is considered to be a matter of transferred intent.:63-64
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.
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 §19·02 (modern offense element)
The traditional common law definitions and the modern definitions approach the crime from different angles.
In the common law approach, the definition includes:
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:
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.
The levels of mens rea and the distinction between them vary among jurisdictions. Although common law originated from England, the common law of each jurisdiction with regard to culpability varies as precedents and statutes vary.
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, and confused mess. This was one of several factors that led to the development of the 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 the US in clarifying the discussion of the different modes of culpability. The following levels of mens rea are found in the MPC::60-62
Except for strict liability, these classes of mens rea are defined in Section 2·02(2) of the MPC.
Since 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 use the aforementioned culpability scheme but relies instead 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. They must intend to commit the full offence.
Mens Rea in the Indian Penal Code 1860 sets out the definition of offences, the general conditions of liability, the conditions of exemptions from liability and punishments for the respective offences. Legislatures had not used the common law doctrine of mens rea in defining these crimes. However, they preferred to import it by using different terms indicating the required evil intent or mens rea as an essence of a particular offence.
Guilt in respect to almost all offences created under the IPC is fastened either on the ground of intention, knowledge or reason to believe. Almost all the offences under the IPC are qualified by one or other words such as 'wrongful gain or loss', 'dishonesty', 'fraudulently', 'reason to believe', 'criminal knowledge or intention', 'intentional cooperation', 'voluntarily', 'malignantly', 'wantonly', 'maliciously'. All these words indicate the blameworthy mental condition required at the time of commission of the offence, nowhere found in the IPC, its essence is reflected in almost all the provisions of the Indian Penal Code 1860. Every offence created under the IPC virtually imports the idea of criminal intent or mens rea in some form or other.
The general rule under common law and statutory law is that "ignorance of the law or a mistake of law is no defense to criminal prosecution." However, in some cases, courts have held that if knowledge of a law, or if intent to break a law, is a material element of an offense, then a defendant may use good faith ignorance as a defense:
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:
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.
However, a law must be reasonably clear; it must be worded so that a reasonable layman can comprehend the specific prohibited acts. Otherwise, the law may be unconstitutional pursuant to the Vagueness doctrine.
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 makes no such admission. 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 rule), 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.
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.
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.
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.:257:81 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. In common law, for example, 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" or "depraved indifference" murder.:82
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.