In criminal law, mens rea (; Law Latin for "guilty mind") is the mental state of a defendant who is accused of committing a crime. In common law jurisdictions, most crimes require proof both of mens rea and actus reus ("guilty act") before the defendant can be found guilty.
Introduction 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". 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 as a result 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. The types of mental states that apply to crimes vary depending on whether a jurisdiction follows criminal law under the common law tradition or, within the United States, according to the Model Penal Code. 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 In some jurisdictions, the terms mens rea and actus reus have been replaced by alternative terminology.
Levels of mens rea within the United States Under the tradition of common law, judges would often require a "bad state of mind" in addition to an action or omission (actus reus) to find a criminal guilty. Over time, culpable mental states (mens rea) became varied among different types of crimes. Such crimes and mental states might include, for example, "malice" for murder, "fraudulence" for fraud, "willfulness and corruption" for perjury, and so on. The crime of manslaughter, further, might not even require a "bad mind" but simply a "negligent" one. Regardless of how the requirements are categorized, the Supreme Court has explained mens rea requirements for crimes are "universal" and essential to "mature systems of law", even going so far as to say that this belief undergirds notions of free will and morality. Within the United States, there is no single encompassing criminal law. Criminal laws are passed and enforced by the states‚ or the federal government, but each of these criminal "codes" vary and may or may not draw from the same theoretical sources.
State criminal law The vast majority of criminal prosecutions in the United States are carried out by the several 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. Concepts like "general intent" and "specific intent" dominated classifications of mental states in state common law, but 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. Nevertheless, states continue to use mental states beyond or besides those listed in the Model Penal Code. Supreme Court Justice Stephen Breyer described the distinction between the two systems in his dissent in Delling v. Idaho:
Case One: The defendant, due to insanity, believes that the victim is a wolf. He shoots and kills the victim. Case Two: The defendant, due to insanity, believes that a wolf, a supernatural figure, has ordered him to kill the victim. In Case One, the defendant does not know he has killed a human being, and his insanity negates a mental element necessary to commit the crime. In Case Two, the defendant has intentionally killed a victim whom he knows is a human being; he possesses the necessary mens rea. In both cases the defendant is unable, due to insanity, to appreciate the true quality of his act, and therefore unable to perceive that it is wrong. But … the defendant in Case One could defend the charge by arguing that he lacked the mens rea, whereas the defendant in Case Two would not be able to raise a defense based on his mental illness.
Federal criminal law 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 connected to the constitutional powers may be pursued by the federal government. Nevertheless, the Supreme Court holds that required mens rea is an essential element of federal criminal offenses. 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, and the Supreme Court has applied mental states such as "willfully."
Model Penal Code Because the landscape of criminal law varied from state to state, the American Law Institute (which issues "restatements" of American legal jurisprudence) declined to issue a restatement of criminal law in favor of a "model" code for states to issue new, standardized criminal law. This Model Penal Code ("MPC") was completed in 1962, and received praise from legal scholars for its reformulation of criminal law. Although not all states follow the criminal law as constructed within the MPC, over 34 states had adopted part or substantially all of the MPC as law by 1983. The federal government has not adopted the MPC, although it has attempted to do so for many decades. The formulation of mens rea set forth in the Model Penal Code has been highly influential throughout the United States in clarifying the discussion of the different modes of culpability since its publication. The following levels of mens rea are found in the MPC §2.02(2), and are considered by the United States Supreme Court to be the four states of mind that give rise to criminal liability:
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