Mens rea

Mens rea (from Latin, "bad mind") is the mental state that, in combination with the actus reus ("bad act"), produces criminal liability. According to Western jurisprudence, there must be a simultaneous concurrence of both actus reus and mens rea for a crime to have been committed; except in crimes of strict liability. However, mens rea does not necessarily mean that the actor "intended" to commit the forbidden act. Mens rea can result from negligent behavior. As it is impossible to know what a defendant has thought, mens rea is inferred. Mens rea is not the same thing as motive. Crimes of strict liability, such as traffic offenses, do not require mens rea to exist in order for criminal charges to be made.

The Model Penal Code, a publication of the American Law Institute, defines mens rea in particularly clear terms and has been adopted by much of the United States:

Purpose: the actor's conscious object is to commit the proscribed act.
Knowledge: the actor knows that the proscribed act will certainly occur as a result of his actions.
Recklessness: the actor is aware of a substantial and unjustifiable risk that the proscribed act will occur as a result of his actions.
Negligence: the actor reasonably should know that the proscribed act will occur as a result of his actions.

Contrast these four categories with the Common Law mens rea definitions:

Specific Intent: the actor intends to commit the proscribed act. E.g. premeditated murder. Roughly equivalent to the Purpose and Knowledge standards of the Model Penal Code.
General Intent: the actor intends a bad act, but not necessarily the act that is subsequently committed. Roughly equivalent to the Recklessness and Negligence standards of the Model Penal Code.
Strict Liability: the actor has no intent to commit any offense but is held liable nonetheless. The Model Penal Code does not recognize strict liability as a mens rea for which imprisonment is appropriate.

A concept that is somewhat misleading for its over-simplification is so-called presumed knowledge of the law. A more appropriate saying is that generally, ignorance of the law is not an excuse. No one is "presumed" to know the law; the reality is that knowledge of the particular statute prohibiting certain conduct is not usually an element of the crime created by that statute, and therefore knowledge of the law is not required for punishment.

Examples of mens rea in statutes

Model Penal Code: A person commits murder if he (1) purposely or knowingly (2) causes the death of a human being.

Common Law: (a) It shall be unlawful for a person to cause the death of a human being with malice aforethought. (b) A violation of this section is murder in the second degree.

See generally: Criminal law