Mens rea means criminal intent. An act must be blameworthy. An act must be done with criminal intent or be an act of gross negligence or recklessness. Mens rea is one of two components of a crime, the other being actus reus. Establishing the mens rea of an offender is usually necessary to prove guilt in a criminal trial. The prosecution typically must prove beyond reasonable doubt that the defendant committed the offense with a culpable state of mind. Justice Holmes famously illustrated the concept of intent when he said “even a dog knows the difference between being stumbled over and being kicked.”
A criminal offense requires a criminal intent. The requirement of a criminal intent is based on moral blameworthiness, a conscious decision to knowingly engage in criminal conduct or to act in a reckless or negligent fashion. Mens rea consists of four states of mind. The most culpable is purposely, and then knowingly, recklessly, and negligently. Strict liability offenses require an actus reus, but do not incorporate a mens rea requirement. These typically are public welfare offenses or crimes that protect public safety by regulating food, drugs, and transportation.
Most states use the Model Penal Code's
classification for various mentes reae. The Model Penal Code organizes and
defines culpable states of mind into four hierarchical categories:
acting purposely - the defendant had an underlying conscious object to act
acting knowingly - the defendant is practically certain that the conduct will cause a particular result
acting recklessly - The defendant consciously disregarded a substantial and unjustified risk
acting negligently - The defendant was not aware of the risk, but should have been aware of the risk
Thus, a crime committed purposefully would carry a more severe punishment than if the offender acted knowingly, recklessly, or negligently. Model Penal Code greatly impacted the criminal codes of a number of states and continues to be influential in furthering discourse on mens rea.
On Culpability and Crime: The Treatment of Mens Rea in the Model Penal Code - Herbert Wechsler, Columbia Law School, ANNALS of the American Academy of Political and Social Science, Vol. 339, No.1 - When conduct has the external attributes of a crime, should further mental elements be required for conviction, and, if so, what should they be?
Decisions have too frequently been imprecise in analysis and inconsistent in results, yielding a multitude of single instances which in the aggregate dilute the moral force that should attach to condemnation of behavior considered criminal. Criminal liability may justly be based only upon conduct which includes a voluntary act or the omission to perform an act which the actor was physically able to perform. Act or omission are essential prerequisites to liability, but they are not sufficient to establish culpability.
The Model Penal Code proposes four concepts to describe the kinds of culpability which are sufficient to establish liability: purpose, knowledge, recklessness, or negligence. The doctrine of strict liability, ignorance and mistake of fact, intoxication, and ignorance and mistake of law are pertinent to establishing liability.
One of the most contentious problems of the penal law concerns the criteria that should determine when individuals whose conduct would otherwise be criminal ought to be exculpated on the ground that they were suffering from mental disease or defect when they acted.
The Model Penal Code would exculpate the person who, as a result of mental disease or defect, lacked substantial capacity either to appreciate the criminality of his conduct or to conform his behavior to the requirements of law.
States categorized mental states into crimes which required "general intent" and "specific intent." However, due to the confusion that ensued over how to describe "intent," most states now either use the Model Penal Code's four-tiered classification, or the malice distinction.