Actus reus

Lore & Background

Actus reus, Latin for “guilty act,” is one of the two core elements required to prove a crime in common law jurisdictions, the other being mens rea, or “guilty mind.” In the United States, it is sometimes called the external or objective element of a crime. The principle derives from a statement by Edward Coke: actus non facit reum nisi mens sit rea, meaning an act does not make a person guilty unless their mind is also guilty. For an actus reus to exist, there must generally be an act, defined across various common law jurisdictions as a bodily movement, whether voluntary or involuntary. However, the U.S. Supreme Court ruled in Robinson v.

California that mere status—such as being a drug addict—does not constitute an act and therefore cannot be criminal. Commentator Dennis Baker notes that an act can consist of commission, omission, or possession. Omission involves a failure to act where a duty is imposed by law, such as under contract, assumed care, creation of a dangerous situation, or official position.

In English law, no Good Samaritan rule exists, so liability for omission requires a duty of care. Possession, though not traditionally an act under common law, is treated as a voluntary act in some jurisdictions like the United States, fulfilling the requirements for actus reus. Voluntariness is essential: conduct must be willed.

Involuntary conduct includes reflexes, convulsions, movements during unconsciousness or sleep, hypnotic behavior, or any movement not a product of the actor’s effort. For instance, in People v. Decina, a driver who suffered a seizure while knowing his susceptibility was held criminally liable because his decision to drive was voluntary, even if the collision itself was not. Automatism—behavior of which a person is unaware and over which they have no control—must be eliminated to prove voluntariness.

Reader's Guide

The concept of actus reus is central to criminal liability, ensuring that no one is punished solely for thoughts or status. The U.S. Supreme Court in Robinson v. Actus reus requires a voluntary act, though jurisdictions differ on what constitutes voluntariness. The Model Penal Code lists involuntary conduct as including reflexes, convulsions, movements during unconsciousness or sleep, conduct during hypnosis, and movements not product of the actor's effort.

Cases such as People v. Decina illustrate that knowingly engaging in dangerous conduct despite a known risk of involuntary episodes can establish culpability. The defense of automatism—behavior of which a person is unaware and over which they have no conscious control—may negate actus reus. The principle that 'what goes on inside a person's head is never enough in itself to constitute a crime' underscores the importance of actus reus in protecting individual liberty.