Actus Reus

Actus reus, Latin for "guilty act," is the requirement that a crime must involve a voluntary physical act or, in limited circumstances, a culpable omission. The principle serves as a fundamental safeguard against punishing mere thoughts, intentions, or a person's status. Under both common law and the Model Penal Code, criminal liability requires an act that is the product of the defendant's conscious will and effort.

An act is considered voluntary when it results from a conscious exercise of the will. Reflexive movements, convulsions, acts performed while unconscious or asleep, and conduct during a hypnotic state are generally not considered voluntary acts. Importantly, if a course of conduct includes even one voluntary act, the actus reus requirement is satisfied. The MPC defines an act broadly but excludes reflexes, movements during unconsciousness, and conduct during seizures from the definition of voluntary acts.

Omissions, or failures to act, can satisfy the actus reus requirement only when the defendant had a legal duty to act. Legal duties arise from statutes (such as filing tax returns), contractual obligations, special relationships (parent-child), voluntary assumption of care, or the creation of a peril. The actus reus requirement also prohibits status crimes under the Eighth Amendment, as the Supreme Court held in Robinson v. California that punishing a person for being an addict, rather than for the act of using drugs, constitutes cruel and unusual punishment.