Actus Reus (Voluntary Act)

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'Actus Reus' refers to the requirement that the defendant have carried out a voluntary act. It is important to distinguish such acts from thoughts, words, states of possession, or status; involuntary acts; and omissions.

Actus Reus is one of four elements that must exist in order for a crime to have occurred. The other three are:

Model Penal Code

The American Law Institute's Model Penal Code, which has been influential in the standardization of criminal law in the United States, sets out the following provision with respect to the act requirement:

1. A person is not guilty of an offense unless his liability is based on conduct which includes a voluntary act or the omission to perform an act of which he is physically capable.

2. The following are not voluntary acts within the meaning of this Section:

  • (a) a reflex or convulsion;
  • (b) a bodily movement during unconsciousness or sleep;
  • (c) conduct during hypnosis or resulting from hypnotic suggestion;
  • (d) a bodily movement that otherwise is not a product of the effort or determination of the actor, either conscious or habitual.

3. Liability for the commission of an offense may not be based on an omission unaccompanied by action unless:

  • (a) the omission is expressly made sufficient by the law defining the offense; or
  • (b) a duty to perform the omitted act is otherwise imposed by law.

4. Possession is an act, within the meaning of this Section, if the possessor knowingly procured or received the thing possessed or was aware of his control thereof for a sufficient period to have been able to terminate his possession.

What Are Not Acts

Thoughts, Words, Possession and Status

Acts are distinct from thoughts, words, possession and status.

  • Thoughts alone can never be punished as crimes. Lawmakers are hesitant to impose controls on what people may think, and such laws would also present profound problems of proof and enforcement.
  • Words usually cannot constitute acts. However, depending on the jurisdiction and the crime, an agreement to commit a crime may be enough to constitute conspiracy, and words of encouragement may constitute aiding and abetting a crime.
  • In many countries, possession holds a unique position, as it does not constitute an act, but may be criminalized. In some countries, however, including the United States, possession has been legally defined as a voluntary act. Possession usually includes only knowing, conscious possession.
  • A defendant's status or condition cannot constitute an act. In the United States, the Supreme Court has held that a law making addiction a crime to be an unconstitutional cruel and unusual punishment.

Involuntary Conduct

Acts must be voluntary. Reflexes, bodily movements during sleep or unconsciousness, and conduct while under hypnosis do not constitute acts. However, conduct leading to the involuntary state may suffice, e.g. an individual who has himself hypnotized for the purpose of committing a crime. Similarly, becoming extremely intoxicated before crashing a car would surely satisfy the act requirement.


Omissions do not constitute acts. For example, an individual who chooses not to save a stranger drowning in front of him usually cannot be held criminally liable, at least in the Anglo-American legal tradition. Reasons for this approach include the intuition that the difference between acting and failing to act should be legally important, the fact that laws criminalizing inaction would be impermissibly vague, and the difficulty of deciding which of various people should be held liable when more than one person chose not to act.

  • There are a number of exceptions to this rule. For example, when the defendant and victim have a special relationship, such as when the defendant is the victim's parent, a duty to act may exist. A duty may also arise from contract; if two individuals have a relationship of mutual dependence, such as mountain climbers; if the danger to the victim was caused by the defendant; if the defendant has undertaken to assist the victim; if a statute imposes a duty to act; or if the defendant is a public official with a duty to act, such as a police officer.