Core principles of criminal liability - Actus reus

Learning Outcomes

After studying this article, you will be able to explain the meaning and function of actus reus in criminal law, distinguish between conduct, result, and state of affairs crimes, identify when omissions can give rise to liability, and apply the rules of causation. You will also be able to define and use key terms with precision and answer SQE1-style questions on this topic.

SQE1 Syllabus

For SQE1, you are required to understand the core principles of criminal liability relating to actus reus. Focus your revision on:

• the definition and role of actus reus in criminal offences
• the distinction between conduct, result, and state of affairs crimes
• when omissions can give rise to criminal liability
• the rules of factual and legal causation, including intervening acts and the thin skull rule
• the requirement of voluntariness in criminal acts

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

1. What is the general rule regarding liability for omissions in criminal law? Name two exceptions.
2. What is the difference between factual causation and legal causation?
3. True or false? A person can be criminally liable for an involuntary act.
4. What is the effect of the thin skull rule in criminal law?

Introduction

Actus reus is a fundamental concept in criminal law. It refers to the physical element of an offence—the conduct, omission, or state of affairs that, together with the required mental element (mens rea), makes a person criminally liable. For most crimes, the prosecution must prove both actus reus and mens rea. Understanding actus reus is essential for analysing criminal liability and answering SQE1 questions.

The Meaning of Actus Reus

Actus reus means the "guilty act." It is the external component of a crime, separate from the mental state of the defendant. The actus reus may be a positive act, a failure to act (omission), or simply being in a particular situation.

Key Term: actus reus The external element of a criminal offence, consisting of conduct, omission, or a state of affairs, required by law for liability.

Voluntary Acts

Criminal liability generally requires a voluntary act. The law does not punish people for actions over which they have no control.

Key Term: voluntary act An act performed with conscious control by the defendant.

Involuntary Acts

If a person's conduct is involuntary—such as a reflex action, an act while unconscious, or movement during a seizure—they are not usually criminally liable.

Automatism

Automatism is a defence where the defendant's actions were not voluntary due to an external factor, such as a blow to the head or a medical episode.

Key Term: automatism A state in which a person has no conscious control over their actions, potentially providing a defence to criminal liability.

Omissions

The general rule is that there is no liability for failing to act. However, there are important exceptions where the law imposes a duty to act.

Key Term: omission A failure to act when there is a legal duty to do so, which can give rise to criminal liability in certain circumstances.

When Does a Duty to Act Arise?

A duty to act may arise in the following situations:

• Statutory duty (e.g., failing to provide a breath specimen)
• Contractual duty (e.g., a lifeguard failing to save a swimmer)
• Special relationship (e.g., parent and child)
• Voluntary assumption of care (e.g., caring for an elderly neighbour)
• Creation of a dangerous situation (e.g., starting a fire and failing to put it out)

Worked Example 1.1

Scenario: Jamie is a train station employee responsible for closing a gate at a level crossing. He forgets to close the gate, and a car is hit by a train, killing the driver. Is Jamie criminally liable?

Answer: Yes. Jamie had a contractual duty to act. His omission (failing to close the gate) led to death, and he can be liable for manslaughter.

Types of Crimes: Conduct, Result, and State of Affairs

Criminal offences can be classified by the nature of their actus reus.

Conduct Crimes

These require proof of a particular behaviour, regardless of the outcome. Example: perjury (lying under oath).

Result Crimes

These require that the defendant's conduct causes a specific result, such as death or injury.

Key Term: result crime An offence where liability depends on the defendant's conduct causing a particular consequence.

State of Affairs Crimes

Some offences require only that the defendant is found in a particular situation, regardless of any voluntary act. Example: being in possession of illegal drugs.

Causation

For result crimes, the prosecution must prove that the defendant's conduct caused the prohibited consequence.

Key Term: causation The requirement that the defendant's conduct must be linked to the prohibited consequence in a result crime.

Factual Causation

The "but for" test: Would the result have occurred but for the defendant's conduct?

Key Term: factual causation The test of whether the consequence would have happened "but for" the defendant's conduct.

Legal Causation

Legal causation asks whether it is fair to hold the defendant responsible. The defendant's act must be a significant (more than minimal) cause of the result.

Key Term: legal causation The requirement that the defendant's conduct is a substantial and operating cause of the result, and that no intervening act breaks the chain.

Intervening Acts (Novus Actus Interveniens)

An intervening act may break the chain of causation if it is independent and unforeseeable.

Key Term: novus actus interveniens A new and independent act or event that breaks the chain of causation between the defendant's conduct and the result.

The Thin Skull Rule

The defendant must take the victim as found, including any pre-existing vulnerabilities.

Key Term: thin skull rule The principle that the defendant is liable for the full extent of harm caused, even if the victim has an unusual weakness or condition.

Worked Example 1.2

Scenario: Alex punches Sam, who has a rare blood disorder. Sam dies from minor bleeding that would not have harmed a healthy person. Is Alex liable for Sam's death?

Answer: Yes. The thin skull rule applies. Alex is responsible for the death, even though Sam was unusually vulnerable.

Worked Example 1.3

Scenario: Priya sets fire to rubbish near a building. A sudden, unforeseeable storm causes the fire to spread and destroy the building. Is Priya liable for the damage?

Answer: No. The storm is a novus actus interveniens—an unforeseeable event that breaks the chain of causation.

Exam Warning

For SQE1, always distinguish between factual and legal causation. Remember that an intervening act must be both independent and unforeseeable to break the chain.

Revision Tip

When analysing actus reus, identify: (1) the conduct or omission, (2) any required result, (3) whether there is a duty to act, and (4) whether causation is an issue.

Summary

Key Point Checklist

This article has covered the following key knowledge points:

• The actus reus is the external element of a criminal offence and may be a positive act, omission, or state of affairs.
• Liability for omissions arises only where there is a legal duty to act.
• Result crimes require proof that the defendant's conduct caused the prohibited consequence.
• Factual causation uses the "but for" test; legal causation requires a substantial and operating cause.
• The chain of causation may be broken by a novus actus interveniens.
• The thin skull rule means the defendant is liable for the full harm caused, even if the victim is unusually vulnerable.
• Criminal liability generally requires a voluntary act.

Key Terms and Concepts

• actus reus
• voluntary act
• automatism
• omission
• result crime
• causation
• factual causation
• legal causation
• novus actus interveniens
• thin skull rule