Core principles of criminal liability - Actus reus

Learning Outcomes

This article examines actus reus in criminal liability, including:

• Actus reus as the external element of crime, distinguishing conduct, circumstance, and result components, and applying this breakdown to structure SQE1 problem answers.
• The differences between conduct crimes, result crimes, and state of affairs offences, with clear examples and indicators to spot each type in multiple-choice questions.
• The requirement of voluntariness in criminal acts and situations where involuntary movements, automatism, reflexes, or seizures will prevent criminal liability.
• The circumstances in which omissions give rise to liability, the recognised duty categories, and how to identify a duty to act on exam facts.
• Causation in result crimes, using the factual "but for" test and the legal "substantial and operating cause" test, including multiple causes.
• The effect of intervening acts (victim conduct, third-party actions, medical treatment, and natural events) on breaking or preserving the chain of causation.
• Application of the thin skull rule to physical and psychological vulnerabilities and to victims who refuse medical treatment, and its impact on the extent of liability.
• Contemporaneity (coincidence) rules, including continuing acts and single transaction reasoning, to align actus reus and mens rea where they do not occur at the same moment.
• The role of possession, status, and state-of-affairs offences, and how strict liability elements influence analysis of the actus reus.

SQE1 Syllabus

For SQE1, you are required to understand actus reus in criminal liability, with a focus on the following syllabus points:

• 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
• coincidence of actus reus and mens rea: continuing acts and single transaction principles
• categories of intervening acts: victim responses, third-party interventions, medical treatment, and natural events

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.

Actus reus is not limited to positive acts. The external elements of offences may comprise:

• conduct (acts or omissions)
• circumstances (facts that must exist, such as “belonging to another” in theft)
• result (a prohibited consequence, such as death in homicide)

Some offences can be committed by omission where the law imposes a duty to act. For result crimes, causation must link the conduct to the prohibited result. Finally, the coincidence principle requires that actus reus and mens rea align; courts sometimes achieve this through “continuing acts” and “single transaction” analyses.

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.

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.

Actus reus often combines elements:

• conduct (e.g., applying unlawful force for battery)
• circumstances (e.g., property “belonging to another” in criminal damage and theft)
• result (e.g., “actual bodily harm” in ABH or “grievous bodily harm” in s.20)

Many offences are primarily conduct-based, but they may still include circumstance elements. For example, burglary involves conduct (entry) and circumstances (as a trespasser) and, depending on the subsection, either ulterior intent (s.9(1)(a)) or commission of an ulterior offence (s.9(1)(b)).

Voluntary Acts

Criminal liability generally requires a voluntary act. The law does not punish people for actions over which they have no control. Voluntariness means the defendant’s muscles are controlled by the conscious mind at the time of the act.

Typical involuntary scenarios include:

• reflex actions (e.g., swatting a wasp and striking someone inadvertently)
• convulsions or seizures causing movement
• acts performed while unconscious or semi-conscious (e.g., sleepwalking with an external trigger)

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. The mere presence of an impulse or strong emotion does not mean an act is involuntary; there must be a true loss of control over bodily movements.

Courts look closely at whether apparent actions are the product of the defendant’s will. A loss of conscious control from an external cause may negate voluntariness. By contrast, choosing to act while intoxicated does not usually engage involuntariness.

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. The loss of control must be total, not partial.

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

Although the syllabus does not require the detail of the substantive defence, voluntariness analysis often touches on automatism because criminal liability presupposes voluntary conduct.

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.

There is no general Good Samaritan law: moral duty alone is insufficient. Liability for nonfeasance arises only where a recognised legal duty exists and is breached.

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 or failing to stop after an accident)
• Contractual duty (e.g., a level crossing gatekeeper failing to close the gate)
• Special relationship (e.g., parent and child; carer and vulnerable person)
• Voluntary assumption of care (e.g., assuming responsibility for an elderly or infirm person)
• Creation of a dangerous situation (e.g., starting a fire and failing to take reasonable steps to control or summon help)

These categories can overlap. In practice, the question is whether the defendant fell below the duty imposed by law in the situation: for instance, a person who voluntarily undertakes care must exercise reasonable care once the undertaking begins, and a person who creates danger must take reasonable steps to mitigate it.

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. This helps structure analysis.

Conduct Crimes

These require proof of a particular behaviour, regardless of the outcome. Example: perjury (wilfully making a false statement under oath). Common assault (causing another to apprehend immediate unlawful violence) is also a conduct offence, though it can be charged alongside result-based offences when harm occurs.

Result Crimes

These require that the defendant's conduct causes a specific result, such as death or injury. ABH (s.47), s.20 GBH/wounding, s.18 GBH with intent, murder, manslaughter, and criminal damage are result crimes. For such offences, causation must be established from conduct to consequence.

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. Examples include possession offences (e.g., possession of controlled drugs), being “in charge” of a vehicle while unfit, or membership of a proscribed organisation. Liability arises from the prohibited status or situation, sometimes with strict liability elements.

Causation

For result crimes, the prosecution must prove that the defendant's conduct caused the prohibited consequence. This is a two-stage inquiry: factual causation and legal causation.

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? If the answer is “yes,” factual causation is not established; if “no,” factual causation is present.

This test can be applied to positive acts and omissions where a duty exists. Multiple concurrent causes do not defeat factual causation; several acts may operate together to produce the result.

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 and remain an operating cause at the time of the consequence.

“More than minimal” is often described as “more than slight or trifling.” The defendant need not be the sole cause. The chain of causation may, however, be broken by an intervening act if it amounts to a new, independent cause.

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.

Factors that may break (or not break) the chain include:

• Victim’s acts: A response that is reasonable and proportionate to the threat (e.g., jumping from a moving vehicle to escape) typically does not break the chain. Only a wholly “daft” or unforeseeable reaction will.
• Third-party acts: A free, deliberate and informed intervention may break the chain. By contrast, instinctive or compelled acts (e.g., police firing in self-defence) are not “free” in this sense.
• Medical treatment: Ordinary negligence usually does not break the chain. Only treatment that is so palpably wrong and so overwhelming that the original injury becomes insignificant will do so.
• Natural events: A genuinely unforeseeable “act of God” may break the chain; foreseeable natural developments (e.g., tides) ordinarily will not.

Intervening Acts (Novus Actus Interveniens)

An intervening act may break the chain of causation if it is independent and unforeseeable, or if it is a free, deliberate and informed act that supersedes the defendant’s contribution.

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—physical, psychological, or religious beliefs (such as refusing medical treatment). The rule applies to both the extent of harm and the victim's decisions about treatment, provided the original injury remains an operating cause.

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.

Worked Example 1.4

Scenario: Ben accidentally starts a small fire in his bedsit by falling asleep with a cigarette. He wakes, sees the smouldering mattress, and goes to another room without attempting to extinguish the fire or raise the alarm. The building is damaged. Is Ben liable?

Answer:
Yes. By creating a dangerous situation, Ben had a duty to take reasonable steps to prevent harm. His omission maintains liability for criminal damage.

Worked Example 1.5

Scenario: Dana aggressively drives with Taylor in the passenger seat. Taylor, fearing imminent harm, opens the door and jumps from the slow-moving car, suffering a broken arm. Does Dana’s conduct cause the injury?

Answer:
Yes. Taylor’s reaction was a proportionate response to the threat and was foreseeable. The chain is not broken; Dana’s driving is a substantial and operating cause of the injury.

Worked Example 1.6

Scenario: Omar stabs Lee, who is taken to hospital. A doctor administers the wrong medication, which worsens Lee’s condition, leading to death. Is Omar still liable for homicide?

Answer:
Likely yes. Ordinary medical negligence does not usually break the chain. Unless the treatment was so palpably wrong and overwhelming that Omar’s original injury ceased to be a substantial cause, Omar remains legally causative.

Worked Example 1.7

Scenario: Rosa pushes Vijay during an argument. While Vijay is on the ground, a third party with a personal grudge arrives and deliberately stabs Vijay fatally. Is Rosa liable for the death?

Answer:
No for the death. The third party’s free, deliberate and informed intervention breaks the chain. Rosa remains liable for the original assault but not for the fatal consequence.

Exam Warning

• Distinguish clearly between factual causation (“but for”) and legal causation (“substantial and operating cause”).
• An intervening act must be both independent and unforeseeable (or free, deliberate and informed) to break the chain. Instinctive self-defence by third parties and reasonable victim reactions do not usually do so.
• The thin skull rule applies to pre-existing conditions and treatment decisions; refusal of medical treatment does not break the chain if the original injury remains operative.
• In omission cases, confirm the offence can be committed by omission and identify a recognised duty category before proceeding to breach and causation.

Revision Tip

When analysing actus reus, proceed systematically:

• identify the conduct (act or omission), any required circumstances, and any required result
• for omission cases, ask: is the offence capable of commission by omission; does a recognised duty arise; was it breached?
• for result crimes, apply factual and legal causation and test for intervening acts (victim, third party, medical, natural events)
• consider whether the thin skull rule affects the scope of liability
• check contemporaneity: if mens rea is debated, consider continuing acts and single transactions to align actus reus and mens rea

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.
• Conduct, circumstances, and result elements often combine to form the actus reus.
• Liability for omissions arises only where there is a legal duty to act and a breach of that duty.
• Result crimes require proof that the defendant's conduct caused the prohibited consequence through factual and legal causation.
• Intervening acts include victim responses, third-party interventions, medical treatment, and natural events; only certain types break the chain.
• The thin skull rule means the defendant is liable for the full harm caused, even if the victim is unusually vulnerable or refuses treatment.
• Criminal liability generally requires a voluntary act; true involuntary movements negate liability.
• Coincidence of actus reus and mens rea may be achieved through continuing acts and single transaction reasoning where appropriate.

Key Terms and Concepts

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