Criminal Law

Criminal Law Revision: Actus Reus, Mens Rea and Defences

A sharp revision guide to the building blocks of criminal liability, with real cases and exam technique baked in.

Every criminal law problem question follows the same skeleton: did the defendant do the prohibited act, did they have the required state of mind, and is there anything that gets them off the hook? Get those three stages right and you can handle almost anything the examiner throws at you.

Actus Reus: More Than Just "The Act"

Actus reus translates roughly as "guilty act", but that phrase misleads students into thinking it just means a physical action. It covers three possible conduct types: a positive act, an omission, or a state of affairs. Each raises different issues.

Positive Acts and Causation

For most offences, the defendant does something: punches, takes, damages. The analysis here is usually quick unless causation is in dispute. Causation is the part students most often rush past, and it's also where examiners love to hide marks.

There are two stages:

1. Factual causation: Apply the "but for" test. But for the defendant's act, would the result have occurred? If the answer is no, factual causation is established. R v White [1910] 2 KB 124 is the classic: the defendant put poison in his mother's drink, she died of a heart attack before ingesting enough to kill her. He failed the but-for test on the murder charge because she would have died anyway.

1. Legal causation: The defendant's act must be an operating and substantial cause of the result. It doesn't have to be the only cause or even the main cause, but it must be more than minimal. The thin skull rule applies: you take your victim as you find them.

Novus actus interveniens breaks the chain. A third party's free, voluntary, and informed act can do this, as can the victim's own conduct, though courts set the threshold high for victim conduct. An act is not free and voluntary if it was a reasonable response to danger created by the defendant.

Omissions

The general rule is that there is no criminal liability for a failure to act. English law does not criminalise the bystander who watches someone drown and does nothing. But there are well-established exceptions, and problem questions frequently exploit them.

A duty to act arises from:

• Contract: R v Pittwood (1902) where a gatekeeper's failure to close a level crossing gate killed someone.
• Special relationship: Parents and children, spouses in some circumstances.
• Voluntary assumption of care: R v Stone and Dobinson [1977] QB 354, where the defendants took in a vulnerable relative and then failed to summon help when she deteriorated.
• Creation of a dangerous situation: R v Miller [1983] 2 AC 161. Miller accidentally started a fire, realised it, and did nothing. The House of Lords held he was under a duty to take reasonable steps to counteract the danger he had created.
• Public office: A police officer's duty, for example.

In an exam, if someone dies and the defendant didn't actively cause it, ask yourself: was there a duty? If yes, was there a breach? Then move to mens rea.

State of Affairs Offences

These are strict liability-adjacent and sometimes come up in discussions of the actus reus requirement. R v Larsonneur (1933) is the uncomfortable example: a French woman was deported to Ireland, then forcibly brought back to England by Irish police, and convicted of being an alien found in the UK without leave. The courts upheld the conviction even though she had no control over being there. It's controversial, but it's examined.

Mens Rea: Intention, Recklessness, and Everything Below

Mens rea is the mental element. The specific type required varies by offence, so you must always identify what the statute or common law demands before you start applying tests.

Intention

Direct intention is straightforward: the defendant's purpose or aim was to bring about the result. The difficulty comes with oblique intention, where the result was not the defendant's aim but was a near-certain consequence of what they did.

The modern test comes from R v Woollin [1998] 3 WLR 382. The jury is not entitled to find intention unless the defendant foresaw the result as a virtual certainty (not merely probable), and the result was a virtual certainty given the defendant's act. If both conditions are met, the jury may infer intention. It is a rule about inference, not a definition of intention as such. That distinction matters for accuracy in your answer.

Recklessness

After R v G [2003] UKHL 50 overruled Metropolitan Police Commissioner v Caldwell [1982] AC 341, the test for recklessness in English criminal law is now subjective. The defendant must themselves have been aware of the unjustifiable risk and must have gone ahead and taken it anyway. Caldwell's objective test, which had criminalised defendants who simply failed to think, was scrapped.

In practice, recklessness covers a huge range of offences from criminal damage to some assault charges. Make sure you apply the R v G subjective test unless you're dealing with an offence that has its own statutory definition of recklessness.

Negligence and Strict Liability

Some offences require only negligence, gross negligence manslaughter being the key example. The test from R v Adomako [1994] 3 All ER 935 asks whether the defendant's breach of duty was so bad that it should be judged criminal by a jury. Negligence in civil law is not enough.

Strict liability offences require no mens rea as to at least one element. They are almost always statutory. Sweet v Parsley [1970] AC 132 is the House of Lords authority on how courts approach whether Parliament intended strict liability. The presumption is against strict liability for truly criminal offences; it is more easily rebutted for regulatory offences.

Coincidence of Actus Reus and Mens Rea

The conduct and the mental state must coincide. They don't have to coincide at a single precise moment, and courts have stretched this with the "continuing act" theory. Fagan v Metropolitan Police Commissioner [1969] 1 QB 439 is the exam favourite: the defendant accidentally drove onto a police officer's foot, then deliberately left the car there. The actus reus (being on the foot) was continuing, so the mens rea formed during it was sufficient.

Defences: Structure Matters as Much as Knowledge

Defences divide into two types, and you need to know which is which because it affects who bears the burden of proof and what the consequence of success is.

• Complete defences result in acquittal: duress, self-defence, consent (in appropriate circumstances), automatism, insanity.
• Partial defences to murder only reduce the conviction to voluntary manslaughter: loss of control (Coroners and Justice Act 2009, s.54) and diminished responsibility (Homicide Act 1957, s.2 as amended).

Self-Defence and the Use of Force

The defence covers both self-defence at common law and defence of others, and now sits alongside the Criminal Justice and Immigration Act 2008, s.76. The test has two stages:

1. Did the defendant genuinely believe force was necessary? This is assessed subjectively, including any mistaken belief, provided the mistake was genuine.
2. Was the force used objectively reasonable in the circumstances as the defendant believed them to be?

The defendant is not expected to weigh the precise level of force with exact precision in the heat of the moment. A householder defending against an intruder has slightly more latitude under s.76(5A), added by the Crime and Courts Act 2013, though the force still cannot be grossly disproportionate.

Duress

Duress requires a threat of death or serious injury, the threat must be directed at the defendant or a person for whom they have responsibility, a sober person of reasonable firmness sharing the defendant's characteristics must have responded the same way, and there must have been no reasonable opportunity to escape the threat. Duress is not available for murder (R v Howe [1987] AC 417) or attempted murder (R v Gotts [1992] 2 AC 412).

Duress of circumstances follows the same test but arises from surrounding circumstances rather than a direct threat from a person.

Intoxication

This is the defence that trips up students most often. The rules depend on whether the offence requires specific intent or basic intent, and whether the intoxication was voluntary or involuntary.

• Voluntary intoxication is a defence to specific intent offences (murder, theft, s.18 GBH) if it meant the defendant lacked the required mens rea. It is not a defence to basic intent offences (manslaughter, s.20 GBH, assault) because getting voluntarily drunk is treated as recklessness in itself.
• Involuntary intoxication, where the defendant did not know they were being intoxicated, can be a defence to any offence, but only if it prevented the defendant from forming the mens rea. If they still formed the mens rea while involuntarily intoxicated, the defence fails.

Insanity and Automatism

These sit together because both involve a claim that the defendant was not in control of their actions, but they lead to very different outcomes.

Insanity under the M'Naghten Rules (1843) requires: a defect of reason, arising from a disease of the mind, such that the defendant either did not know the nature and quality of their act or did not know that what they were doing was wrong. A successful plea leads to a special verdict of not guilty by reason of insanity, which can still result in a hospital order. The disease of the mind is a legal, not medical, concept: epilepsy has been held to qualify, as has sleepwalking in some cases.

Automatism is a complete defence and results in full acquittal. It requires a total destruction of voluntary control, caused by an external factor. A reflex action, a muscle spasm, or being in a hypoglycaemic state caused by insulin (an external agent) can qualify. The distinction between internal and external cause matters: an internal cause usually means insanity, not automatism.

Putting It Together in an Exam

The structure an examiner rewards is consistent and methodical:

1. Identify the offence and its elements.
2. Work through actus reus: conduct, causation if relevant, any omission issues.
3. Work through mens rea: identify the precise mental element required, apply the correct test.
4. Consider defences: any on the facts? Run the test in full.
5. Conclude briefly on liability.

Don't skim causation because a question looks "obvious". Don't skip defences because you think they'll fail. Show the examiner you know the full test even when applying it leads to a clear answer.

For homicide questions specifically, always check whether the facts raise any partial defences before you finish with murder. Loss of control and diminished responsibility are commonly tested and often present in the same facts that suggest murder.

The Caselaw criminal law topic hub is worth bookmarking for case summaries as you work through the offences. And if you want to test your application under exam conditions, past papers with model answers are the most reliable way to find out whether your structure holds up under time pressure.