Criminal Law: General Defences

A defence can reduce or defeat criminal liability even where the actus reus and mens rea are made out. This guide covers the four most heavily examined general defences: self-defence, duress, intoxication and insanity. Each has tightly defined limits, and the marks come from applying those limits precisely.

1. Self-Defence and the Use of Force

A person may use reasonable force to defend themselves or another, to protect property, or to prevent crime. The common-law defence is now clarified by section 76 of the Criminal Justice and Immigration Act 2008. Two questions arise: did the defendant genuinely believe force was necessary, and was the force used reasonable?

Necessity is judged on the facts as the defendant honestly believed them to be, even if that belief was mistaken (R v Gladstone Williams), though a belief induced by voluntary intoxication cannot be relied on. Reasonableness is judged objectively against those believed facts.

Householder cases

In a householder case under s.76(5A), force used against a trespasser in the home is unreasonable only if it was grossly disproportionate, a more generous standard than the "disproportionate" limit that applies in ordinary cases.

2. Duress by Threats

Duress excuses a defendant who commits a crime because of a threat of death or serious injury. The test is a two-stage one (from R v Graham, approved in R v Hasan): did the defendant reasonably believe they had good cause to fear death or serious injury, and would a sober person of reasonable firmness sharing the defendant's characteristics have responded the same way?

The defence is tightly confined. There must be no safe avenue of escape, the threat must be effectively immediate, and a defendant who voluntarily joins a criminal association where they could foresee being pressured to commit crimes cannot rely on it (R v Hasan).

3. Intoxication

Intoxication is not a defence in itself; it is relevant only to whether the defendant formed the required mens rea. The key distinction is between specific-intent and basic-intent offences.

Where voluntary intoxication negates the specific intent (for example, for murder), the defendant may still be convicted of the basic-intent fallback (manslaughter). Involuntary intoxication is treated more leniently and can negate mens rea more broadly.

4. Insanity

The defence of insanity is governed by the M'Naghten Rules. The defendant must prove that, at the time of the act, they were suffering from a defect of reason caused by a disease of the mind, such that they did not know the nature and quality of the act, or did not know that it was wrong. A successful plea leads to the special verdict of not guilty by reason of insanity.

5. Worked Example

RULE: necessity is judged on the facts as D believed them (Williams), and the force must be reasonable on those facts. APPLICATION: D genuinely believed he faced an armed attacker, so necessity is assessed on that basis, and a single proportionate blow to escape is likely reasonable. CONCLUSION: self-defence is likely to succeed and D is not liable.

Examiner Insights

Conclusion

General defences are won on their limits. Apply the believed-facts rule to self-defence, the two-stage test to duress, the specific or basic intent split to intoxication, and the M'Naghten criteria to insanity, and always identify the precise element in issue.