Even once a prosecutor proves all the elements of an offence, there might be a reasonable justification for the conduct. In these cases, a defendant might have a “defence”, and can be found innocent where they would otherwise have been found guilty.

There are many different defences that can be raised. Some of the most common defences are described below.



Self-defence can be argued when the accused was trying to protect themselves or others from attack. It can include situations where the person is protecting their property or preventing unlawful trespass. To establish self-defence the accused must show that:

  • They believed their conduct was necessary in order to defend themselves, and
  • Their actions were a reasonable response in the circumstances as they perceived them.

The conduct cannot be unreasonable or excessive. For instance, if someone pokes you in the chest, you have no right to belt them over the head with a baseball bat.



Duress can be argued when the defendant was forced to commit the offence because they were threatened by another person. Unlike self-defence, this isn’t likely to happen during an attack. An example of a situation where duress may apply is a person threatening to hurt someone you care about unless you rob a bank.  To prove duress, the defendant must show that:

  • Because of a threat of death or serious injury,
  • The defendant gave into the threats and committed the crime demanded, and
  • A person of ordinary firmness and strength of will, and of the same maturity, sex and position of the accused, would have done the same.



Necessity can be argued when the defendant was forced to commit the offence in order to avoid worse consequences caused by a situation outside of their control. This might include emergency situations, or pushing a person out of the way of oncoming traffic.

In a famous British case, after being shipwrecked on an island for three weeks without food, two sailors killed and ate the cabin boy. They were charged with murder, and defended themselves by arguing necessity. The sailors were unsuccessful, and it is still generally held that necessity cannot be used against murder. To be used successfully today, a defendant must show that:

  • They committed the offence to avoid imminent death or serious injury,
  • They honestly and reasonably believed there was a situation of imminent danger, and
  • Their actions were not disproportionate to the imminent danger.


Extreme provocation

Extreme provocation is a partial defence which can only be used for a charge of murder. If provocation is proven, the charge will be reduced to manslaughter. Extreme provocation is a defence when:

  • The act of the accused was in response to conduct of the deceased towards the accused,
  • The conduct was a serious indictable offence,
  • The conduct caused the accused to lose self-control, and
  • The conduct would have caused an ordinary person to lose self-control to the extent of causing death or serious injury.

This doesn’t apply when the “conduct” was a non-violent sexual advance, or where the accused encouraged the conduct as an excuse to use violence.

Unlike most defences, it is the responsibility of the prosecution to show that the murder was not in response to extreme provocation.



The use of intoxication as a defence is increasingly restricted. Previously, if an offender was drunk at the time of the offence it was assumed that their mental capacity, and thus culpability, was reduced. However, this approach allowed people to drink to excess and evade full responsibility for their crimes.

Nowadays, intoxication can only be used as a defence in “specific intent” offences, where the defendant intended the type of harm they caused.  However, even for these offences, intoxication cannot be used if the defendant:

  • Decided to commit the offence, and then had a drink first, or
  • Decide to drink, in order to improve their commitment to committing the offence.

In relation to other all offences, intoxication can only be used as a defence if it was not self-induced. If someone spiked the defendant’s drink, and they committed assault, the intoxication could be relevant.


Mental illness

It is possible to find the accused not guilty because of mental illness. To establish the defence of mental illness, the accused must show that:

  • At the time of the offence they suffered from a “defect of reason or disease of the mind” (mental illness), and
  • Because of that illness they did not understand the nature of the act, or understand it was wrong.

If the defendant is found not guilty by way of mental illness, the judge must then determine whether they can be safely released. If the judge believes that the defendant poses a risk either to themselves or the community, they will be transferred to the Mental Health Review Tribunal. The Tribunal will determine whether they should be detained in a medical facility, released, or undergo treatment.


Honest and reasonable mistake of fact

Ignorance of the law is not a defence, however genuine mistake of fact can be a defence for strict liability offences. Strict Liability offences do not require any proof of mental fault or criminal intent. For example, you can be charged with speeding even if you didn’t know you were speeding.

To prove this defence the defendant must show that:

  • They held an honest and reasonable, but incorrect, belief in the existence of certain facts, and
  • Had the belief been correct, their conduct would not have amounted to a criminal offence.

This defence is often argued for aggravated sexual assault where the victim is under the age of 16. If the defendant genuinely believed that the victim was over 16, this could be used as a defence if they could show that their belief was honest and reasonable.


Proving a defence

Under the common law there are two main “standards of proof”:

  • The higher standard is “beyond reasonable doubt”, and
  • The lower standard is “on the balance of probabilities”.

The prosecution must prove that the accused committed a crime beyond reasonable doubt. However if the accused wants to raise a defence, they must prove it on the balance of probability. This means that the accused must show the court that their defence is more likely than not to be true.


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