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  • Writer's pictureAustin & Carnley Solicitors

When does Murder become Manslaughter by Diminished Responsibility?

Murder is one of the most serious criminal offences that a defendant can be charged with. If convicted, a judge must impose a mandatory life sentence and follow guidance regarding the minimum term that the defendant has to serve before they are eligible for parole. Diminished responsibility is a partial defence to murder which reduces a defendant’s culpability from murder to ‘voluntary’ manslaughter. The defence carries the burden to prove this applies on the balance of probabilities.

Diminished Responsibility

When a defendant pleads diminished responsibility they will not be convicted of murder if they can prove that they were suffering from an abnormality of mental functioning, which provides an explanation for their acts or omissions in doing or being a party to the killing, which arose from a recognised medical condition and substantially impaired the defendants ability to understand the nature of their conduct; to form a rational judgement; and to exercise self-control.

If a defendant successfully pleads diminished responsibility, the charge of murder will be reduced to voluntary manslaughter.

Abnormality of mental functioning

There is no legal definition for this term, but this element basically requires the defendant’s medical condition to have an impact on their mind. In case law this has been described as ‘a state of mind so different from that of ordinary human beings that the reasonable person would describe it as abnormal’. Medical evidence often assists with proving this element of the defence.

Arose from a recognised medical condition

The link between the defendant’s abnormality of mental functioning and a recognised medical condition is one of the important parts of this defence. The court will take into account conditions recognised by the medical profession such as schizophrenia and bipolar.

Substantially impaired the defendant’s ability to

1.     Understand the nature of their conduct – the partial defence is satisfied by the defendant having a substantial impairment meaning that they can’t understand their conduct and the impact of this conduct on the victim.

2.     Form a rational judgment – this is the most likely route for a victim of abuse who kills their abuser as the pressure of their situation substantially impairs their ability to make rational decisions.

3.     Exercise control – this is most likely to used where a defendant cannot live up to the standard of normal self-restraint.

Explanation for the Defendant’s acts or omissions

The abnormality must explain the killing. This is the final element and requires the previous three elements to combine and cause, or significantly contribute, to murder. This is by far the most difficult element of the defence to prove. Even with the assistance of medical experts, there can be multiple causes and/or motives for the murder.

Impact of Diminished Responsibility  

If the defence can successfully prove on a balance of probabilities that diminished responsibility applies in their case, the defendant will be convicted of manslaughter. The maximum sentence for manslaughter is life imprisonment, but unlike with murder, this is not mandatory. The sentencing range can be anywhere between 3 and 40 years imprisonment depending on the degree of responsibility retained. The defendant can serve this sentence in prison although a Hospital Order will usually be imposed for diminished responsibility cases.

Hospital Orders

When a defendant is convicted of manslaughter by diminished responsibility, the court will usually impose a Hospital Order if they think it’s an appropriate sentence and if treatment is available for the defendant’s mental disorder. The defendant will be sent to a medium or high security psychiatric hospital. These orders are reviewed every 6 months. After the first year the order is then reviewed annually by the Responsible Clinician.

A Restriction Order can be paired with the Hospital Order, which restricts the release of the defendant from the Hospital Order until the Secretary of State thinks it’s fit to release them. This has the effect of making the Hospital Order indefinite and is used for the most dangerous offenders as they are unlikely to ever be released.


The partial defence of diminished responsibility is one of the most complex areas of criminal law. Further challenges apply including the burden being placed on the defence to prove the elements of diminished responsibility on a balance of probabilities.

Here at Austin and Carnley we have vast experience and expertise within Criminal Law and will be able to advise if you, or someone you know, finds themselves subject to criminal proceedings.



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