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In criminal cases, psychiatric evidence plays a distinct role from civil cases. Rather than assessing damages or parenting capacity, criminal psychiatrists address questions fundamental to criminal responsibility: Was the defendant mentally unfit to stand trial? Did mental illness reduce their responsibility for the offence? Are they dangerous? What disposal (sentence or hospital order) is appropriate?
Fitness to plead: the Pritchard criteria
The foundational test for fitness to plead (also called fitness to stand trial) comes from the case of Pritchard (1836). A defendant is considered fit to plead if they can:
- Understand the nature of the charge against them (what offence they are accused of)
- Understand the nature and object of the proceedings (that they are in a court, facing trial, that they may be convicted or acquitted)
- Instruct their solicitor and counsel (communicate with their legal representatives and understand legal advice)
- Follow the evidence presented during trial (understand what witnesses say and what that means for the case)
- Make a rational decision about pleading (understand the consequences of pleading guilty or not guilty and make a rational choice)
If a defendant, because of mental illness or disability, cannot satisfy these criteria, they are unfit to plead. Unfitness is determined by the court (usually before trial) following a medical examination. If a defendant is found unfit, the case does not proceed to full trial; instead, a judge conducts a fact-finding hearing to determine whether the defendant committed the act alleged. If satisfied, the court may impose a hospital order, guardianship order, or supervision order under the Mental Health Act.
Fitness to plead is not the same as insanity (which is about criminal responsibility at the time of the offence) or diminished responsibility (which reduces the seriousness of murder). It is about the defendant’s current capacity to participate in trial.
Who instructs the expert and when
The defence solicitor usually instructs a psychiatrist early in the case if fitness is in question. A defendant may be referred to psychiatric services before trial, and their treating psychiatrist’s assessment may feed into the expert evidence. Alternatively, the defence may instruct a psychiatrist specifically to assess fitness. If the defence raises fitness, the prosecution may instruct their own psychiatrist to provide a second opinion. The court can direct a joint assessment or order a court-appointed expert.
Fitness must be resolved before trial proceeds. If a defendant raises fitness (or if the court is alerted to concerns about fitness), the court will order an assessment before the trial date.
Other criminal psychiatric questions
Diminished responsibility: Under Section 2 of the Homicide Act 1957, a defendant charged with murder may argue that an abnormality of mental function (caused by an identified medical condition) substantially impaired their ability to do one of three things:
- Understand the nature of their conduct
- Understand that it was wrong
- Form a rational decision about doing it
If this defence succeeds, murder is reduced to manslaughter. A psychiatrist will assess whether the defendant has a recognised medical condition (diagnosed according to ICD-11 or DSM-5), whether that condition caused an abnormality of mental function, and whether the abnormality substantially impaired responsibility at the time.
Insanity defence: The M’Naghten rule (from 1843) sets out the insanity defence: a defendant is not responsible for an act if, because of mental illness, they did not know the nature or quality of the act, or did not know it was wrong. This defence is rarely used (about 20–30 cases per year in England and Wales) because conviction for an offence other than murder is usually a better outcome than a not guilty by reason of insanity verdict.
Sentencing reports: After conviction, a psychiatrist may be instructed to prepare a report for sentencing. The report assesses the defendant’s mental state, any connection between mental illness and the offence, likelihood of treatment engagement and response, and risk of harm to the public. This informs whether the judge should impose a custodial sentence, a community order with mental health requirements, or a hospital order.
Mental Health Act disposals: Under Part III of the Mental Health Act 1983, instead of sending a convicted person to prison, a court may impose a hospital order (sending the person to psychiatric hospital for treatment), a guardianship order, or a community order with a treatment requirement. A psychiatrist’s evidence on the person’s mental disorder, treatability, and suitability for hospital care informs this decision.
Role of prison healthcare and external assessment
Some defendants are assessed by psychiatrists employed by the prison service. Others are assessed by NHS psychiatrists or independent psychiatrists instructed specifically for litigation. Courts often prefer reports from NHS services where possible, as this ensures the person can be transferred to hospital for treatment if ordered. Independent experts are also used, particularly by the defence.
Disclosure and procedural rules
Criminal Procedure Rules Part 19 governs expert evidence in criminal proceedings. Similar to civil rules (CPR Part 35), experts have a duty to the court. An expert’s report must comply with the rules and include a statement that the expert understands their duty. Unlike civil cases where single joint experts are now common, criminal proceedings more often involve separate defence and prosecution experts because the adversarial nature of criminal trials requires each side to test the other’s evidence.
Disclosure rules in criminal cases require that prosecution experts’ reports are served on the defence. The defence is entitled to instruct their own expert to examine the same person and prepare a rebuttal report if they disagree.
Responsibility and risk
Criminal psychiatrists must address risk: Is the defendant dangerous? How likely are they to reoffend? This is a complex question because psychiatry cannot predict individual behaviour with certainty. Experts must explain the limits of their predictions and the degree of uncertainty. A defendant with psychotic illness and a history of violence may carry higher risk than a non-violent offender, but prediction remains probabilistic.
Assessment by NHS services vs. private experts
A defendant in the criminal justice system can be assessed through NHS psychiatric services (at the Crown Court’s direction, or through the probation service). Alternatively, the defence may instruct a private psychiatric expert. NHS reports are often free or low-cost (funded through the court); private experts’ fees are paid by the defence (or through legal aid if the defendant is in scope).
Key points
- Fitness to plead requires a defendant to understand the nature of the charge, the nature of the proceedings, instruct their legal team, follow evidence, and make a rational decision about pleading
- The Pritchard criteria from 1836 still define fitness; if a defendant cannot meet them because of mental disorder, they are unfit to plead
- Diminished responsibility (a partial defence to murder under the Homicide Act 1957) is based on abnormality of mental function substantially impairing responsibility
- The insanity defence (M’Naghten rule) is rarely used in modern practice
- Sentencing psychiatrists assess mental disorder, treatability, and risk of harm to inform sentencing decisions and potential Mental Health Act disposals
- Criminal experts have a duty to the court under Criminal Procedure Rules Part 19
- Assessment can be through NHS psychiatric services or private experts
Related: Find a medico-legal expert