And psychologists assessing people facing criminal charges are “acutely aware of the issue of faking”.
Professor Nigel Eastman said it is more usual during trials for either a judge or jury to decide on a person’s mental state at the time of the alleged offence. This is known in legal circles as the “mad or bad” argument.
The decision the judge or jury reaches in cases involving killing determines whether manslaughter on the grounds of diminished responsibility is returned, or a plea is accepted by the prosecution, he said.
Prof Eastman told BBC Breakfast: “Forensic psychologists are acutely aware of the issue of faking. Whether someone can fake depends on the nature of the disorder.
“There are some types which are very difficult to fake. It is very difficult to keep up a faking exercise when someone is in hospital for a period of time.”
Dr Mike Harris, who heads Rampton secure hospital, said: “There is always that risk that people might not be genuine, and our staff are actually aware of it.
“If you can observe a patient 24 hours a day you can see whether the symptoms are real.”
When admitted, patients are held in single rooms on wards, in isolation, with contact only with medical and nursing staff.
There are bars on the windows and patients are not allowed to leave their room in the early days, for fear they could be a danger to others or at risk themselves from other patients.
Watching patients interact with others is important in determining their mental state, he added.
“Sectioning” under the Mental Health Act 1983 occurs when doctors decide someone is suffering from a mental disorder and sent to a secure unit for assessment.
Part of the act provides for the detention of people who are suffering from severe personality or mental disorders or impairments.
In the case of Rampton high security hospital, where caretaker Ian Huntley was taken to and charged with murder on Tuesday, about three-quarters of its 400 patients have committed “very serious crimes”.
They include the so-called ‘Angel of Death’, the Nottinghamshire nurse Beverley Allitt, who was convicted of attacked 13 children and killed four others.
Section two of the Act provides for a patient to be held for 28 days and assessed by at least two clinical psychologists.
In the case of a suspected criminal, the order could be discharged if deemed appropriate and court proceedings started in the normal way.
If not, then section three of the Act provides for extensions in secure units of six months.
Cases of defendants either pleading guilty to manslaughter due to diminished responsibility or being convicted of it by a jury are relatively common.
Among the most high profile in recent years are the cases of Eden Strang and Christopher Clunis.
In November 1999, Strang stripped naked and attacked 11 members of the congregation at St Andrew’s Roman Catholic Church in Thornton Heath, south London, with a Samurai sword.
He was cleared of seven counts of attempted murder and assault at the Old Bailey after psychologists said he was suffering from schizophrenia at the time and was therefore not responsible for his actions.
A judge ordered him to be detained under the Mental Health Act for an indefinite period to “protect the public from serious harm”.
He was released earlier this year.
Clunis stabbed to death 27-year-old musician Jonathan Zito at Finsbury Park underground station in north London in December 1992.
He was sentenced a year later to be detained indefinitely at Rampton high security hospital after admitting manslaughter due to diminished responsibility.
The court heard he had failed to take his medication.