Image caption The 32-year-old was due to stand trial

Self-confessed mass killer Anders Behring Breivik is more likely to be detained in a mental institute rather than prison, after psychiatrists in Norway declared him psychotic. How is this kind of assessment carried out?

Breivik, who admits killing 77 people and injuring 151 others in July, has been declared a paranoid schizophrenic after months of assessment.

A report by two court-appointed psychiatrists says he was psychotic at the time of the shootings and in subsequent interviews. Their findings must now be reviewed by a panel from the Norwegian Board of Forensic Medicine.

If approved, it could mean he is detained indefinitely in a psychiatric hospital and will not stand trial.

So how are such assessments made? And how accurate can they really be?

Professor Tarjei Rygnestad, who heads the Norwegian Board of Forensic Medicine that will assess the report on Breivik, says psychiatrists use extensive interviews, and relevant information from other agencies like the police, to evaluate the mental state of the accused.

"They talk about not only the incident but also the time leading up to the incident and afterwards. What happened? Was he in some way displaying psychotic symptoms, hearing voices leading him? Was he on drugs?"

The answer Psychiatrists conduct extensive interviews

They are trained to identify symptoms of psychosis

People accused of a murder have to prove they did not know that killing was wrong at the time

They are also assessed around the clock

Experts say it's very hard to fool the professionals, but possible Norway killer is declared insane

They are looking for genuine psychotic symptoms or signs of a reduced level of consciousness, he says.

"If he is considered legally psychotic and been medically diagnosed as such, the accused will not be brought to the court for a criminal case. He will be sentenced to a high-security psychiatric unit.

"The principle [upheld here] is that he had no conscious choice of action due to his psychotic illness, he had no consciousness to choose."

Being psychotic can limit your ability to perform a task, he says, but not always. And while meticulous preparations such as those made by Breivik might suggest a sane mind, it doesn't necessarily rule out psychosis.

"Is he faking or not? This is always born in mind. Always, always. Especially in forensic psychiatry because if he can fake psychosis he won't be sentenced. So this is a crucial point but I think it's difficult because these are experienced psychiatrists."

Famous insanity cases in US John Hinckley, Jr (above): Still in psychiatric care after trying to assassinate President Ronald Reagan, acquitted due to reason of insanity

Still in psychiatric care after trying to assassinate President Ronald Reagan, acquitted due to reason of insanity Jeffrey L Dahmer: Murdered 17 men and boys, and failed in attempts to plead insanity

Murdered 17 men and boys, and failed in attempts to plead insanity Lorena Bobbit: Cut off her husband's penis, found not guilty due to insanity

Cut off her husband's penis, found not guilty due to insanity Ted Kaczynski: So-called Unabomber was diagnosed as a paranoid schizophrenic but still stood trial and convicted on 10 counts

In the US, Jared Loughner, the man accused of shooting congresswoman Gabrielle Giffords and killing six others, is being held in a mental health facility while doctors assess whether he is able to stand trial.

There is an insanity defence in the US, based on the M'Nachten Rules that originated in the UK, although some states have abolished it.

It requires defendants to show they were unable to tell right from wrong at the time of the offence. The standard is tough to meet and experts say it features in only 1% of cases, and is only successful in 0.3%.

"It's a tiny number of people," says Mark Heyrman, a professor of law in Chicago with a speciality in mental health. "Probably only a couple of hundred people in a year and we have hundreds of thousands of felony prosecutions each year."

The so-called Unabomber, Ted Kaczynski, who conducted a deadly nail bombing campaign over 20 years, was diagnosed as a paranoid schizophrenic but still declared competent to stand trial, and convicted in 1996.

The Hinckley effect It became harder to successfully plead insanity after Hinckley case in 1982 caused outrage

Prosecutors no longer had to prove defendant sane, but accused must prove insanity

So the defendant bears the burden of proof

To be insane, accused must be unable to tell right from wrong

So even if everyone agreed a crime was caused by a mental illness, if the accused knew it to be wrong he can't be acquitted due to insanity Source: Prof Mark Heyrman, Univ of Chicago

In the UK, there is a pre-trial assessment of whether someone is fit to plead, and then there are two possible defences at a trial for someone who is mentally ill - insanity, which is very rare, and diminished responsibility, which is more common.

A team of mental health professionals - psychiatrists, psychologists, nursing staff and occupational therapists - take part in assessments, says Dr Simone Fox at the psychology department of the Royal Holloway, University of London. It's not just the interviews - the nature of which should remain secret, she says - but the round-the-clock monitoring and medical history that play a part.

"How are they presenting on the ward? What are they like at night-time? What are they saying? It's to see if there's a consistent picture, because what they are saying at interview could be for the sake of a trial. You would have to be very good to fool everyone."

Richard Charlton, chairman of the Mental Health Lawyers Association in the UK, says he has represented people who have killed other people believing they are saving them from the devil, but courts have not found them to be insane.

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If they knew that killing was wrong, even if God told them to do it, then that's not enough to give them a defence of insanity although it might be sufficient to give them a diminished responsibility defence, he says.

Insanity requires a higher test, and one example might be someone who threw a baby on to a fire in the belief that the baby was a piece of wood and not a baby.

In 20 years, Mr Charlton says there have been only a handful of people who may have faked mental illness. But psychiatric detention is not always desirable.

"When you're locked up in hospital, you don't have any end date. The care is better than in prison but it's not a soft option."