'Unlawful homicide' includes murder, manslaughter, and infanticide. The legal definition of murder requires that the crime was premeditated and the accused was fully responsible for the act. If these conditions are not fulfilled, the offence becomes manslaughter.

There are 800-900 homicides per year in England and Wales. In about 75 per cent of cases, killer and victim are well known to each other. Alcohol and other substances are often a factor, both in the background and at the time of the offence. Up to 15 per cent of killers commit suicide soon after their crime. A recent review (Oyebode, 2006) indicates that there are about 40 homicides per year by people in contact with mental health services in the 12 months prior to the event. (To put this in context, this is roughly the same number as those done by (non-psychiatric) offenders on probation.) However, surveys indicate that up to 50 per cent of killers could be diagnosed with a mental disorder of some kind - mainly personality disorder and substance misuse, although depressive disorder and psychosis also occur.

The legal finding of diminished responsibility (see below) applies in less than 5 per cent of homicides. The law classifies diminished responsibility, infanticide, homicide in a failed pact of suicide, and not guilty by reason of insanity (see below), as 'abnormal homicides'. Fewer than one in six homicides is classified as 'abnormal'. The presence of a mental disorder may permit a murder charge to be reduced to manslaughter on the grounds of diminished responsibility. A verdict of murder always carries a sentence of life imprisonment, but manslaughter may receive any sentence, including a hospital order under the Mental Health Act 1983, or even a non-custodial sentence.

A rare plea, in practice reserved for murder cases, is 'not guilty by reason of insanity' when the offender fulfils the MacNaughten Rules; that is, he either did not know the nature and quality of his act, or did not know that it was wrong. A deluded patient is assumed to be under the same degree of responsibility as if the delusions were true. If this plea is successful, the accused is sent to a psychiatric hospital under the equivalent of section 37 of the Mental Health Act.

About half of those accused of murder claim amnesia for the event, but this is not an adequate defence, nor is voluntary intoxication with alcohol or drugs. Automatism, in epileptics or sleepwalkers, can constitute an acceptable defence.

Infanticide is a defence when a child less than a year old is killed by its mother who is 'suffering from a mental imbalance attributable to the effects of giving birth or to the consequent lactation'.

Mental disorders which can contribute to homicide are as follows:

• personality disorder

• alcohol and drug misuse

psychoses, usually with delusions, as depressed people may kill their children or other close relatives because of a delusion that they were going to suffer an even worse fate. Psychosis may lead to homicide through paranoid delusions; even here, the victim is often known; stranger killing (e.g. http://www.zitotrust. co.uk/), the type so feared by the general public - or at any rate so prominent in the media - is rare. Puerperal psychosis accounts for some, but not all, cases of infanticide.

• morbid jealousy

learning disability, in which frustration may be expressed by violence

• epileptic automatism: a rare phenomenon

• automatism during sleep, in which individuals prone to sleepwalking or night terrors occasionally kill during sleep - a rare defence hard to prove

• organic brain disease.

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