THE KILLER stalking London's gay community has dispelled any notion that serial killing is largely an American phenomenon. After all, the most notorious of all was Jack the Ripper, who killed six prostitutes in 1888. One was stabbed 39 times, another was disembowelled. The head of the last was partially severed from her body, her heart was removed and placed on her pillow and her entrails draped over a picture frame.

But even this is tame compared with some modern multiple murderers, such as the American Jeffrey Dahmer, who is said to have cannibalised 17 black homosexual victims.

One obstacle to understanding what motivates serial killers is that the deep fear they provoke leads to 'panic theorising'. An article in the Times recently suggested that serial killers share personality characteristics with Mozart and Picasso. Dubious theories multiply because so much of the behaviour is apparently inexplicable. For example, the London homosexual killer is believed to have telephoned the police to boast about his crimes.

One explanation for that behaviour was provided by Douglas Daniel Clark, 43, the 'Sunset Slayer', on death row in San Quentin, California, convicted in 1983 for the murder of six women. He claimed that serial killers commit their crimes because they want to be captured and eventually executed. He said that they derive a perverse thrill from the thought of their own capture and death.

Although this remains a controversial theory, it is true that much has been learnt from psychiatrists' analysis of the claims that such criminals make about their own motivations. Police investigating the current case in Britain will be examining this data, much of it from the FBI's National Center for the Analysis of Violent Crime, which featured in the film The Silence of the Lambs.

Given the circumstances in which the five London victims have been found, the first hypothesis is that this killer is a sexual sadist. Sexual sadism was first described by the psychiatrist Richard von Krafft-Ebing (1840-1902). He named this disorder after studying the Marquis de Sade (1740-1814), who combined sexual acts with domination, degradation and violence.

One of the clearest descriptions of sexual sadism comes from reports by American psychiatrists on the writings of a man who sodomised and then murdered his victims. He made audio tapes of the sexual torture of his wife, and drew up plans for constructing an S & M (sado-masochist) 'play area', including an incinerator.

This man wrote: 'The wish to inflict pain on others is not the essence of sadism. One essential impulse: to have complete mastery over another person, to make him/her a helpless object of our will, to become the absolute ruler over her, to become her God, to do as one pleases.

'To humiliate her, to enslave her, are means to this end, and the most important radical aim is to make her suffer, since there is no greater power over another person than that of inflicting pain on her, to force her to undergo suffering without her being able to defend herself.'

The motivations described by this man are consistent with the behaviour of most sexual sadists studied by the FBI. This research into the established patterns of previous sexual sadists will be used to build an 'offender profile' of the man being hunted in London.

About 30 per cent of the people studied by the FBI's psychologists were police buffs, who showed an obsessive interest in police activities and paraphernalia. This ranged from collecting literature dealing with police technology to owning police uniforms and counterfeit identifications. The London serial killer seems to be preoccupied with the police. Psychologists argue that a criminal's fascination with police activities and paraphernalia reflects his strivings for power.

Professionals who study the sexual criminal commonly use a classification scheme based on the criminal's attitude towards power. They are divided into four types: the power-reassurance seeker, who is trying to compensate for disturbing thoughts about his sexual inadequacy; the power-assertive type, who is expressing mastery and dominance; the anger-retaliatory type, who expresses hostility and rage; and the anger-excitation type, who experiences pleasure and excitement in response to his victim's suffering.

The FBI's studies of sexual sadists throw up other findings that may be relevant to the London case. For instance, 90 per cent of them used a 'con' to approach the victim, some kind of pretext, such as requesting or offering assistance, for instance asking directions. A small number used 'surprise' approaches, like grabbing or striking victims. More than three-quarters of the cases featured bondage, tallying with the reports of sado-masochism in the London case.

In the American research, 73 per cent of the men studied had performed anal rape on the victim; 70 per cent had forced the victim to perform fellatio; 40 per cent used foreign objects to penetrate the victim.

According to the victims, 87 per cent of the perpetrators had an unemotional, detached attitude during the offences. All of the offenders intentionally tortured their victims, and 61 per cent of the murders by sexual sadists were by asphyxiation, a higher proportion than in murders in general. This seems to have been the method of the London killer.

But perhaps the most disturbing of all the findings of the FBI's studies into the sexual sadist is the failure of most security measures to prevent these crimes. As one offender said when asked how people could prevent being raped: 'There's a lot of steps you can take to help eliminate the average criminal, who is just spontaneous and reckless and careless, but if somebody wants somebody bad enough it's nearly impossible to prevent.

'They could have the best security in the world, they could have guards and dogs and everything else. But if you have the time and the patience, the opportunity is going to arise when you can hit somebody.'

The writer is a clinical lecturer at the Institute of Psychiatry.