Forensic psychologist Paul Britton probably knows more about how a killer's mind works than the forces of the FBI and Interpol combined. Now he has written an autobiography, containing graphic accounts of his work. Emma Cook asks him why
Afternoon tea at the Langham Hilton Hotel in London and 10 minutes into the conversation the forensic psychologist Paul Britton is already contemplating the existence of pure evil. "It is very, very infrequent," he reflects, in unnervingly quiet, methodic tones, "but I have come across a person for whom no other description is really suitable. By that I mean someone who hurts and exploits other people, knows what they're doing, relishes it, revels in it and doesn't want to stop. And that seems to be a characteristic from their earliest beginnings."

He pauses and smiles just ever so slightly, poised for the next question. Suddenly things don't seem quite so insulated here among the plush surroundings of genteel shoppers and their cosy, high-pitched chit-chat. As the pianist tinkles away in the corner, Britton explains how extreme sickness invariably inhabits a normal guise. His skill is to go beyond the guise, to understand and inhabit the psychopathic state of mind.

"Looking at the scene of the crime, you can be sure that whoever did it has left a clear trace of who they are. It's always that which leads you to them."

The details of crimes that Britton has steeped himself in may seem like a different world, yet he insists that it's all much closer than we dare to contemplate. "In the West case, very few people would have known what was going on; they may have seemed like 'animals', but only in hindsight. That's the way we protect ourselves, because we have to ask what separates 'us' from 'them'. What is it? It is only the things you've experienced and the decisions you've made."

If you really felt the divide was so gossamer thin, then Britton's first- hand account of his work, The Jigsaw Man, would be pretty unpalatable. As it is, the descriptions of some of the murders make for a harrowing read.

Britton, aged 50, is the first forensic psychologist in Britain to work closely with the police, building up a psychological profile of a murderer and often giving advice on how to question a suspect once he is arrested. He has been at the centre of some of the decade's most high-profile cases, including Fred and Rose West, Jamie Bulger, Naomi Smith, Rachel Nickell and the abduction of baby Abbie Humphries.

In each case, Britton works backwards through the offender's life, building up a picture of his family, friends and relationships. "In a sense, I have to imagine that this man is in my consulting-room chair and I'm conducting a one-sided interview. If I know exactly what motivated him to kill a woman, I can put a precise shape to his personality functioning."

A subject's childhood invariably holds many of the clues, which is why Britton believes that people are rarely innately sadistic, self-serving or remorseless. Upbringing and formative influences are, he says, the key to most of the sickness he has witnessed. "How does Fred West, for example, go from being a child to the sort of person he was? You see someone who wasn't valued as a child, someone who never learnt that other people counted, that learnt people were just playthings."

West's partner, not surprisingly, shared a similar childhood. "Rose West had to come from a particular background. She herself had been victimised in one way or another and had not learnt the lesson of self-worth."

The same could be said of Robert Thompson and Jon Venables, whose separate histories shaped their friendship and their joint actions that led to Jamie Bulger's murder. The significant difference with two such young offenders was how they reacted when confronted with those actions. Britton, who worked on the case, took great care to advise the police on how to question the offenders, encouraging them to be sensitive and gentle, not to show any hostility or criticism. "Most of the officers were parents. They had to focus not on tracking down some adult depraved offenders, but on two youngsters. They were dealing with two young, frightened victims who could be emotionally traumatised. There were important points along the way, like how they'd respond when sexual matters were raised, and how they'd want to blame the other, and so on."

Britton is also skilled at working with the media, helping to convey the right signals to the offender. With the Abbie Humphries kidnapping, it was essential to communicate a tone of sympathy in the public appeals.

"In those first 48 hours we had to let that woman understand that she wasn't seen as a monster. So when the time came for police to knock on her door, she wouldn't run off. She knew she was going to be OK. But it's an enormous strain - you know there's a child out there, and if you're wrong, you may lose it."

Although Britton's book deals objectively with the strategic operations behind each crime, the descriptions of the victims and the killers' motivations can be disturbingly detailed. Compelling as it is, at some points you begin to wonder what purpose the in-depth accounts can really serve, except to satisfy the general appetite for this sort of genre; The Silence of the Lambs, Cracker, Prime Suspect et al.

But Britton believes that The Jigsaw Man needs to be out there, partly because of the lessons we can learn from it. "The most important fact in the book is it's not the killers who are depicted as heroic, it's the investigative process and the victims. It's designed to show that if you are contemplating anything, you're not going to get away with it. If that message is hammered home, then it's worthwhile."

There is also a value in emphasising that the offenders are not "monsters", but more victims of their own circumstances. Which Britton does very effectively. "Once you understand their backgrounds, the things they haven't had in childhood, you see that the outcome is an inevitable consequence for them."

Interestingly, when The Silence of the Lambs was released, a Home Office official asked Britton if he thought the film would increase the number of serial killings - and his answer was yes. "It's not a case that a film causes that increase," he says, "but some people with certain propensities will look at the attention being given to serial killing through the media and think, 'I want some of that.' "

It seems to be a depressingly familiar pattern, one that he sees time and time again in his consulting room; a desperation for some sort of external approval. Britton explains, "People will say, 'I don't want to be a nobody. I feel I'm invisible - nobody sees me for what I am.' The notion of being famous for something dreadful is as attractive to some people as being well known for a social achievement."

Although Britton is never judgemental about the personalities he analyses, he admits that the contrast between his clinical and forensic commitments can be upsetting. "I spend a great deal of time working with offenders. I see so many people who injure others. On the other hand, I see so many people who've been injured. It's that relationship between working with offenders and seeing the dreadful things that have happened to victims that I find most difficult."

It is also galling when one of his profiles fails to catch the killer until it is too late. Often Britton will never hear about the outcome of a particular crime, and has to presume that it has been solved. "One dreadful example was the Green Chain rapes, where I was able to give a clear analysis of who they were dealing with, and I assumed they moved forward with it," he says. A couple of years later, he was working on the Bissett case, the murder of a mother and daughter in south-east London, and realised it was the same man. "I was thinking, hang on, we know who this is. That really hurts."

Britton displays the sort of professional detachment you'd expect in his line of work; all his observations are precise, considered and methodical. Yet he's clearly affected by the cases he is asked to deal with. "I've been told I used to laugh more," he says. Certainly there's a sort of sad resignation in his tone, which runs through the book, too. He agrees. "Well how could I be otherwise? Every time you have to go [to a case], it leaves you with a very bad feeling in your stomach, before and afterwards. They all affect me one way or another. It's hard; you always leave something of yourself behind"n

'The Jigsaw Man', by Paul Britton, Transworld, pounds 16.99.