Dr Park Dietz has an uneasy relationship with publicity, and it's not just because of his deep, abiding familiarity with the psychological make-up of serial killers, sexual deviants, stalkers and mass murderers.

For years, America's foremost criminal profiler - who has testified at the trials of such criminals as John Hinckley, Jeffrey Dahmer and Andrea Yates - would not let reporters give details of his home or office. He did not want his photograph in newspapers or magazines.

The reason, which he is now happy to reveal from his harbour-front office in Newport Beach, is a stalker was after him - a man who switched obsessions from a prominent British performer (whom Dietz declines to name) after seeing Dietz's name twice in the media and deciding they shared some bond. The stalker was arrested a few years ago and imprisoned on other charges, which has given Dietz at least a few years' breathing room.

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Dietz believes such an obsession could only have been the result of a slew of television shows glorifying criminal profilers such as himself -such as Millennium, Profiler, The X-Files and NYPD Blue. He receives slightly unhinged fan mail, from people who, he says, either feel they are victims in need of his help or simply want to be him. He sees ways in which both saturation news coverage of shocking crimes and their fictional depiction in drama series can spur further acts of real-life violence.

And yet Dietz is also very much a creature of the media - understandable, given his association with just about every high-profile murder case of the past several decades. He appears regularly on television news shows. He has been profiled in all sorts of magazines and for the past several years has been a consultant on the cop show Law & Order and its offshoot Law & Order: Criminal Intent, which has just started airing a new season in Britain. (The character of Detective Robert Goren, played by Vincent D'Onofrio, is based on Dietz, and they share what Dietz describes as an "idiot savant knowledge of obscure things that pop up in the relation to crimes".)

On Law & Order, he has two relatively modest goals. The first is to make sure that nothing in the scripts he sees gives away information that might be valuable to a real-life criminal. "If I raise something with the writers, something that could foreseeably help a criminal, they always change it," Dietz notes.

His second goal is to defend the integrity of his learning as a forensic psychiatrist. Little irks him more than the fad for university courses in profiling and forensics which, he says, are inspired more by television than the real world of crime-fighting, are taught by unqualified people and base themselves on "foolish or plagiarised books". Real profiling, he insists, is the fruit of years of experience as either a detective or, as in his case, a crime scene analyst. He prides himself on helping the show's writers to come up with plausible motivations for the characters, especially the more deviant ones. "It doesn't have to be probable behaviour," he allows, "it just has to be possible."

He takes an equally severe attitude to news shows. Twice, he appeared on CNN in the middle of a sensational murder case and warned the network that if it didn't tone down their coverage it would lead to further crimes. On another occasion, he told a production team from 20/20, a magazine show on ABC, that he would not participate in a programme reconstructing a workplace shooting because he feared their approach would encourage copycats. The programme went out on a Friday; by the following Tuesday there had been two fresh mass murders in other parts of the United States.

"Here's my hypothesis," he said. "Saturation-level news coverage of mass murder causes, on average, one more mass murder in the next two weeks." The reason, he says, has something to do with the USA's size. In a country so large the likelihood of one or two people snapping becomes quite high.

"It's not that the news coverage made the person paranoid, or armed, or suicidally depressed," Dietz said. "But you've got to imagine this small number of people sitting at home, with guns on their lap and a hit list in their mind. They feel willing to die. When they watch the coverage of a school shooting or a workplace mass murder, it only takes one or two of them to say - 'that guy is just like me, that's the solution to my problem, that's what I'll do tomorrow'. The point is that the media coverage moves them a little closer to the action.

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"Is that causation? Legally, maybe not. Epidemiologically, yes," he said.

Dietz is just as outspoken on the subject of films of television dramas that have a propensity to spawn real-life crime. For him, the key issue here is the association of violence with sexual titillation - or, as he puts it, giving the teenage boys in the cinema an erection "while we kill the woman". That's the point at which cultural artefacts don't just give ideas to criminals who are likely to offend anyway, but help form them as criminals. "Sexual deviations are acquired," Dietz argued. "And the reason why is, often, the pairing of normal sexual arousal in boys with fortuitous imagery that becomes associated with erotic arousal."

Dietz famously argued that Jeffrey Dahmer suffered from paraphilia - a sexual excitement associated with the mutilation of his victims. In Britain, in the years following the Second World War, clubs and magazines sprang up devoted to gas mask fetishisation, which Dietz is convinced can be traced back to teenage boys watching girls they fancied in air-raid shelters. No such fad has existed in the USA, which was not bombed during the war. Dietz finds himself in agreement with Stanley Kubrick, who pulledA Clockwork Orange from distribution in Britain following a spate of crimes mimicking the sexual violence of the teenage gang at the centre of the plot.

Dietz has had some modest successes in making his case - most notably, persuading the pulp detective magazine industry to change its covers in the 1980s after he published a paper warning of the dangers of showing half-naked women being bound or tortured. The magazines switched to pictures of half-naked women wielding guns instead. For the most part, though, he has failed to make an impression on the entertainment industry. Since the 1980s, Dietz has unsuccessfully pushed for a so-called "detumescence period" - a three-minute gap between sexually arousing images and acts of violence on screen, especially in films aimed at teenagers.

His frustration with the broader industry helps explain his willingness to be associated with Law & Order. As he acknowledged, with some gratitude: "They have listened to me."

The Hallmark's channel crime season continues tonight with the UK premieres of Law & Order: Criminal Intent daily at 8pm and with Law and Order: Special Victims' Unit weekdays at 10pm

How criminal profiling works

The art of criminal profiling consists of two major components: picking clues from a crime scene and using them to build up a psychological profile of the offender to figure out what kind of person he (or she) is and where he might be tracked down and caught; then, after the culprit is in custody, conducting extensive interviews to determine his mindset at the time of the crime or crimes.

The first tends to be the realm of the experienced cop, or, more commonly, FBI agent - the feds have a greater opportunity to see a variety of extreme crimes such as mass infanticide or serial killing and so recognise their characteristics, while a city homicide detective, however experienced, is likely to see only one or two such events in his career.

The extensive interviews are what Park Dietz is known for, particularly when it comes to determining, on behalf of a prosecution or defence team, whether the defendant in a criminal trial should be considered insane or not. That, in turn, is often a matter of interpreting state laws, which can define insanity as anything from not knowing that a certain action is a criminal offence (a relatively narrow definition) to behaviour arising from mental illness (a relatively broad one). "Sanity and insanity are such misleading terms," Dietz said - which is why he gets paid a lot of money to unpick one from the other.