Just who do they think they are?

"Trust me, I'm a doctor", said Paul Bint, thrusting his hands down his girlfriend's bra to feel for lumps on her breasts. He would probably have said the same thing as he put 12 stitches into a patient's head wound if the hapless man felt the need to ask. Of course he didn't. Nor did the man with a collapsed lung whom Bint attended. Nor the parents he told their daughter would live after a road accident, just before she died. Nor the unfortunate at whose heart by-pass operation he tried to assist.

It is one of those things we take on trust - that the chap in the white coat with the stethoscope in the hospital treatment room is a qualified doctor. Just as the 150 passengers on the Charing Cross to Ramsgate commuter train assumed that the young man at the controls was a qualified driver, or at the least an approved trainee. But Paul Bint was not a doctor, and Nicholas Gray, aged 18, was not a train driver. Gray was sentenced to two years' probation last week for posing as one and taking the controls of the train.

We all have fantasies. But most of us keep them where they belong, inside our heads. Increasingly, though, significant numbers of our fellow citizens are beginning to inflict theirs on the rest of us. Recent months have turned up a clutch of counterfeit doctors, several sham SAS men and a staggering 20 cases of bogus paramedics in the London Ambulance Service area alone. There has been the phoney priest who celebrated mass at a Catholic cathedral and then used his clerical identity to enter a woman's home and sexually assault her. There was the Scotsman who dressed as a policeman and subjected cars to roadside checks. There has even been a man who got a job as a secretary by pretending to be a woman - and was only unveiled when he got drunk during a romantic weekend with his boss. That last incident was in Moscow. But it's probably only a matter of time before it, like the others, surfaces here.

Lying, said Oscar Wilde at the end of the last century, is amongst most people - barristers excluded - a decaying art. Oscar, thou shouldst be living. Lying has been transformed from a mere art into a science.

We live with three kinds of liars, according to Dr Bill Thompson, a forensic criminologist at Reading University; sufferers of obsessive desire, delusion and incarnate fantasy. Obsessive desire is that of Nicholas Gray, the wannabe train driver. "He doesn't think he's a train driver - he knows he isn't one - but his obsession transcends the practicalities," says Thompson. "This is common enough in modern society: many fishermen are similar obsessives - you'd have to be to sit in the rain on a river bank for eight hours watching a float bob up and down. But this type only come to public attention when they break the law."

Or violate conventional patterns of behaviour. Brian McKinnon was the 32-year-old man who returned to school posing as Brandon Lee, a 17-year- old schoolboy, in a ruse to get into medical school (and it is still his intention to go there, he revealed last week). "He didn't personate a doctor, just a schoolboy," says Thompson. "He was prepared to do the work to become a doctor." He spent an intensely nervous year at school constantly terrified he would be uncovered.

But there were elements of fantasy, too. He invented a whole life - a father who was a regius professor of zoology and mezzo-soprano mother killed in a car crash: his real father was a fireman, and his mother a nurse. "I suppose I had watched too much Mission: Impossible," he said afterwards. "Only if he finally fails to fulfil his ambition would you find out whether he goes further into delusion or fantasy," says Thompson.

Delusion is the province of the stalker. "These are erotomaniacs who convince themselves their victim is in love with them," he says. Radio presenters and DJs are particularly prone to their attentions, and Frances Lawrence, widow of the murdered headmaster Philip Lawrence, has also been trailed by one. "They can go crazy when thwarted. They can even kill - as one did last week with the ex-Miss Poland."

But the fantasist is, he says, the most dangerous. "People with obsessive desire want to put in the effort; fantasists don't," he says. "Desire people, when cornered, will own up; fantasists won't, so the unskilled man who dresses in a white coat and walks round hospitals will say, when pushed, give the patient the drug, regardless of the consequences". One such was the character who persuaded his mistress into a suicide pact, claiming he was an SAS colonel who was about to be assassinated by the IRA. She died, but he lived. He turned out to be not an SAS man but an employee of Eastern Electricity Board who lived with his wife and two children in a semi outside Peterborough.

The military fantasist is one of the most common types. It is as if the more inadequate the personality the greater his need for an alter ego of epic proportions. In this category falls the man who told his local paper he was a mercenary who had fought in the Congo (when he would in reality have been only 12), in the Gulf war, and as a CIA agent in Vietnam and Nicaragua. He also convinced the South Shields Gazette he was involved in a million-dollar plot against the Shah of Iran. They ran it all under the headline "My Life as a Hitman". Next day the intoxicated protagonist robbed the local Barclays and stole pounds 7,495. He was arrested when staff recognised his picture from the newspaper.

More serious was the case of Jamie Petrolini and Richard Elsey who, two years ago, were convicted of murder for the stabbing of an Egyptian chef while pretending to be on an SAS training mission. They are not isolated examples. At the SAS headquarters in Hereford and at the Army's Intelligence Centre at Ashford files are kept on people who have a record of pretending to be part of the unit.

Bill Thompson blames the media. "We live in a fantasy-inducing society. TV and films turn the most mundane event into a drama. Police work is not about tedious systematic slog. It's about the breakthrough, the brainwave, the confrontation, the arrest. It's about the instant hit, the high. You have to teach them to get off that - to learn about the high of hard work and perseverance".

Perhaps. But there must be more to it than that. Society has become more fragmented. The emphasis upon individualism and self-help, the Thatcherite notion that there is no such thing as society, and our redefining of ourselves as consumers rather than citizens - all these have created a world where self and solipsism are celebrated. In it, alienation has become a norm. The sad characters who roam our streets pretending to speak into walkie- talkies are as much a measure of that as of a caricature media culture which heightens, exaggerates and dramatises everything it touches.

"Such people are not mad. Those with proper psychotic mental illnesses tend to have delusions - they believe themselves to be God or the Queen - which are shifted rather than shifting with circumstances as are those of fantasists," says Dr Bridget Dolan, a forensic psychologist at St George's Hospital, London. Nor are they necessarily acting out of criminal motivation - though the two can be mixed, as with the paedophilic women posing as health visitors with the aim of "examining" babies or a conwoman like "Lady" Rosemary Aberdour, who stole pounds 3m from the charity for which she worked and squandered it on a fantasy existence of posh cars, swanky parties, society balls, champagne and jewellery. She was really not an aristocrat but the daughter of a radiographer from Essex.

But true fantasists are different creatures. Dr Dolan calls these "pseudologues" after the condition Pseudologia Fantastica first described by the German psychologist Delbruck in 1891. He defined it as fantastic lies woven into a complex system of deception. The object of his study was an Austrian maid who wandered Europe successfully assuming the roles of a Romanian princess and a member of the Spanish royal family.

"The pseudologue is not mentally ill but someone with a personality disorder," says Dr Dolan. "The difference is that one is like an ice-cream (the person) with a cherry (the illness) on the top, where the latter is like a raspberry ripple." In this condition the lies are built on a matrix of truth, the intent is self-aggrandisement and the tendency usually manifests itself in early adulthood.

It is this syndrome which lies behind enigmatic historical pretenders like the woman who for 50 years maintained she was the Grand Duchess Anastasia, only surviving daughter of the last Tsar; it was not until after her recent death that she was exposed by DNA tests to be a fraud. It was there too in the case of Judith Ward, who spent 17 years in jail as an IRA bomber before being released on appeal. "Evidence at her appeal related to her previous use of false assertions to enhance her self-esteem - these claims related to romantic themes and her involvement with Ireland, Irish people and Irish causes," says Dr Dolan.

"Pseudologues are usually neither mentally ill nor deluded. Often they are intelligent, gifted, worldly, knowledgeable, of superior verbal ability and otherwise capable of success," she adds. And their fantasies shift and develop. One 48-year-old man who was so convincing in his insistence that he was a doctor that experienced clinicians found it hard to fault him. He was rumbled only when he also claimed to be a dentist, an accountant and a barrister. And Paul "Trust me" Bint, as well as posing as a doctor, pretended to be the grandson of Lord Forte, a relative of Lord Havers and the "Earl of Arundel". He also claimed to be a dancer with the Royal Festival Ballet and tricked his way into expensive cars, including a Rolls- Royce, a Porsche, and an pounds 84,000 Ferrari.

"Bint was very successful at his deceptions," says Dr Dolan. So much so that at one of his trials the judge recognised the real risk of treatment allowing him to acquire further medical knowledge which could lead to him posing as a psychiatrist.

The field of medicine is a particular magnet for the pseudologue. One former US army-registered nurse treated 440 patients as a British hospital doctor and was uncovered only when one died. Another man posed as a gynaecologist for six months in Lancashire and, after being unmasked, then talked his way into a top job as a hospital administrator at Guy's.

The problem of bogus doctors is such that the Economic and Social Research Council funded Dr Harry Collins, professor of sociology at Southampton University, and a colleague Dr Joanne Hartland to study 100 cases of such personation. "Hardly any were caught because of deficiencies in their medical practice," says Dr Collins. "They were caught shoplifting or making an administrative mistake which caused people to look again at their CV.

"We found that people do not expect you to be anything other than what you say. If you do anything which is inconsistent, colleagues will 'repair' for you - justifying your deficiencies by saying that you were trained in another part of the world or whatever. Also once you are accepted you are given on-the-job training as a matter of course. It's much easier to bluff your way into most professions than people assume, though it's also probably much less damaging."

Yet how can society operate except on trust, asks Dr Dolan. "It is not feasible to check everything. Anyway most people have different personas - at work, at home, in the pub, at a party. It's how we express the different facets of our character. We keep our roles - as a father, as a businessman - quite distinct and are comforted by our success in doing so."

But then should you believe any of this expert opinion? "It might be," warns Professor Collins, "that I'm only pretending to be a sociologist."