This weekend's incident involving Madonna - she had to be escorted off a plane at London's Heathrow Airport after receiving a threatening phone call - is the second time the superstar is known to have been harrassed by an obsessed fan.

In 1995, Robert Hoskins was prosecuted after scaling the security wall surrounding her Hollywood Hills home.

Madonna: This weekend's incident is the second time she has been harrassed

But it is also just another in a long line of similar cases. Brad Pitt, Steven Spielberg, Stevie Nicks, Linda Ronstadt, Jerry Lewis, Ashley Judd and Monica Seles are just a few of the stars who have been targeted in recent years and months.

The tactics employed by celebrity stalkers can vary, from sending letters to full-scale break-in of their homes.

But, according to one UK psychologist, in nearly all cases, their extreme behaviour is a perversion of an otherwise natural and healthy interest in well-known figures.

"Fans who become stalkers are trying to satisfy a drive to become totally immersed in whoever they are into, regardless of the consequences and the risk of dangerous isolation," says Dr Sandy Wolfson, Principal Lecturer in Psychology at the University of Northumbria.

Dr Wolfson says that normal fan worship can have a number of positive results, including reducing tension and aiding social-interaction.

Steven Spielberg feared for his life and that of his family

When things go wrong, however, the opposite becomes true, as the fan-turned-stalker removes themselves from reality into an imaginary world populated solely by the star.

In January this year, a woman was arrested after she was found in actor Brad Pitt's Santa Monica home. She wanted to be close to him and dressed up and slept in his clothes.

The woman's actions pushed her into a minor transgression of the law. But, according to forensic psychologist Ian Steven, the type of obsession that compels a stalker can escalate.

"It is very difficult to treat because the reason for it is often rooted so far in the past. It can be treated and managed but often the root of the problem remains and can reappear as is shown when a stalker re-offends on release from prison," Steven said in a recent interview.

Last year, a man was jailed for 25 years after he was found outside Steven Spielberg's Los Angeles home with a "rape kit" consisting of tape, handcuffs, nipple clamps, and a knife.

The film director told the court how the stalker had carried out extensive research about him and his family. Despite the conviction, Spielberg believed the man's obsession would remain.

Monica Seles: Forced off the tennis circuit after being stabbed

Dr Wolfson also believes that the seeds of stalking are deep-rooted and that it need only take something small, like a minor family conflict, to tip a potential obsessive over the edge.

She adds that the line between threats and causing real harm is just as easily crossed.

"When an obsessed fan becomes violent, it is because something has ultimately led them to completely lose their grip and control of what is happening around them," she explains.

In Los Angeles, an estimated 15% of stalking cases turn violent - but sometimes with fatal consequences.

Famous cases include former Beatle John Lennon who was shot dead by Mark Chapman in New York in 1980. In 1989, rising actress Rebecca Schaeffer was also gunned down by obsessed fan Robert Bard. Tennis star Monica Seles was stabbed by a crazed Steffi Graf fan in 1993 in Munich.

On rare occasions, the celebrity has actually had a relationship with a stalker. NYPD Blue actress Andrea Thompson was stalked by a designer on the series with whom she had broken off a relationship.

John Lennon lost his life to a crazed stalker

However, most fans, says Dr Wolfson - who has researched some of the world's most ardent, such as those at Star Trek conventions - are level-headed, responsible individuals.

What's more, stalking can happen to all sorts of people, both male and female, in all walks of life.

Figures up to April this year showed that in the UK more than 2,500 stalking cases were brought before the courts, 1,250 people were found guilty and 165 jailed over a period of six months.

Research carried out in the US last year estimated that over one million women and 370,000 men were stalked each year.