Image caption Police snipers opposite Kreditbanken where Jan-Erik Olsson held workers hostage for six days

Forty years ago, the term Stockholm Syndrome was coined at the end of a six-day bank siege. What is it and why is it cited time and again in hostage situations?

Most people know the phrase Stockholm Syndrome from the numerous high-profile kidnapping and hostage cases - usually involving women - in which it has been cited.

The term is most associated with Patty Hearst, the Californian newspaper heiress who was kidnapped by revolutionary militants in 1974. She appeared to develop sympathy with her captors and joined them in a robbery. She was eventually caught and received a prison sentence.

But Hearst's defence lawyer Bailey claimed that the 19-year-old had been brainwashed and was suffering from "Stockholm Syndrome" - a term that had been recently coined to explain the apparently irrational feelings of some captives for their captors.

More recently the term was applied in media reports about the Natascha Kampusch case. Kampusch - kidnapped as a 10-year-old by Wolfgang Priklopil and held in a basement for eight years - was reported to have cried when she heard her captor had died and subsequently lit a candle for him as he lay in the mortuary.

Image caption Natascha Kampusch was kidnapped as a 10-year-old by Wolfgang Priklopil

While the term is widely known, the incident that led to its coinage remains relatively obscure.

Outside Sweden few know the names of bank workers Birgitta Lundblad, Elisabeth Oldgren, Kristin Ehnmark and Sven Safstrom.

It was 23 August 1973 when the four were taken hostage in the Kreditbanken by 32-year-old career-criminal Jan-Erik Olsson - who was later joined at the bank by a former prison mate. Six days later when the stand-off ended, it became evident that the victims had formed some kind of positive relationship with their captors.

Stockholm Syndrome was born by way of explanation.

Patricia "Patty" Hearst 19-year-old American newspaper heiress Patty Hearst was kidnapped and held hostage by little-known group the Symbionese Liberation Army (SLA) in 1974

She was apparently brainwashed into accepting their ideas

In April 1974 she was caught on CCTV helping the group to rob a bank

She went on the run, but was caught by the FBI

Hearst was sentenced to seven years in prison, but was released after three years

She was pardoned in January 2001 by President Bill Clinton 1974: Newspaper heiress kidnapped

The phrase was reported to have been coined by criminologist and psychiatrist Nils Bejerot. Psychiatrist Dr Frank Ochberg was intrigued by the phenomenon and went on to define the syndrome for the FBI and Scotland Yard in the 1970s.

At the time, he was helping the US National Task Force on Terrorism and Disorder devise strategies for hostage situations.

His criteria included the following: "First people would experience something terrifying that just comes at them out of the blue. They are certain they are going to die.

"Then they experience a type of infantilisation - where, like a child, they are unable to eat, speak or go to the toilet without permission."

Small acts of kindness - such as being given food - prompts a "primitive gratitude for the gift of life," he explains.

"The hostages experience a powerful, primitive positive feeling towards their captor. They are in denial that this is the person who put them in that situation. In their mind, they think this is the person who is going to let them live."

But he says that cases of Stockholm Syndrome are rare.

So, what went on in the bank on Stockholm's Norrmalmstorg square that enabled the captives to experience positive feelings towards their captors, despite fearing for their lives?

In a 2009 interview with Radio Sweden, Kristin Ehnmark explained: "It's some kind of a context you get into when all your values, the morals you have change in some way."

It was Ehnmark that, according to reports, built up the strongest relationship with Olsson. There were even erroneous reports afterwards that the pair had become engaged.

Image caption Employees taken hostage in the bank's vault by Jan-Erik Olsson

In one phone call from the bank's vault to the country's prime minister Olof Palme, Ehnmark begged to be allowed to leave the bank with the kidnappers. One of Olsson's demands had been the delivery of a getaway car in which he planned to escape with the hostages. The authorities had refused.

Telling Palme that she was "very disappointed" with him, Ehnmark said: "I think you are sitting there playing chequers with our lives. I fully trust Clark and the robber. I am not desperate. They haven't done a thing to us. On the contrary, they have been very nice. But you know, Olof, what I'm scared of is that the police will attack and cause us to die."

American journalist Daniel Lang interviewed everyone involved in the drama a year later for the New Yorker. It paints the most extensive picture of how captors and captives interacted.

The hostages spoke of being well treated by Olsson, and at the time it appeared that they believed they owed their lives to the criminal pair, he wrote.

On one occasion a claustrophobic Elisabeth Oldgren was allowed to leave the vault that had become their prison but only with a rope fixed around her neck.

She said that at the time she thought it was "very kind" of Olsson to allow her to move around the floor of the bank.

Safstrom said he even felt gratitude when Olsson told him he was planning to shoot him - to show the police understood he meant business - but added he would make sure he didn't kill him and would let him get drunk first.

"When he treated us well, we could think of him as an emergency God," he went on to say.

Stockholm Syndrome is typically applied to explain the ambivalent feelings of the captives, but the feelings of the captors change too.

Image caption Police officers wearing gas masks escort Jan-Erik Olsson from the bank

Olsson remarked at the beginning of the siege he could have "easily" killed the hostages but that had changed over the days.

"I learned that the psychiatrists I interviewed had left out something: victims might identify with aggressors as the doctors claimed, but things weren't all one way," wrote Lang.

"Olsson spoke harshly. 'It was the hostages' fault,' he said. 'They did everything I told them to do. If they hadn't, I might not be here now. Why didn't any of them attack me? They made it hard to kill. They made us go on living together day after day, like goats, in that filth. There was nothing to do but get to know each other.'"

The notion that perpetrators can display positive feelings toward captives is a key element of Stockholm Syndrome that crisis negotiators are encouraged to develop, according to an article in the 2007 FBI Law Enforcement Bulletin. It can improve the chances of hostage survival, it explained.

But while Stockholm syndrome has long been featured on police hostage negotiating courses, it is rarely encountered, says Hugh McGowan, who spent 35 years with the New York Police Department.

McGowan was commanding officer and chief negotiator of the Hostage Negotiation Team, which was set up in April 1973 in the wake of a number of hostage incidents that took place in 1972 - the bank heist that inspired the film Dog Day Afternoon, an uprising that came to a violent end at Attica prison in New York and the massacre at the Munich Olympics.

"I would be hard pressed to say that it exists," he says. "Sometimes in the field of psychology people are looking for cause and effect when it isn't there.

"Stockholm was a unique situation. It occurred at around the time when we were starting to see more hostage situations and maybe people didn't want to take away something that we might see again."

He acknowledges that the term gained currency partly because of the bringing together of the fields of psychology and policing in the field of hostage negotiating.

There are no widely accepted diagnostic criteria to identify the syndrome, which is also known as terror-bonding or trauma bonding and it is not in either of the two main psychiatric manuals, The Diagnostic and Statistical Manual of Mental Disorders and the International Statistical Classification of Diseases and Related Health Problems (ICD).

But the underlying principles of how it works can be related to different situations, say some psychologists.

"A classic example is domestic violence, when someone - typically a woman - has a sense of dependency on her partner and stays with him," says psychologist Jennifer Wild, a consultant clinical psychologist at the University of Oxford.

"She might feel empathy rather than anger. Child abuse is another one - when parents emotionally or physically abuse their children, but the child is protective towards them and either doesn't speak about it or lies about it."

Forty years on and the term is evoked nearly every time an abductee is found after many years out of public sight. Some argue that its very nature implies a criticism of the survivor - a weakness perhaps.

In a 2010 interview with the Guardian, Kampusch rejected the label of Stockholm Syndrome, explaining that it doesn't take into account the rational choices people make in particular situations.

"I find it very natural that you would adapt yourself to identify with your kidnapper," she says. "Especially if you spend a great deal of time with that person. It's about empathy, communication. Looking for normality within the framework of a crime is not a syndrome. It is a survival strategy."

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