It is unlikely that the name of Scandinavian criminal Jan-Erik Olsson would be remembered by very many today, were it simply for the bank robbery he attempted in the Swedish capital Stockholm on this day in 1973.

But when the robbery went awry and Olsson took members of the bank’s staff hostage, a chain of events were set in motion leading to the discovery of a strange psychological phenomenon, which would become known as Stockholm Syndrome.

When the 32-year-old began his assault on the capital’s Kreditbanken, police were called and gunfire was exchanged. Olsson grabbed four bank workers and held them captive for more than five days as he attempted to negotiate his escape. Joined by former prison mate Clark Olofsson, Olsson asked for three million Swedish Kronor, more guns, bulletproof vests, and a getaway car.

Olsson called prime minister Olof Palme to assure him that the hostages would die if his demands were not met. The following day, Palme received another call; this time, it was from hostage Kristin Ehnmark, who strangely pleaded for the bank workers to be allowed to escape with their captors.

After the standoff ended on August 28 when police used gas to stun the robbers, it became apparent that the hostages had formed a positive bond with the two men. They said they were well treated by Olsson, and that believed they owed their lives to the criminals.

[Read more: May 5, 1980 - SAS storm Iranian Embassy to end six-day siege in front of millions of TV viewers]

They repeatedly claimed they were more frightened by potential police action than the robbers; their obvious empathy with them aroused academic interest and the term Stockholm Syndrome was coined to for the phenomenon, which would go on to be defined by psychiatrist Dr Frank Ochberg.

"The hostages experience a powerful, primitive positive feeling towards their captor,” said Ochberg in explaining the condition. “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."

Stockholm Syndrome would famously be cited a year later when American heiress Patty Hearst was kidnapped by a group of domestic terrorists and - after being ‘brainwashed’ into accepting their ideas – was later caught on CCTV cameras helping them rob a bank.