It's actually quite rare:

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.