'Jihadi John': Why do we give criminals nicknames?

Assuming that he has web access, Mohammed Emwazi, the British Islamic State militant revealed last week to have featured in several terrorist videos, would be able scan headlines last summer to learn what his nickname could have been. On 21 August, the day after the film circulated showing the killing of James Foley, the journalist's masked murderer was called variously "The Beatle", "The Ali G Jihadi", "John the Executioner" and "Jailer John". But only one name would stick.