A new film starring Benedict Cumberbatch, The Courier, tells the story of a salesman, Greville Wynne, caught up in the murky world of espionage during the Cuban Missile Crisis. This follows recent news that David Smith, a 57-year-old and apparently normal security guard at the British embassy in Berlin, had been spying for Russia. So why do seemingly ordinary people become spies?

In 1988, the KGB defector Stanislav Levchenko described an American mnemonic, Mice, which stands for "money", "ideology", "coercion/compromise" and "ego". Susceptibility to these factors, he claimed, was a target's key weakness that could be exploited.