Almost all crime dramas on television suffer from the same cliché. A gruff, old cop, who thinks nothing of breaking the rules, is paired with a naive, young recruit, who insists on doing everything by the book. This clash between characters – typified by Gene Hunt and Sam Tyler in the BBC’s Life on Mars – may be a cliché but it represents a real disagreement about how to fight crime. We should not worry too much about the misuse of police powers, say some, if it cuts crime. On the contrary, say others, we cannot trust the police, because they will abuse civil liberties.

In reality what lies behind criminal behaviour and what works in policing is far more complex. But the news this week that ...