The Italian police’s overheated interpretation of Knox’s behaviour was a particularly pungent manifestation of a universal trait, one that frequently leads criminal investigators and juries astray: overconfidence in our ability to read someone else’s state of mind simply by looking at them. This is not a uniquely modern error, born of pop psychology books. Shakespeare was wary of it. In Macbeth, he has Duncan remark how hard it is “to find the mind’s construction in the face”. It’s a warning that law enforcement officers often seem unable, or unwilling, to heed.