Those who study culture — or think about public policy in relation to it — often wrestle with the classic post hoc dilemma: did a work or movement in popular culture influence events in real life, or did it simply reflect the zeitgeist?

Were, say, ‘video nasties’ responsible for an uptick in violence and sadism in a generation of British youth? The Daily Mail seemed to think so, although today their hysterical headlines appear faintly ridiculous.