On an uneventful day, five passers-by in busy Oxford shopping street suddenly stop and look upwards. They have spotted a camera mounted on a nearby roof, pointed straight at them. But these aren’t strangers who have suddenly realised that Big Brother is watching them. They are actors, who are taking part in a natural experiment that looks at how information spreads through crowds of people.

Many psychologists have studied how humans and other animals follow each others’ gazes. But the vast majority of these studies have been done in laboratory settings, in which volunteers sit face-to-face. Very few scientists have studied natural crowds of people.

Stanley Milgram (he of the famous obedience experiment) was one of the first. In 1969,