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Stephen Reicher is a social psychologist at St Andrews University who has spent decades understanding how people behave when in a group. To do so, he's often had to immerse himself among the subjects of his studies, from the Bristol riots in 1980 to the millions of Hindu pilgrims who go to the Magh Mela. Stephen Reicher talks to Jim al-Khalili about the positive and the negative sides to a crowd and the role of a leader of a crowd. He explains how he gave up a place to read medicine, to the annoyance of his parents, to study psychology. Now, he says, his mother would be proud of him as he's publishing research on the health benefits of attending mass gatherings.