Why do crowds move us?

Being part of a crowd can embolden us: we might raise our voices in protest at a march, or snort-laugh more at a sold-out comedy gig. But in the popular imagination, big crowds are often associated with danger - with hysteria and violence. From the Hajj to a heavy metal concert, how does being surrounded by others change our behavior?

Dessa talks with a psychologist, a mathematician, an activist, and a legendary DJ to find out how we are moved by crowds.