Asked by: Julia Nielson, Cardiff
They may wish to fit in by imitating others, feel constrained by being observed, become excited by the noise and actions of the crowd, or may respond to local conditions and culture.
An odd example is that in different countries pedestrians step right or left when meeting others on a crowded street. This is unrelated to which side of the road they drive on and appears to emerge spontaneously and then stick in each culture.
Perhaps the most worrying behaviour is when people feel less responsible for helping someone in trouble when there are others around. This ‘bystander effect’ is quite rare but more likely the larger the crowd.