Imagine that you witness someone collapse in a crowded street. You might assume that the more people who are present, the more likely it is that the victim will receive help. Surprisingly, decades of psychological research suggest the opposite is often true. This puzzling phenomenon is known as the “bystander effect”: the more people who witness an emergency, the less likely any single individual is to step forward and help.

The effect was first studied seriously after a number of well-publicised cases in which people in trouble were apparently ignored by large numbers of onlookers. Psychologists identified two main reasons for it. The first is what they call the “diffusion of responsibility”. When we are alone and see someone in need, we feel that the responsibility to act rests entirely on us. But when we are one of many, that responsibility feels divided, and each person assumes that someone else will do something – with the result that nobody does.

The second reason is social influence. In an uncertain situation, we look to others to work out how to behave. If everyone around us appears calm and does nothing, we conclude that the situation is not really an emergency, or that action is not expected of us. Ironically, everyone may be looking at everyone else for a signal, and everyone may be drawing the same mistaken conclusion.

Understanding the bystander effect is not just an interesting piece of psychology; it can genuinely save lives. Experts advise that if you are ever the person in trouble, you should not shout to the crowd in general, but point directly at one individual and ask them specifically for help. By doing so, you overcome the diffusion of responsibility and make it clear that the task is theirs. Equally, simply knowing about the effect makes us more likely to overcome it ourselves and to be the one who steps forward when others hesitate.