The bystander effect refers to the phenomenon where people are less likely to help in an emergency when others are present. The more bystanders there are, the lower the chance that any one person will intervene.
Psychologists attribute this to diffusion of responsibility (“someone else will help”) and social influence (we look at others’ reactions to decide how to act). A tragic example was the case of Kitty Genovese in 1964, where reports suggested many neighbours witnessed her attack, but few intervened.
The bystander effect highlights the importance of awareness—simply recognising this tendency makes people more likely to step forward and help when needed.