Bystander Effect
The Bystander Effect is the social psychological phenomenon in which individuals are less likely to offer help to a victim when other people are present. The more bystanders witness an emergency, the less likely any individual is to intervene — due to diffusion of responsibility ("someone else will handle it") and pluralistic ignorance ("no one else is reacting, so maybe it's not serious"). First documented by John Darley and Bibb Latané following the Kitty Genovese murder in 1964.