Noun
bystander effect (plural bystander effects)
(psychology) The phenomenon that someone is less likely to help another if other potential helpers are present than otherwise.
Research has shown that the bystander effect is often motivated by diffusion of responsibility, when different people witnessing an emergency all assume someone else will help. Philip Zimbardo
Research also suggests that the number of bystanders witnessing distress or suffering affects the likelihood of helping (the Bystander effect ). Source: Internet