The bystander effect, or bystander apathy, is a social psychological theory that states that individuals are less likely to offer help to a victim when there are other people present. First proposed in 1964, much research, mostly in the lab, has focused on increasingly varied factors, such as the number of bystanders, ambiguity, group cohesiveness, and diffusion of responsibility that reinforces mutual denial

The theory was prompted by the murder of Kitty Genovese about which it was wrongly reported that 38 bystanders watched passively. Recent research has focused on "real world" events captured on security cameras, and the coherency and robustness of the effect has come under question. More recent studies also show that this effect can generalize to workplace settings, where subordinates often refrain from informing managers regarding ideas, concerns, and opinions.

Social psychology research