Bystander effect refers to the finding that the greater the number of bystanders who witness an emergency, the less likely any one of them is to help. It is the phenomenon that underlies many examples of failing to help strangers in distress: the larger the group a person is in (or thinks he is in), the less likely he is to come to a stranger's assistance. One reason is diffusion of responsibility (no one thinks it is his responsibility to act).
Other /More definition:
Bystander effect refers to the tendency for the presence of other people to inhibit helping. It is the finding that a person is less likely to provide help when there are other bystanders. Moreover, it is the social phenomenon in which individuals are less likely to provide needed help when others are present than when they are alone. It is the tendency for people to help less when they know others are present and capable of helping. The effect was initially thought to be the result of apathy and a selfish unwillingness to get involved, but research suggests a number of cognitive and social processes, including diffusion of responsibility and misinterpretation that help is not needed, contribute to the effect.

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