People will often intervene if another person is in need of help: this is known as bystander intervention. They are less likely assist if many people are present. This propensity to passivity is called the bystander effect, or bystander apathy.
The failure of a large number of witnesses to act during the half-hour murder of Kitty Genovese in 1964 shocked the world, and was the impetus for investigation into the bystander effect. For this reason, the name Genovese syndrome or Genovese effect was used to describe the effect at the time.
The killing in 1995 of Deletha Word in front of a crowd who failed to intervene appears to have been another case of the bystander effect on a large group of people.
A 1968 study by Darley and Latane first demonstrated the bystander effect in the laboratory, and it has been studied ever since.
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In an unrelated use, the term bystander effect is also used in radiobiology to describe the effects of radiation on cells that are adjacent to those directly affected by radiation.