Bystander Effect or Diffusion of Responsibility
the vicious murder of Kitty Genovese in Kew Gardens, NY committed with view of at least 38 witnesses, none of whom intervened, led John Darley and Bibb latane to research this.
the larger the number of people who witness an emergency situation, the less likely anyone will be to intervene. The larger the group who witness a problem, the less responsible any one individual feels to help.
People tend to assume someone else will take action.