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When a young woman was killed in a brutal attack in New York in 1964, it was reported that not one of 38 witnesses intervened. Although no evidence was found to support the story, it prompted decades of research to try and explain why bystanders do or don’t help in risky situations. Dr Rachel Manning from Buckingham University, UK, explains how the case led to a theory called the ‘bystander effect’, which suggests that people are less likely to help if more people are present. (Image: Woman running at night, Credit: Getty Images.)