The Bystander Effect: why crowds do nothing

In 1964, Kitty Genovese was attacked outside her New York apartment while, according to early reports, 38 neighbours watched and did nothing. Social psychologists John Darley and Bibb Latané used the case to demonstrate the bystander effect: the more witnesses present, the less likely any individual is to intervene, because responsibility becomes diffused across the group. Studies replicated this in labs, showing that a person alone will help 85% of the time, while one of five bystanders helps only 31% of the time.

Source: Darley & Latané, Journal of Personality and Social Psychology (1968)