The bystander effect is a well-established theory in psychology which states that people are less likely to respond to an emergency if other people are present. This theory was formulated and tested in response to the circumstances surrounding the murder of Kitty Genovese in New York City in 1964. The New York Times reported that thirty-eight people witnessed the crime, and none of them did anything to prevent it.
Although the accuracy of The New York Times report has since been challenged, the bystander effect is now well-attested. There are several theories claiming to explain it but most are based on the idea of diffused responsibility. If you are the only witness to an emergency, then you are the only person who can prevent it. If there are thirty-seven other people present, it is natural to assume that one of the other witnesses is better-equipped to handle the situation than you are.
Various studies have been conducted to establish the bystander effect. The earliest were arranged by John M. Darley and Bibb Latané, who used a number of different scenarios. One involved a woman in distress, like the Kitty Genovese murder. Other involved a fire, with smoke being pumped into a room where students were working. Darley and Latané found that, in all circumstances, the presence of others made subjects slower to notice the emergency and less likely to respond.