In 1964, in New York City, a woman named Kitty Genovese was chased and repeatedly stabbed by an assailant, who took 35 minutes to kill her. The woman's screams went unheeded by at least 38 people who watched from their windows. No one tried to stop the attacker; no one even made a quick, anonymous telephone call to the police until after the attacker had left. When the bystanders were questioned later, they could not explain their inaction.
As you can imagine, people were appalled and shocked by the bystanders' response to the Genovese murder. Commentators said that the apparent indifference of the bystanders demonstrated that society had become cold and apathetic.
The fact that Kitty Genovese's attack went unreported is not remarkable because 38 were present; it is precisely because so many people were present that the attack was not reported.
This is known as the phenomenon of bystander intervention - the actions of people witnessing a situation in which someone appears to require assistance. In such situation, the presence of other people who are doing nothing inhibits others from giving aid. Almost everyone would help the victim when they were the only witness; however, when there appeared to be other witnesses, they were much less likely to try to help. In addition, those who did try to help reacted more slowly if other people were present.
Don't just be a bystander. Intervene.
(Carlson, N. R. & Heth, C. D.  Psychology the Science of Behaviour. Toronto, ON: Pearsons Canada.)