The Bystander Effect

The bystander effect or Genovese syndrome is a social psychological phenomenon that refers to cases where individuals do not offer any means of help in an emergency situation to the victim when other people are present. The probability of help has in the past been thought to be inversely related to the number of bystanders; in other words, the greater the number of bystanders, the less likely it is that any one of them will help. The mere presence of other bystanders greatly decreases intervention. This happens because as the number of bystanders increases, any given bystander is less likely to notice the incident, less likely to interpret the incident as a problem, and less likely to assume responsibility for taking action.[1]
<http://psychology.about.com/od/socialpsychology/a/bystandereffect.htm>