The bystander effect is a social psychological phenomenon in which individuals are less likely to offer help in an emergency situation when other people are present. The probability of help is inversely proportional 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.
—Safety Canada, January 2004, "Don't Just Stand There - Do Something"
Blog postsBeware the UnsurprisedEvolving to Extinction— On howevolutioncould be responsible for the bystander effect.