the bystander effect
i explained this to my daughter when she was probably in jr. high--to make her aware of it so she can override it and do the right thing. even when it comes to calling 911 for a car that is stranded on the side of the road. (maybe they don't have a phone to call, maybe their phone is dead, etc., and don't think anyone else will make the call for them because they all might be thinking the same thing)
"The bystander effect occurs when the presence of others hinders an individual from intervening in an emergency situation. Social psychologists Bibb Latané and John Darley popularized the concept following the infamous 1964 Kitty Genovese murder in Kew Gardens, New York. Genovese was stabbed to death outside her apartment three times, while bystanders who reportedly observed the crime did not step in to assist or call the police. Latane and Darley attributed the bystander effect to the diffusion of responsibility (onlookers are more likely to intervene if there are few or no other witnesses) and social influence (individuals in a group monitor the behavior of those around them to determine how to act). In Genovese's case, each onlooker concluded from their neighbors' inaction that their own help was not needed."
http://www.psychologytoday.com/basics/bystander-effect