Diffusion of responsibility is a term used in the social sciences to describe phenomena in which none of the members of a large group take a particular action or take responsibility for anything that occurs. The diffusion of responsibility phenomenon can take many different forms. It occurs, for instance, when a large group of people watches a crime occur but does nothing to prevent it or to get help. In a different situation, underlings who commit an illegal act may claim to have just been following orders while those in charge defend themselves by saying that they only issued the orders but did not act. In both of these cases, no one person or group of people actually takes responsibility or action, and the group effectively "absorbs" it.
There are several different sociological phenomena which fall into the category of diffusion of responsibility. One example, groupthink, occurs in highly-cohesive groups of people who work very closely with each other on a regular basis without much variance in the composition of the group. It is commonly observed that, in the interest of reaching a unanimous decision, members of such groups often fail to discuss possible problems or alternatives. Another phenomenon, the bystander effect — or Genovese syndrome — occurs when individuals do not offer aid in emergency situations when they know that other people are present. Social psychologists believe that individuals look to other people to determine how to act in such situations, so they do nothing when they observe that everyone else is doing nothing.
It is important to note that diffusion of responsibility only applies to very large groups. A group of three or four people is much more likely to react to witnessing a crime than a group of three or four hundred people. Individuals in a smaller group know that everyone has the same perspective on the event so they cannot convince themselves that they aren't simply misinterpreting the situation. Additionally, people in smaller groups can usually talk about how to handle a situation, while there are too many people in large groups for any discussion to be useful.
Many different factors can prevent diffusion of philosophy. If a single member of the group takes charge and acts on a situation, the diffusion of responsibility tends to end. Diffusion of responsibility is also less likely to occur when the situation can actually affect one or more of the members of the group. People are much more likely to act when they have a personal stake in what happened.