Definition: Diffusion of responsibility occurs when people are in groups. The more people that are present, the less accountable any of those people feel for their actions. Simply put, the responsibility is diffused - or spread - over the group.
Due to diffusion of responsibility, people of all ages will often act differently in a group situation than they would on their own. For example, they might be less likely to step in to help a sick or injured person than they would if they were alone, a phenomenon called the bystander effect. The classic example of the bystander effect is the case of Kitty Genovese, in which a murder was witnessed by many people yet no one intervened.
As a tween's social life ramps up, he tends to spend more time in groups and to more frequently experience diffusion of responsibility. For instance, diffusion of responsibility may affect a tween's schoolwork, with the tween doing less work on a group project or academic task. This phenomenon is called social loafing.
All in all, the tween may feel less responsible for his actions when in a group and is more likely to engage in uncharacteristic behavior.
Related concepts: Bystander effect, social loafing
Rathus, Spencer. Psychology: Concepts and Connections, Brief Version, 8th Edition. 2006. Belmont, CA: Wadsworth Publishing.