Social loafing

In social psychology, social loafing is the phenomenon of people exerting less effort to achieve a goal when they work in a group than when they work alone. A researcher in the 1970s blindfolded participants and told them they were going to play tug-of-war against another team. When they were told they had 3 others pulling with them, they pulled 18% less strenuously than when they were told they were alone.