The correct answer is social loafing
The size of a group affects its behavior. Minors are more agile in completing tasks, but a larger group, oriented to solve a certain problem, is more successful in relation to minors. One of the discoveries related to the size of a group is a phenomenon called social loitering, that is, a tendency to work less when you are in a group than when you are working individually.
Social loafing can be explained by the belief that the other members are not meeting their goals, by the dispersion of responsibility, since the results are not measured individually, but in a group. In this situation, members tend to be opportunistic. In short, there may be a drop in the efficiency of groups when they feel that individual contribution cannot be measured.