This is often caused by social loafing, a theory that says individual members of a group will actually put in less effort, because they believe other members will make up for the slack.
Awen iddilil net social loafing , ta-ite t-allaɣet as addinat n taqqimt a taj in assahat andarrat fal as ordahan addinat iyyad asidu i anmichray