When a large group is asked to complete a task, individual members often neglect their responsibilities, because it is impossible for any one person to receive credit or blame. this phenomenon is called?

This is social loafing. People feel like there are others who will be willing to pick up the slack and make sure that the task gets completed, else everyone will be held responsible. This leads to a number of people failing to do what is required of them, which could jeopardize the entire task if too few people do cover the lost productivity.