The tendency for the presence of other people to increase performance on easy tasks and impair performance on difficult tasks is known as and is when a person exerts less effort they work in a group than when working alone.

Social facilitation; social loafing                                                                                  

Explanation:

Social facilitation: In social psychology, the term "social facilitation" is described as a phenomenon that states that different individuals tend to perform well on a particular task when they are being surrounded with other peoples. It was first discovered by "Norman Triplett" during 1898 and labeled as social facilitation by a psychologist named Floyd Allport.

Social loafing: In social psychology, the term "social loafing" is also referred as "ringelmann effect" as it was first proposed by Max Ringelmann during 1913, and is described as a phenomenon in which an individual tends to exert less effort on a specific task if he or she is in a group as compared to the scenario in which he or she works alone.

In the question above, the given statement signifies the concept of social facilitation and social loafing.