British economist A.C. Pigou was instrumental in developing the theory of externalities. The theory examines cases where some of the costs or benefits of activities "spill over" onto third parties. When it is a cost that is imposed on third parties, it is called a negative externality. When third parties benefit from an activity in which they are not directly involved, the benefit is called a positive externality. The study of such situations, a part of welfare economics, has been an active area of research since Pigou's efforts early in the twentieth century.