in economics, a theory developed at the end of the 19th century by a number of writers, including John Bates Clark and Philip Henry Wicksteed, who argued that a business firm would be willing to pay a productive agent only what he adds to the firm's well-being or utility; that it is clearly unprofitable to buy, for example, a man-hour of labour if it adds less to its buyer's income than what it costs. This marginal yield of a productive input came to be called the value of its marginal product, and the resulting theory of distribution states that every type of input will be paid the value of its marginal product. (See distribution theory.)