Theory of Marginal Productivity, vulgar bourgeois economic concept which claims that the source of value is the productivity of the "production factors" (labour, capital and land). It appeared in the first half of the 19th century, and was most conclusively elaborated by the American economic! J. B. Clark (late 19th century). The concept is based on the theory of production factors. According to Clark, each production factor is involved in the process of production and is therefore productive. By contrast, Marxist political economy says that productivity is the attribute of concrete labour which produces use value. In Clark’s view, each factor of production participates in creating a product’s value to the extent of its marginal productivity, i. e., the amount of the "marginal product" it creates. The "marginal product" is the increase in output resulting from increasing this production factor by one unit, with all other factors being unchanged. According to this theory, the "marginal product" determines the “fair” incomes paid to each of the factors. Thus, the "marginal product" of capital is interest. The workers’ wages are determined by the "marginal product of labour”. According to this concept, an increase in the number of people working at an enterprise tends to reduce the productivity of labour of each newly employed worker, given the unchanged amount of capital and same technical level. The entrepreneur stops employing workers when a worker is unable to produce the amount of commodities needed to provide for his existence. The productivity of this particular worker is "marginal productivity”, and the marginal product he produces is “natural”, or “fair”, payment for his work. Thus, the amount of one’s wages is made dependent on productivity and employment levels. The more workers who are employed, the lower the productivity and the lower the wages. According to this reasoning, unemployment is caused by workers’ demanding wages which exceed "marginal product”. Thus, wages are taken out of the context of social and class relationships and are divorced from capitalist relations of production, those of exploitation of labour by capital. They are presented as the "natural price of labour”, as a non-historical category. The theory of marginal productivity is widely employed by reformist ideologists to justify their concepts of wages under capitalism.