Explain why the marginal cost of production must increase if the marginal product of the variable resource is decreasing. A decrease in the marginal productivity of a variable resource means that the additional output from each additional unit of the variable resource is getting smaller. Because each additional unit of the variable resource adds the same amount to the total cost of production (i.e., this added cost equals the unit price of the variable resource), the marginal cost of each additional unit of output must be increasing.