Diminishing returns to an Input (diminishing marginal product) a diminishing returns to an input an increase in the quantity of an input leads to a decline in the marginal product of that input, holding the levels of all other inputs fixed a Other things held constant, as more of a variable input is used in production, its marginal productivity will decline after a certain point.Diminishing Returns to an Input (diminishing marginal product) ◼ diminishing returns to an input: an increase in the quantity of an input leads to a decline in the marginal product of that input, holding the levels of all other inputs fixed ◼ Other things held constant, as more of a variable input is used in production, its marginal productivity will decline after a certain point