For a firm that hires factors in a perfectly competitive market, factor demand curve is essentially the marginal revenue product curve. Monopsony and other imperfectly competitive firms with some degree of market control over the factor market do not have a clear-cut factor demand curve. In other words, factor price and quantity demanded may or may not be inversely related.

Factor Demand Curve

The exhibit to the right displays a typical factor demand curve. This particular curve is the demand for labor by Waldo's TexMex Taco World, a hypothetical Shady Valley restaurant specializing in the production of Super Deluxe TexMex Gargantuan Tacos (with sour cream and jalapeno peppers).

The number of workers is measured on the horizontal axis and the wage paid per worker is measured on the vertical axis. This factor demand curve indicates that Waldo is willing to hire 4 workers at $35 each, 5 workers at $25 each, and 6 workers at $15 each. The wage declines with an increase in the number of workers employed, because extra workers contribute less and less to total production and to total revenue.

The most important feature of this factor demand curve is the negative slope. Waldo's TexMex Taco World is willing and able to pay workers a higher wage if they employ fewer workers. However, if they employ more workers, then they are willing and able to pay a lower wage. The primary reason for this inverse relation between wage and quantity demanded is the productivity of the workers and the law of diminishing marginal returns.

As Waldo's TexMex Taco World employs more workers IN A GIVEN RESTAURANT, marginal product declines, each additional worker contributes less to total product as the law of diminishing marginal returns kicks in. However, with less production to sell, Waldo's TexMex Taco World generates less revenue. In other words, the law of diminishing marginal returns causes a decline in marginal physical product, and consequently a decline in marginal revenue product. But if workers generate less revenue, then Waldo's TexMex Taco World is willing and able to pay a lower wage.