The marginal revenue productivity theory of wages states that wages are paid in accordance to the marginal revenue product of labor. As long as the incremental revenue of an additional worker is greater or equal to the cost of the additional worker a firm will hire the additional worker. However, no firm will higher an additional worker if the incremental revenue is less than the cost of said worker.
More information:
https://courses.byui.edu/econ_150/econ_150_old_site/lesson_10.htm
http://staffwww.fullcoll.edu/fchan/Micro/5demand_of_a_factor.htm
