The Marginal Productivity Theory of Distribution (MPTD) claims that in a free-market economy the demand for a factor of production will depend upon its marginal product - where "marginal product" is defined as the change in total product that is caused by, or that follows, the addition or subtraction of the marginal unit of the factor used in the production process, with all other inputs held constant. From its inception in the early nineteenth century the MPTD has been claimed by some economists to be a solution to the ethical problem of distributive justice, i.e. to be a means of...