A principle stating that as more and more of a variable input is combined with a fixed input in short-run production, the marginal product of the variable input eventually declines. This is THE economic principle underlying the analysis of short-run production for a firm. Among a host of other things, it offers an explanation for the upward-sloping market supply curve. How does the law of diminishing marginal returns help us understand supply? The law of supply and the upward-sloping supply curve indicate that a firm needs to receive higher prices to produce and sell larger quantities. Why do they need higher prices?