How does the law of diminishing returns affect a firm's cost of production?

In economics, diminishing returns refers to production in the short run (also called diminishing marginal returns) is the decrease in the marginal output of a production process as the amount of a single factor of production is increased, while the amounts of all other factors of production stay constant. The law of diminishing returns states that in all productive processes, adding more of one factor of production, while holding all others constant ("ceteris paribus"), will at some point yield lower per-unit returns . The law of diminishing returns does not imply that adding more of a factor will decrease the total production, a condition known as negative returns, though in fact this is common.

The short run is a time period where at least one factor of production is in fixed supply. We normally assume that the quantity of plant and machinery is fixed and that production can be altered through changing variable inputs such as labour, raw materials and energy.

In the short run, the law of diminishing returns states that as we add more units of a variable input to fixed amounts of land and capital, the change in total output will at first rise and then fall. Diminishing returns to labour occurs when marginal product of labour starts to fall. This means that total output will be increasing at a decreasing rate.

The marginal cost of supplying an extra unit of output is linked with the marginal productivity of labour. The law of diminishing returns implies that marginal cost will rise as output increases. Eventually, rising marginal cost will lead to a rise in average total cost.