Law of Diminishing Marginal Returns vs Diseconomies of Scale
Law of Diminishing Marginal Returns and Diseconomies of Scale are two Production & Costs concepts in AP Economics that students often mix up. The law of diminishing marginal returns states that adding more of a variable input to fixed inputs eventually yields smaller increases in output. Diseconomies of scale occur when long-run average total cost increases as output increases. Here is how they compare side by side.
As a firm adds workers to a fixed amount of capital, marginal product may rise at first but eventually falls. This causes marginal cost to rise, shaping the upward-sloping part of the cost curves. It applies only in the short run, when at least one input is fixed.
This results from coordination problems, communication breakdowns, or bureaucracy as a firm becomes too large. It causes per-unit costs to rise, reducing efficiency and profitability at higher output levels.
Diminishing Marginal Returns vs Diseconomies of Scale: Different Time Frames, Different Curves
| Law of Diminishing Marginal Returns | Diseconomies of Scale | |
|---|---|---|
| Time frame | Short run, with at least one input fixed | Long run, with every input variable |
| What is changing | One variable input added to an unchanged plant | All inputs raised together in the same proportion |
| What it measures | Falling marginal product of the added input | Rising long run average total cost as the firm expands |
| Curve affected | Turns short run MC and AVC upward | Turns the long run ATC curve upward on its right side |
| Usual cause | Crowding of the variable input onto a fixed input | Coordination, monitoring and communication problems in a larger organization |
| Can the firm escape it | Yes, by building a bigger plant once it has time | No, since the plant is already free to change |
| What it directly claims | A statement about physical output, not about cost | A statement about cost per unit at larger scale |
Diminishing returns happen inside a plant that cannot change
Hold capital fixed at one bakery with one oven and start adding bakers. Total output runs 10, 24, 36, 44 and 48 loaves for one through five bakers. The marginal product of each baker is therefore 10, 14, 12, 8 and 4 loaves. The second baker raises output more than the first did, because two people can split mixing and baking instead of one person doing both. From the third baker onward each new hire adds less than the hire before, which is the point where diminishing marginal returns begin. Nothing here has gone wrong and nobody has become lazy. The oven simply has not grown, so each extra pair of hands has less equipment to work with. Two details students routinely miss. First, diminishing marginal returns start while total output is still rising, so a falling marginal product is entirely compatible with more loaves coming out. Second, the law says nothing about money until you attach an input price. Multiply through by a wage and it becomes the rising portion of marginal cost. See /glossary/marginal-product for that translation. These figures are illustrative.
Diseconomies of scale need every input to change at once
Now let the bakery change everything. Suppose one configuration uses 10 workers and 2 ovens to make 100 loaves, with workers at 50 dollars and ovens at 200 dollars for the period. That costs 500 plus 400, or 900 dollars, so average total cost is 9 dollars a loaf. Double both inputs to 20 workers and 4 ovens and suppose output rises only to 190 loaves. The bill is now 1,000 plus 800, or 1,800 dollars, and average total cost is 1,800 divided by 190, which is about 9.47 dollars a loaf. Cost per loaf went up as the firm got bigger, which is diseconomies of scale. Notice what made this different from the bakery above: no input was held fixed, so crowding around a single oven cannot be the explanation. The usual explanations are managerial. A bigger operation needs more layers of supervision, information travels through those layers slowly and gets distorted on the way, and decisions that one owner used to make alone now require meetings. On a diagram this shows up only on the right hand side of the long run average total cost curve, and it never appears on a short run curve at all. See /glossary/economies-of-scale for the falling stretch that normally comes first.
Frequently asked questions
What is the difference between diminishing marginal returns and diseconomies of scale?
Diminishing marginal returns is a short run idea about adding more of one input to a fixed plant and getting smaller output gains, while diseconomies of scale is a long run idea about cost per unit rising when every input expands together. One is about output from one input and the other is about cost at a larger overall size.
Can a firm face diminishing marginal returns and economies of scale at the same time?
Yes, because the two describe different time frames and are perfectly compatible. Inside today's plant the next worker can add less output than the last while a larger plant, once built, still produces at a lower cost per unit.
Which one explains the upward sloping part of short run marginal cost?
Diminishing marginal returns does, because as each extra worker adds less output the cost of squeezing out one more unit rises. Diseconomies of scale belong to the long run curve and have no effect on the shape of a short run marginal cost curve.
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