Average Product vs Law of Diminishing Marginal Returns
Average Product and Law of Diminishing Marginal Returns are two Production & Costs concepts in AP Economics that students often mix up. Average Product is the total output produced per unit of a variable input, typically labor. The law of diminishing marginal returns states that adding more of a variable input to fixed inputs eventually yields smaller increases in output. Here is how they compare side by side.
It is found by dividing total product by the quantity of the variable input used. Average product rises when marginal product is above it and falls when marginal product is below it.
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.
Average Product vs the Law of Diminishing Marginal Returns: A Ratio and a Turning Point
| Average Product | Law of Diminishing Marginal Returns | |
|---|---|---|
| What it is | A number you calculate at each level of the input | A claim about how the marginal product column behaves |
| Which curve it describes | The average product curve | The marginal product curve, specifically where it peaks |
| Needs a fixed input | No, it can be computed for any input combination | Yes, it only holds when at least one input is held fixed |
| Where its turning point sits | Later, where marginal product crosses it from above | Earlier, at the worker after the maximum of marginal product |
| What a decline means | Each worker on average is producing less than before | Each extra worker is adding less than the previous worker added |
| Cost-curve counterpart | Its peak is the minimum of average variable cost | Its onset is where marginal cost starts to rise |
Diminishing returns does not mean output is falling
The word returns refers to the increment, not the total. In the schedule above, total output rises at every one of the six workers, from 9 all the way to 78, and it keeps rising after diminishing returns has set in at the fourth. What shrinks is the size of each addition. Adding a seventh worker who brings total output down to 77 would be a different situation entirely, called negative marginal returns, and it marks the point where an extra worker is actively in the way. Average product at that seventh worker is 11, still comfortably positive, which shows that the three quantities move independently: total output down, marginal product below zero, average product positive and falling. A rational firm never hires into negative marginal returns, since it would pay a wage for output it loses. It will often hire well past the onset of diminishing returns, because the fourth, fifth and sixth workers each still add output worth having. Diminishing returns is a statement about the shape of a curve, not a signal to stop hiring, and the hiring decision comes from comparing the value of what a worker adds against the wage.
Reading the exam wording: which worker, which peak
Multiple-choice questions phrase this in a handful of predictable ways, and one production table answers all of them differently. Diminishing marginal returns begin at the first worker whose marginal product is lower than the previous worker's, so find the peak of the marginal product column and take the next row. Output per worker peaks where marginal product falls below average product, which happens later. Total product peaks where marginal product reaches zero, later still. Three answers, one table, and the wording is the only thing separating them. One further distinction is worth holding onto. Diminishing marginal returns is a short-run idea that requires a fixed input, while diseconomies of scale describes long-run average cost rising when every input, plant included, has been scaled up together. A firm that doubles both its workforce and its factory space is not experiencing diminishing marginal returns at all, because nothing was held fixed. If a question describes a fixed plant and a variable workforce, it is testing diminishing returns. If it describes building a bigger facility, it is testing returns to scale. Work the underlying arithmetic at /calculate/marginal-product.
Frequently asked questions
Does the law of diminishing marginal returns mean average product falls?
The law of diminishing marginal returns describes marginal product, not average product, so average product can keep rising well after diminishing returns has begun. Average product rises as long as the newest worker adds more than the current average, even when that addition is smaller than the previous worker's addition. Average product only turns down at the later point where marginal product crosses below it.
At which worker do diminishing marginal returns begin?
Diminishing marginal returns begin at the first worker whose marginal product is smaller than the previous worker's marginal product. With marginal products of 9, 15, 18, 16, 12 and 8, the peak is the third worker, so diminishing returns begin with the fourth. Locating the maximum of average product instead gives the wrong worker, because average product peaks one hire later.
Can total output rise while marginal returns are diminishing?
Total output rises throughout the range where marginal returns are diminishing, as long as marginal product stays positive. Each extra worker adds less than the one before, and adding less is still adding. Total output only falls once marginal product turns negative, which is a separate stage called negative marginal returns and one no profit-seeking firm has a reason to enter.
Live Production Costs graph. Drag the curves, or open the full version.
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