Diminishing Marginal Returns

Adding early workers to a workshop can increase marginal product through specialization. One prepares materials while another operates equipment. Eventually, however, the fixed space and machines become crowded.

The law of diminishing marginal returns says that when successive units of a variable input are added to fixed inputs, marginal product eventually falls, holding technology and input quality constant.

Diminishing does not mean negative

If marginal product falls from 30 to 24, the new worker still adds 24 units. Total product rises, just more slowly. Total product falls only when marginal product becomes negative.

Three stages help read the graph:

1. MP may initially rise as specialization improves coordination.
2. MP eventually falls but remains positive; TP rises at a decreasing rate.
3. If crowding becomes severe, MP can turn negative and TP falls.

This is a short-run principle because at least one input is fixed. Expanding the kitchen or changing technology shifts the production function; it does not describe another movement along the old function.

From production to cost

If each worker receives the same wage $w$, the labor cost of extra output depends inversely on marginal product:

When MP rises, each additional unit is cheaper to produce. Once diminishing returns reduce MP, marginal cost rises. This link explains the usual upward-sloping part of a firm's short-run marginal-cost curve.

Locate the turning point correctly

With two fixed stoves, cooks add 22, 28, 32, 28, 22, 14, and 6 bowls per hour. Diminishing marginal returns begin with the fourth cook, when MP first falls from 32 to 28. They do not begin when total output falls; total output keeps rising because every listed MP remains positive.

If an eighth cook had MP of $-3$, total output would finally decline. Separating “smaller,” “zero,” and “negative” prevents a common graph-reading mistake.

Change capacity or move along it?

Adding another cook moves along the current short-run production function. Installing a third stove changes a fixed input and shifts the function. Later cooks may become more productive after the shift, but with enough labor the larger kitchen can eventually become crowded too.

This distinction connects production to cost: with a fixed wage, falling MP means each additional unit requires more labor cost, causing MC to rise.