The fourth worker on a job helps less than the first, and the tenth may barely help at all

Add more of one input, like workers or fertiliser, while holding everything else fixed, and each extra unit eventually produces a smaller gain than the one before it. Economists call this the law of diminishing returns; it's why cramming a kitchen with cooks doesn't speed dinner up proportionally, and why farmers can't fix a small field by just dumping on more fertiliser.

— David Ricardo; later formalised by neoclassical economists, Principles of Political Economy — 19th-century agricultural economics, generalised since