Add one more worker to a small kitchen and things get done faster. Add a fifth, a tenth, a twentieth, and the kitchen becomes a crowd. Output may still rise, but each new worker adds less than the one before. This is diminishing returns, one of the oldest ideas in economics. It dates back to David Ricardo and his analysis of farmland in the early 1800s.

The principle applies whenever at least one input is fixed. A farmer with a fixed plot of land can add fertilizer and see yields jump. Keep adding fertilizer and the gains shrink, then stop altogether, then turn negative as chemicals burn the soil. A factory with fixed machinery can hire more workers, but eventually they get in each other's way.

Diminishing returns matter for growth theory. It helps explain why poor countries can grow quickly by adopting existing technology, while rich countries must innovate to keep growing. It also warns against simple solutions. Throwing more money or more people at a problem works only until it does not.

Key points

• At least one input must be fixed
• Marginal output falls as variable inputs increase
• Eventually marginal output can turn negative
• The principle applies to land, labor, and capital
• It underpins the case for innovation-led growth