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M03.L02 — The Law of Diminishing Marginal Returns

Module: Module 03 — Production and Costs
Lesson: L02 of 05
Duration: ~30 minutes
Level: Introductory
Provenance: Principles of Microeconomics (Curtis and Irvine) | MIT OCW 14.01 Principles of Microeconomics

Learning Objective

Explain why marginal product eventually declines in the short run.

Diminishing Marginal Returns

In Australian mining operations, the law states that adding more of a variable input (like labour) to fixed inputs (like equipment) will eventually yield smaller increases in output. This occurs because: 1. Initially, workers can specialize (MP increases) 2. Eventually, workers share fixed equipment (MP decreases but positive) 3. Finally, overcrowding occurs (MP becomes negative)

Production has three stages: - Stage I: MP > AP (AP increasing) - Stage II: 0 < MP < AP (AP decreasing) - Stage III: MP < 0 (TP decreasing)

Rational firms operate in Stage II where MP is positive but diminishing.

Worked Example

Australian Iron Ore Mine

A Pilbara mine with fixed equipment: 1. 0-3 workers: MP increases (specialization) 2. 4-6 workers: MP positive but decreasing (equipment sharing) 3. 7+ workers: MP negative (overcrowding at crushers)

At 5 workers, TP=100 tonnes, MP=10. Adding a 6th worker gives TP=107, MP=7 (diminishing returns).

Common Misconception

"Diminishing returns mean the business is failing."

Actually, it's a natural consequence of fixed inputs. Even successful Australian mines experience diminishing returns when adding too many workers to limited equipment.

Key Takeaways

  • Applies in short run with fixed inputs
  • MP eventually falls due to fixed factors
  • Rational production occurs in Stage II
  • Different from diseconomies of scale (long-run concept)

Practice

  1. Why does MP initially increase before diminishing?
  2. At what stage should a farm operate? Why?
  3. How might an Australian vineyard experience diminishing returns?

Further Resources

Provenance: Principles of Microeconomics (Curtis and Irvine) | MIT OCW 14.01 Principles of Microeconomics