AS & A-Level Economics — Consumer Choice, Efficiency and Externalities

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Original Deckloop economics study material on consumer choice, efficiency and externalities, with 100 practice cards and 20 concept explainers. Includes worked applications and analytical reasoning.

Economics EN A-Level

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Diminishing Marginal Utility

The Law of Diminishing Marginal Utility is a fundamental principle in consumer behaviour stating that as a consumer consumes more and more units of a specific good, the additional satisfaction (marginal utility) derived from each successive unit tends to decrease. This reflects the idea of satiation, where the urgency or desire for further units of a good lessens after initial consumption. While total utility may still be increasing, it does so at a progressively slower rate once marginal utility begins to diminish. This law is crucial for understanding the downward slope of the demand curve. However, it relies on the assumption of cardinal utility, implying utility can be objectively measured and compared, which is a significant limitation. Real-world consumer decisions are also influenced by changing preferences, imperfect information, habits, emotions, and bounded rationality, making precise cardinal measurement challenging. The ordinal utility approach, using indifference curves, offers an alternative by only requiring consumers to rank preferences.

Key points

As consumption of a good increases, its marginal utility eventually falls.
Total utility continues to rise as long as marginal utility is positive, but at a decreasing rate.
The law reflects the concept of satiation or reduced urgency for additional units.
It helps explain why consumers are willing to pay less for additional units of a good.
The concept of utility as objectively measurable (cardinal utility) is a simplifying assumption, as satisfaction is subjective and not directly quantifiable.

Worked example

Question

A baker's total utility from eating croissants is as follows: 1st croissant = 15 units, 2nd = 25 units, 3rd = 32 units, 4th = 37 units, 5th = 40 units. Demonstrate the Law of Diminishing Marginal Utility using this data.

Solution

1. Calculate marginal utility for each additional croissant:
2. MU of 1st = 15 units
3. MU of 2nd = 25 - 15 = 10 units
4. MU of 3rd = 32 - 25 = 7 units
5. MU of 4th = 37 - 32 = 5 units
6. MU of 5th = 40 - 37 = 3 units
7. Observe the trend in marginal utility.

The marginal utility decreases from 15 units (1st) to 10 units (2nd), then to 7 units (3rd), 5 units (4th), and finally to 3 units (5th). This consistent decline in additional satisfaction with each successive croissant demonstrates the Law of Diminishing Marginal Utility.

Common pitfalls

Confusing diminishing marginal utility with negative marginal utility: Diminishing MU means the additional satisfaction is getting smaller but is still positive. Negative MU means consuming more causes dissatisfaction. Total utility still rises when MU is diminishing but positive.
Assuming total utility must fall: Total utility only falls if marginal utility becomes negative. As long as marginal utility is positive, total utility will continue to increase, albeit at a slower rate.
Treating utility as an objectively measurable quantity: While useful for modelling, utility is subjective and cannot be precisely quantified or compared across individuals, limiting the real-world applicability of cardinal utility theory.

Prerequisites

Deck 09 — Total Utility and Marginal Utility: Understanding the definition and calculation of marginal utility is essential to grasp how it changes (diminishes) with increased consumption, as demonstrated in the worked example and cards.