The Law of Diminishing Marginal Utility

The Law of Diminishing Marginal Utility (DMU) is the most fundamental law in the whole of Utility Analysis, and it is the psychological foundation on which the Law of Demand itself rests. It was first stated in a mathematical form by the German economist H.H. Gossen in 1854 (which is why it is also called Gossen's First Law), but it was Alfred Marshall who developed and popularised it in the form in which it is taught today.

Statement of the Law. "As a consumer goes on consuming more and more units of a commodity, one after another, in a given period of time, the marginal utility derived from each successive unit goes on diminishing, other things (income, tastes, price) remaining constant." In simple words: the more of a good a person already has, the less additional satisfaction one more unit of it gives.

Assumptions of the Law. The Maharashtra HSC syllabus lists these assumptions, each of which must hold for the law to operate as stated:

1. Cardinal measurability of utility — utility can be expressed in numerical units (utils).
2. Rationality — the consumer behaves rationally, aiming to maximise satisfaction.
3. Homogeneity of units — all units of the good consumed are identical in size, quality and other characteristics.
4. Continuity of consumption — units are consumed one after another without a significant time gap between them.
5. Constancy of other factors — the consumer's income, tastes and preferences, and the price of the good (and of related goods) remain unchanged during the period.
6. No change in fashion or habit during the period of consumption.
7. Utility is measured in a single, consistent unit (the util) throughout.

Schedule illustrating the Law. Suppose a consumer, over a single sitting, consumes successive cups of tea and reports the following Total Utility (TU) after each cup:

Reading the table: MU is obtained by subtracting the previous TU from the current TU ($MU_n = TU_n - TU_{n-1}$). MU falls steadily from 10 to 8 to 6 to 4 — this steady fall, unit after unit, is exactly what the Law of DMU describes. At the 5th cup, MU becomes zero — TU has reached its maximum (28 utils) and the consumer has reached the point of satiety. At the 6th cup, MU turns negative (−2) — TU actually falls to 26, because a sixth cup, consumed after full satisfaction, causes mild discomfort (disutility) rather than added satisfaction.

<!-- FIGURE-NEEDED: Two-panel diagram (stacked vertically, sharing the same X-axis "Units Consumed" from 1 to 6) illustrating the Law of Diminishing Marginal Utility using the tea-cup schedule above. Top panel: the Total Utility (TU) curve — plot points (1,10),(2,18),(3,24),(4,28),(5,28),(6,26); the curve rises at a decreasing (diminishing) rate, reaches its maximum/flattens between units 4 and 5, then bends downward after unit 5. Bottom panel: the Marginal Utility (MU) curve — plot points (1,10),(2,8),(3,6),(4,4),(5,0),(6,-2); a straight or gently curved downward-sloping line starting at 10, crossing the zero line exactly at unit 5 (directly below where TU peaks in the top panel), and continuing into negative territory at unit 6. Label the X-axis "Units of the good consumed," the Y-axis of the top panel "Total Utility (utils)," and the Y-axis of the bottom panel "Marginal Utility (utils)," with a horizontal zero line marked on the MU panel. -->

Exceptions to the Law. A few situations are commonly cited, in the Maharashtra Std XII syllabus, as apparent exceptions where MU does not diminish in the usual way:

1. Collection of rare articles — a collector's marginal utility from an additional rare coin or stamp may actually increase as the collection nears completion.
2. Miser and money — a miser's marginal utility from an additional rupee may not diminish, and can even rise, because the motive is hoarding, not consumption for want-satisfaction.
3. Alcohol and addictive goods — for an addict, the marginal utility of successive drinks/doses may rise rather than fall, at least up to a point, because craving intensifies with use. Note: this is a genuine, well-documented exception, not a contradiction reversing the law generally.
4. Music, poetry, dance appreciation — a connoisseur's marginal utility from listening to a raga or reading a poem may initially rise with repetition, as understanding and appreciation deepen, before eventually diminishing.
5. Applying the law across different goods — since the law strictly applies to successive units of the same good consumed without a time gap, it does not describe what happens when a consumer switches between different goods (that is instead the concern of the Law of Equi-Marginal Utility, covered next). …