Law of Diminishing Marginal Utility states that as a person consumes additional units of a good or service, the satisfaction (utility) derived from each successive unit decreases, assuming all other factors remain constant. Initially, the first few units provide significant satisfaction, but as consumption increases, the utility of each extra unit diminishes. For example, the first slice of pizza may bring great joy, but by the fifth or sixth slice, the additional satisfaction reduces. This principle underlies consumer behavior and helps explain demand curves, as consumers are less willing to pay the same price for additional units of a product.
Definition
According to the law, other things remaining constant, the marginal utility derived from successive units of a commodity decreases as consumption increases.
The law explains the relationship between total utility (TU) and marginal utility (MU) and helps understand how consumers allocate their limited income among different goods.
Assumptions of the Law of Diminishing Marginal Utility
1. Rational Consumer
The law assumes that the consumer is rational and seeks to maximize satisfaction from available resources. The consumer makes decisions according to needs, preferences, and expected utility. Rational behaviour ensures that consumption decisions are made logically and that each additional unit is evaluated according to its satisfaction. If the consumer behaves irrationally, the relationship between quantity consumed and marginal utility may not follow the normal pattern of diminishing additional satisfaction.
2. Homogeneous Units
The law assumes that all units of the commodity are homogeneous, meaning they are identical in quality, size, shape, and characteristics. For example, successive units of a commodity should provide a comparable basis for measuring satisfaction. If the quality or quantity of different units varies considerably, changes in utility may result from differences between units rather than increased consumption. Therefore, homogeneous units are necessary for accurately observing diminishing marginal utility.
3. Continuous Consumption
The law assumes continuous consumption of successive units without significant time intervals. This means the consumer consumes the commodity within a reasonably short period. Continuous consumption ensures that the consumer’s desire for the commodity does not recover between successive units. If long intervals occur, the consumer may become interested in the commodity again, increasing its marginal utility. Therefore, continuous consumption helps establish the normal relationship between increasing consumption and decreasing additional satisfaction.
4. Constant Taste and Preferences
The law assumes that the consumer’s tastes, preferences, habits, and desires remain constant during consumption. Changes in preferences can influence the satisfaction obtained from successive units independently of the quantity consumed. For example, a sudden increase in preference for a particular product may increase its marginal utility. Therefore, keeping consumer preferences constant allows economists to observe the actual effect of increasing consumption on marginal utility without interference from changing tastes.
5. Constant Income
The law assumes that the consumer’s income and purchasing power remain constant during the consumption process. A change in income may affect purchasing capacity, consumption patterns, and the choice of commodities. If income increases, consumers may alter their consumption or shift toward different goods. By keeping income constant, the law focuses specifically on the relationship between quantity consumed and marginal utility. Thus, stable income is an important condition for applying the law effectively.
6. Reasonable Size of Units
The law assumes that each unit consumed has a reasonable and comparable size. Units should not be excessively large or extremely small because their size directly affects the satisfaction received. For example, equal-sized glasses of water provide a suitable basis for comparing successive utility. If units differ significantly in size, the resulting changes in satisfaction may not represent diminishing utility. Therefore, uniform and appropriate units are necessary for a meaningful application of the law.
7. Independent Utility
The law assumes that the utility derived from the commodity is relatively independent of other commodities being consumed. The satisfaction obtained from successive units should not be substantially influenced by complementary or substitute goods. For example, consuming another commodity along with the product may change its perceived satisfaction. By assuming independent utility, economists can study the effect of increasing consumption of a single commodity without significant interference from other consumption decisions.
8. Normal Mental and Physical Condition
The law assumes that the consumer remains in a normal mental and physical condition throughout consumption. Factors such as illness, extreme thirst, fatigue, excitement, or emotional changes can affect the satisfaction received from successive units. For example, a thirsty person may initially obtain exceptionally high utility from water. As physical conditions change, utility may also change. Maintaining normal and stable conditions helps demonstrate the general tendency of declining marginal utility with increased consumption.
Explanation with Schedule and Diagram:
We assume that a man is very thirsty. He takes the glasses of water successively. The marginal utility of the successive glasses of water decreases, ultimately, he reaches the point of satiety. After this point the marginal utility becomes negative, if he is forced further to take a glass of water. The behavior of the consumer is indicated in the following schedule:
| Units of commodity |
Marginal utility |
Total utility |
| 1st glass |
10 |
10 |
| 2nd glass |
8 |
18 |
| 3rd glass |
6 |
24 |
| 4th glass |
4 |
28 |
| 5th glass |
2 |
30 |
| 6th glass |
0 |
30 |
| 7th glass |
-2 |
28 |
On taking the 1st glass of water, the consumer gets 10 units of utility, because he is very thirsty. When he takes 2nd glass of water, his marginal utility goes down to 8 units because his thirst has been partly satisfied. This process continues until the marginal utility drops down to zero which is the saturation point. By taking the seventh glass of water, the marginal utility becomes negative because the thirst of the consumer has already been fully satisfied.
The law of diminishing marginal utility can be explained by the following diagram drawn with the help of above schedule:
In the above figure, the marginal utility of different glasses of water is measured on the y-axis and the units (glasses of water) on X-axis. With the help of the schedule, the points A, B, C, D, E, F and G are derived by the different combinations of units of the commodity (glasses of water) and the marginal utility gained by different units of commodity. By joining these points, we get the marginal utility curve. The marginal utility curve has the downward negative slope. It intersects the X-axis at the point of 6th unit of the commodity. At this point “F” the marginal utility becomes zero. When the MU curve goes beyond this point, the MU becomes negative. So there is an inverse functional relationship between the units of a commodity and the marginal utility of that commodity.
Importance of the Law of Diminishing Marginal Utility
1. Explains Consumer Behaviour
The Law of Diminishing Marginal Utility helps explain how consumers behave when consuming successive units of a commodity. As consumption increases, the additional satisfaction obtained from each extra unit generally decreases. This explains why consumers may not continue purchasing unlimited quantities of the same product. Instead, they tend to distribute their income among different goods to obtain greater overall satisfaction. Thus, the law provides an important foundation for understanding consumer preferences and consumption patterns.
2. Explains the Law of Demand
The law provides an important basis for understanding the Law of Demand. As consumers purchase additional units of a commodity, the marginal utility of each successive unit tends to decline. Therefore, consumers are generally willing to purchase additional quantities only when the price is lower. This relationship helps explain why the demand curve generally slopes downward from left to right. Hence, diminishing marginal utility contributes to explaining the inverse relationship between price and quantity demanded.
3. Helps in Consumer Equilibrium
The law is useful in explaining consumer equilibrium, where a consumer allocates limited income among different commodities to achieve maximum satisfaction. Consumers compare the marginal utility obtained from different goods with their respective prices. They tend to adjust expenditure until the available income is allocated efficiently. The principle of diminishing marginal utility helps explain why consumers distribute their expenditure among various commodities instead of continuously spending their entire income on a single product.
4. Helps Explain Value in Use
The law helps distinguish between total utility and marginal utility when explaining the value or usefulness of commodities. A commodity may have very high total usefulness while its additional units provide relatively little extra satisfaction. For example, water is essential for life and therefore has substantial total utility, but when a person has consumed enough water, the utility of another glass becomes low. Thus, the law helps explain differences between overall usefulness and additional satisfaction.
5. Basis of Consumer Surplus
The concept of consumer surplus is closely related to diminishing marginal utility. Consumers may be willing to pay different amounts for successive units of a commodity because the satisfaction from additional units generally declines. The difference between what consumers are willing to pay and what they actually pay represents consumer surplus. Diminishing marginal utility therefore helps explain why consumers can obtain an additional benefit or surplus when purchasing commodities at a common market price.
6. Guides Resource Allocation
The law helps consumers achieve efficient allocation of limited resources. Since the marginal utility of a commodity generally decreases as consumption increases, consumers have an incentive to distribute their income among different goods. By shifting expenditure toward goods that provide relatively higher marginal utility, consumers can increase total satisfaction. Therefore, the principle provides an economic explanation for the efficient distribution of scarce income among competing wants and contributes to understanding rational consumption decisions.
7. Useful in Pricing Decisions
The principle of diminishing marginal utility provides useful insights into pricing and sales decisions. Since consumers generally derive less additional satisfaction from successive units, businesses may need to consider consumer willingness to pay when determining prices and sales strategies. Different quantities may require different pricing approaches depending on market conditions. Although actual pricing depends on several factors, including costs, competition, demand, and market structure, diminishing utility contributes to understanding consumer responses to different prices.
8. Helps Understand the Importance of Variety
The law explains why consumers generally prefer variety in consumption rather than spending all their income on one commodity. As more units of the same product are consumed, their marginal utility decreases. Consumers can therefore obtain additional satisfaction by purchasing different goods that satisfy different wants. This principle helps explain diversified consumption patterns and the tendency of consumers to balance expenditure across several commodities to achieve greater overall satisfaction from limited income.
Exceptions or Limitations of the Law of Diminishing Marginal Utility
1. Rare and Collectible Goods
The law may not apply normally to rare and collectible goods such as stamps, coins, antiques, or valuable paintings. A collector may experience increasing satisfaction as more related items are acquired because completing a collection creates additional value. In such cases, the marginal utility of successive units may remain constant or even increase rather than decline. Therefore, the assumption of diminishing satisfaction may not adequately explain consumption motivated by collection, completion, or rarity.
2. Money
The application of the law to money is limited because the marginal utility of money may not diminish in the same way as ordinary commodities. A person with increasing wealth may continue to value additional income, particularly when it can satisfy different future wants. However, economic theory generally assumes that the marginal utility of money decreases as income increases. Therefore, the precise application of the law to money depends on income levels, preferences, and circumstances.
3. Addictive Goods
The law may face limitations when applied to addictive goods. In some cases, repeated consumption can increase the consumer’s desire for additional units rather than immediately reducing it. The satisfaction associated with successive consumption may therefore behave differently from ordinary goods. Examples may include certain habit-forming substances or activities. However, the exact pattern varies according to individual circumstances. Thus, addictive consumption can represent an important limitation to the simple assumption of continuously declining marginal utility.
4. Change in Taste and Preferences
The law assumes that consumer tastes and preferences remain constant during consumption. In reality, preferences may change because of advertising, fashion, social influences, experiences, or changing circumstances. A consumer may develop a stronger interest in a commodity after consuming or experiencing it. Consequently, the marginal utility of subsequent units may not necessarily decline. Therefore, changes in consumer preferences can weaken the applicability of the law under real-world conditions.
5. Long Time Intervals
The law assumes continuous consumption within a reasonably short period. If considerable time passes between successive units, the consumer’s desire may be restored. For example, a person may experience high utility from a meal after several hours without food, even if the same person previously consumed a similar meal. In such situations, the marginal utility of a subsequent unit after a long interval may increase. Hence, time gaps can limit the direct application of the law.
6. Very Small or Large Units
The law assumes that successive units are uniform and reasonably sized. If units are extremely small or significantly different in size, the expected decline in marginal utility may not be observed accurately. For example, comparing a very large first unit with extremely small subsequent units creates an unsuitable basis for measuring satisfaction. The observed difference may result from differences in quantity rather than diminishing utility. Therefore, appropriate and comparable units are necessary for applying the law.
7. Complementary Goods
The law may be difficult to apply independently when commodities are complements and are normally consumed together. The utility of one commodity may depend on the availability or quantity of another commodity. For example, the satisfaction obtained from a printer may depend on the availability of ink. Therefore, changes in the consumption of related goods can influence marginal utility, making it difficult to examine the utility of one commodity completely independently.
8. Abnormal Situations and Circumstances
The law may not operate normally under unusual or exceptional circumstances. Factors such as celebrations, emergencies, extreme weather, emotional conditions, or sudden changes in needs can alter the satisfaction obtained from successive units. For example, the marginal utility of food may become exceptionally high during an emergency involving severe shortage. Such circumstances violate the assumption of normal conditions and may produce consumption patterns that differ from the usual diminishing marginal utility relationship.
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