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The law of diminishing marginal utility in economics describes a familiar and fundamental tendency of human behavior (consumer behavior).

## Introduction, Meaning and Statement of the Law by Alfred Marshall:

The law of diminishing marginal utility states that:

“As a consumer consumes more and more units of a specific commodity, the utility from the successive units goes on diminishing”.

Mr. H. Gossen, a German economist, was first to explain this law in 1854. Alfred Marshal later on re-stated the law of diminishing marginal utility in the following words:

“The additional benefit which a person derives from an increase of his stock of a thing diminishes with every increase in the stock that already has”.

Law of diminishing marginal utility is based upon following three facts (conditions):

First, total wants of a man are unlimited but each single want can be satisfied. As a man gets more and more units of a commodity, the desire of his for that good goes on falling. A point is reached when the consumer no longer wants any more units of that good.

Secondly, different goods are not perfect substitutes for each other in the satisfaction of various particular wants. As such the marginal utility will decline as the consumer gets additional units of a specific good.

Thirdly, the marginal utility of money is constant given the consumer’s wealth.

The basis of this law are the fundamental feature of wants. It states that when people go to the market for the purchase of commodities, they do not attach equal importance to all the commodities which they buy. In case of some of commodities, they are willing to pay more and in some less. There are two main reasons for this difference in demand.

(1) The linking of the consumer for the commodity.

(2) The quantity of the commodity which the consumer has with himself.

The more one has of a thing, the less he wants the additional units of it. In other words, the marginal utility of a commodity diminishing as the consumer gets larger quantities of it. This, in brief, is the axiom of law of diminishing marginal utility.

## Explanation with Example:

This law of diminishing marginal utility can be explained by taking a very simple example.

Suppose, a man is very thirsty. He goes to the market and buys one glass of sweet water. The glass of water gives him immense pleasure or we say the first glass of water has great utility for him. If he takes second glass of water after that, the utility will be less than that of the first one. It is because the edge of his thirst has been blunted to a great extent. If he drinks third glass of water, the utility of the third glass will be less than that of second and so on.

The utility goes on diminishing with the consumption of every successive glass water till it drops down to zero. This is the point of satiety (satisfaction). It is the position of consumer’s equilibrium or maximum satisfaction. If the consumer is forced further to take a glass of water, it leads to dis-utility causing total utility to decline. The marginal utility will become negative. A rational consumer will stop taking water at the point at which marginal utility becomes negative even if the good is free. In short, the more we have of a thing, ceteris paribus, the less we want still more of that, or to be more precise.

“In given span of time, the more of a specific product a consumer obtains, the less anxious he is to get more units of that product” or we can say that as more units of a good are consumed, additional units will provide less additional satisfaction than previous units.

## Table and Diagram (Graph):

The following table and diagram will make the law of diminishing marginal utility more clear.

From the above table, it is clear that in a given span of time, the first glass of water to a thirsty man gives 20 units of utility. When he takes second glass of water, the marginal utility goes on down to 12 units; When he consumes fifth glass of water, the marginal utility drops down to zero and if the consumption of water is forced further from this point, the utility changes into dis-utility (-3).

Here it may be noted that the utility of then successive units consumed diminishes not because they are not of inferior in quality than that of others. We assume that all the units of a commodity consumed are exactly alike. The utility of the successive units falls simply because they happen to be consumed afterwards.

The law of diminishing marginal utility can also be represented by a diagram (graph).

In the figure (2.2), along OX we measure units of a commodity consumed and along OY is shown the marginal utility derived from them. The marginal utility of the first glass of water is called initial utility. It is equal to 20 units. The MU of the 5th glass of water is zero. It is called satiety (satisfaction) point. The MU of the 6th glass of water is negative (-3). The MU curve here lies below the OX axis. The utility curve MM/ falls left from left down to the right showing that the marginal utility of the success units of glasses of water is falling.

## Assumptions:

The law of diminishing marginal utility is true under certain assumptions. These assumptions are as under:

(i) Rationality: In the cardinal utility analysis, it is assumed that the consumer is rational. He aims at maximization of utility subject to availability of his income.

(ii) Constant marginal utility of money: It is assumed in the theory that the marginal utility of money based for purchasing goods remains constant. If the marginal utility of money changes with the increase or decrease in income, it then cannot yield correct measurement of the marginal utility of the good.

(iii) Diminishing marginal utility: Another important assumption of utility analysis is that the utility gained from the successive units of a commodity diminishes in a given time period.

(iv) Utility is additive: In the early versions of the theory of consumer behavior, it was assumed that the utilities of different commodities are independent. The total utility of each commodity is additive.

(v) Consumption to be continuous: It is assumed in this law that the consumption of a commodity should be continuous. If there is interval between the consumption of the same units of the commodity, the law may not hold good. For instance, if you take one glass of water in the morning and the 2nd at noon, the marginal utility of the 2nd glass of water may increase.

(vi) Suitable quantity: It is also assumed that the commodity consumed is taken in suitable and reasonable units. If the units are too small, then the marginal utility instead of falling may increase up to a few units.

(vii) Character of the consumer does not change: The law holds true if there is no change in the character of the consumer. For example, if a consumer develops a taste for wine, the additional units of wine may increase the marginal utility to a drunkard.

(viii) No change to fashion: Customs and tastes: If there is a sudden change in fashion or customs or taste of a consumer, it can than make the law inoperative.

(ix) No change in the price of the commodity: there should be any change in the price of that commodity as more units are consumed.

## Limitations or Exceptions:

There are some exceptions or limitations to the law of diminishing utility.

(i) Case of intoxicants: Consumption of liquor defies the low for a short period. The more a person drinks, the more likes it. However, this is truer only initially. A stage comes when a drunkard too starts taking less and less liquor and eventually stops it.

(ii) Rare collection: If there are only two diamonds in the world, the possession of 2nd diamond will push up the marginal utility.

(iii) Application to money: The law equally holds good for money. It is true that more money the man has, the greedier he is to get additional units of it. However, the truth is that the marginal utility of money declines with richness but never falls to zero.

Summing up, we can say that the law of diminishing utility, like other laws of Economics, is simply a statement of tendency. It holds good provided other factors remain constant.

## Practical Importance and Uses:

The law of diminishing utility has great practical importance and uses in economics and daily life. The law of demand, the theory of consumer’s surplus, and the equilibrium in the distribution of expenditure are derived from the law of diminishing marginal utility.

(i) Basis of the law of demand: The law of marginal diminishing utility and the law of demand are very closely related to each other. In fact they law of diminishing marginal utility, the more we have of a thing, and the less we want additional increment of it. In other words, we can say that as a person gets more and more of a particular commodity, the marginal utility of the successive units begins to diminish. So every consumer while buying a particular commodity compares the marginal utility of the commodity and the price of the commodity which he has to pay.

If the marginal utility of the commodity is higher than that of price, he purchases that commodity. As he buys more and more, the marginal utility of the successive units begins to diminish. Then he pays fewer amounts for the successive units. He tries to equate at every step the marginal utility and the price of the commodity, he must lower its price so that the consumers are induced to buy large quantities and this is what is explained in the law of demand. From this, we conclude that the law of demand and the law of diminishing are very closely inter-related.

(ii) Consumer’s surplus concept: The theory of consumer’s surplus is also based on the law of diminishing marginal utility. A consumer while purchasing the commodity compares the utility of the commodity with that of the price which he has to pay. In most of the cases, he is willing to pay more than what he actually pays. The excess of the price which he would be willing to pay rather than to go without the thing over that which he actually does pay is the economic measure of this surplus satisfaction. It is in fact difference between the total utility and the actually money spent.

(iii) Importance to the consumer: A consumer in order to get the maximum satisfaction from his relatively scare resources distributes his income on commodities and services in such a way that the marginal utility from all the uses are the same. Here again the concept of marginal utility helps the consumer in arranging his scale of preference for the commodities and services.

(iv) Importance to finance minister: Sometimes it is pointed out that the law of diminishing marginal utility does not apply on money. As a person collects money, the desires to accumulate more money increases. This view is superficial. It is true that wealth is acquired for the procurement of goods and services and man is always anxious in getting more and more of money. But what about the utility of money to him? Is it not a fact that as a person gets more and more wealth, its utility progressively decreases, though it does not reach to zero?

For example, a person who earns \$90,000 per month attaches less importance to \$10. But a man who gets \$1000 per month, the value of \$10 to him is very high. A finance minister knowing this fact that the utility of money to a rich man is high and to poor man low bases the system of taxation in such a way that the rich persons are taxed at a progressive rate. The system of modern taxation is therefore, based on the law of diminishing marginal utility.