It is our common observation that when units of a commodity are used continuously the lateral units give less and less satisfaction. In the beginning we have strong desire to use unit’s commodity but with the continuous use the desire for commodity to use diminishes, as the want is satisfied. When the want is satisfied utility reaches to zero and then with the excessive use of units of commodity the utility becomes negative. The law of diminishing marginal utility expresses the relationship between the units of a commodity consumed and the marginal utility of each unit of the commodity.
Suppose thirsty person starts drinking water continuously. The first glass gives him great pleasure, the second glass of wager give his less satisfaction and the satisfaction of the third glass of water is less than the previous one and so on. The additional satisfaction decreases with ever successive glass of water till it goes down to zero and by excessive use of satisfaction may be negatives.
The law bolds only when units are consumed continuously. If there is a gap or interval between the consumption of units then utility may increase instead of diminishing e.g. if we take lunch at 2 P.M and then dinner at 8P.M. at dinner utility has increased instead of diminishing.
The law applies when units consumed are of suitable size e.g. when a person is thirsty and he starts drinking water drop by drop the utility of water will increase instead of diminishing.
It is assumed that the units consumed should be homogeneous. If the lateral units are better than the previous units the utility of lateral units may increase instead of diminishing.
No Change In Consumer Taste
It is assumed that taste of consumer remains unchanged. If the taste of the consumers changed the law does not hold.
Consumer Income Does Not Change
If the income of the consumer increases his purchasing power will increase and demand for goods and services increases which shows more value i.e. greater utility.
The law does not apply on knowledge as a person acquires more and more knowledge his desire for knowledge increases e.g. the knowledge of different language gives more and more utility for a person.
Income and wealth
It is argued that as income of a person increases his desire to earn more income increases, because more income means more wants.
The law does not apply to rare collections. A person who is collecting old stamps, the more, he is able to collect stamps, the greater will be his satisfaction.
Utility of a commodity depends upon fashion, showmen and ostentation. A dress in fashion or used for showmen or ostentation has greater utility and a dress out of fashion has less utility.
The law does not hold well in case of intoxicants. A person who is addict of intoxicants his utility for successive units increases.
Consumer benefits from this law while purchasing goods. When a consumer purchases goods he compares the marginal utility of the commodity purchased and the price paid. If the M.U is greater than the price, the consumer purchases more and stop purchasing when M.U is equal to price.
For Taxation/ Finance Minister
We know that M.U of money for rich is less than the poor. The law provides the basis for taxation. The finance minister while imposing taxes or changing the tax rates benefits from this law. Higher tax rates are imposed on rich and lower tax rate on poor because M.U of money for poor is more whiles less for riches.
The law forms a basis of theory of values. It explains how with an increase in supply (demand remain the same) the values of commodity falls.
Values in Exchanges
The law explains which commodities have value in exchange. Air has value in use but no marginal utility while goods have value in exchange because they possess marginal utility.
Basis for some Economic law /Theories
Economics laws e.g. law of equi-marginal utility has been derived from this law. The concept of elasticity of demand, consumer surplus depends upon this law.
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