Diminishing Marginal Utility

Why each additional unit of something usually brings less satisfaction than the one before, and how this idea explains prices, choices and insurance.

Imagine you are very hungry and eat a samosa. The first one is delicious and satisfying. The second is still good. By the fourth, you may not want any more. This everyday experience illustrates the idea of diminishing marginal utility.

Utility and marginal utility

Economists use utility to mean the satisfaction or benefit a person gets from consuming something. Marginal utility is the extra satisfaction from one more unit.

Diminishing marginal utility means that, as you consume more of something, each additional unit adds less to your satisfaction than the one before.

Why it matters

The idea helps explain many things:

Demand curves slope down: because extra units bring less satisfaction, people are willing to pay less for them, so they buy more only at lower prices.
Variety: people spread spending across many goods rather than buying only one, because the first units of different goods bring more satisfaction than extra units of the same good.
Risk aversion: losing money hurts more than gaining the same amount helps, because each extra rupee adds less to wellbeing.
Redistribution arguments: an extra thousand rupees means more to a poor person than to a rich one.

The water-diamond paradox

Adam Smith puzzled over why water, essential for life, is cheap, while diamonds, which are not essential, are expensive. Marginal utility resolves the paradox. Water is plentiful, so the marginal utility of one more litre is low for most people. Diamonds are scarce, so the marginal utility of one more diamond is high. Prices reflect marginal, not total, value.

Water in the desert

For someone lost in a desert, the first litre of water has enormous value. They might pay almost anything for it. But after drinking several litres, the next litre is worth much less. In a city with piped water, where people already have plenty, one more litre is worth very little, which is why it is cheap.

Limits

Diminishing marginal utility is a general tendency, not a law for every case. Some things, like collections or skills, may bring increasing satisfaction up to a point. And utility cannot be directly measured, so economists infer it from people’s choices.

Confusing total and marginal value

Water has enormous total value, but its marginal value is low when it is plentiful. Prices reflect the value of one more unit, not the total importance of a good.

Key takeaways

Marginal utility is the extra satisfaction from one more unit.
Diminishing marginal utility means each extra unit adds less satisfaction.
It helps explain downward-sloping demand, variety-seeking and risk aversion.
It resolves the water-diamond paradox: prices reflect marginal, not total, value.