There are two sides to the analysis of price and value: the supply side and the demand side. If cost can be said to underlie the supply relationship that determines price, the demand side must be taken to reflect consumer tastes and preferences. “Utility” is a concept that has been used to describe these tastes. As already indicated, the cost-of-production analysis of value given above is incomplete, because cost itself depends on the quantity produced. The cost analysis, moreover, applies only to commodities the production of which can be expanded and contracted. The price of a first-folio Shakespeare has no relation to cost of production; it must depend in some sense on its utility to purchasers as it affects their bids.

The classical economists suggested that this leads to a paradox. They argued that utility could not explain the relative price of fine jade and bread, because the latter was for many consumers essential to life, and hence its utility must surely be greater than that of jade. Yet the price of bread is far lower than that of jade. The theory of marginal utility that flowered toward the end of the 19th century supplied the key to the paradox and provided the basis for today’s analysis of demand. Marginal utility was defined as the value to the consumer of an additional unit of some commodity. If, for example, the consumer is offered a choice between 22 and 23 slices of bread for his family, marginal utility measures how much more valuable 23 slices are than 22. It is clear that the magnitude of the marginal utility varies with the magnitude of, say, the smaller of the alternatives. That is, for a family of four, the difference between seven and eight slices of bread per day can be substantial, if the family will still be hungry in either case. But the difference in value between 31 and 32 slices may be negligible. If 31 slices offer enough for everyone to fill his stomach, a 32nd slice may be worth very little. Moreover, the difference in value between 122 and 123 slices may be negative—a 123rd slice may just add to the family’s disposal problem. These observations lead directly to the plausible notion that marginal utility in some sense diminishes with the base from which one starts the calculation. With only seven or eight slices the marginal utility (incremental value) of an eighth slice is high. With 31 or 32 slices it is lower, and so on. The less scarce a commodity, the lower is its marginal utility, because its possessor in any case will have enough to satisfy his most pressing uses for it, and an increment in his holdings will only permit him to satisfy, in addition, desires of lower priority.

The consumer will be motivated to adjust his purchases so that the price of each and every good will be approximately equal to its marginal utility (that is, to the amount of money he is willing to pay for an additional unit). If the price of an item is P dollars, for example, and the consumer is considering buying, say, 10 units, at which point the marginal utility of the good to him is M (which is greater than P), the consumer will be better off if he purchases 11 rather than 10 units, since the additional unit costs him P dollars. He will keep revising his purchase plans upward until he reaches the point where the marginal utility of the item falls to P dollars. In sum, the consumer’s self-interest will lead him (without conscious calculation) to purchase an amount such that the marginal utility is as close as possible to market price. So long as the consumer selects a bundle of purchases that gives him the most benefit (pleasure, utility) for his money, he must end up with quantities such that the marginal utility of each commodity in the bundle is approximately equal to its price.

It now becomes easy to explain the paradox underlying the relationship between the prices of jade and bread. Because a piece of fine jade is scarce, its marginal utility is high, and consumers are willing to pay comparatively high prices for it. The explanation is perfectly consistent with a utility analysis of demand, so long as one relates price to the marginal utility of the item rather than to its total utility. A family’s bread may be very valuable to it, but, if it has enough, the marginal utility of the bread will be small, and this will be reflected in its low price.

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Figure 1: Relationship between marginal utility and quantity (see text).

Encyclopædia Britannica, Inc.

The relationship between price and marginal utility is important not because it explains issues like the jade–bread paradox but because it enables one to analyze the relationship between prices and quantities demanded. It also, as a practical matter, permits one to judge how well any portion of the price mechanism is working as a device to secure the efficient satisfaction of the wants of the public, within the limits set by available resources. The conclusion that at any price the consumer will purchase the quantity at which marginal utility is equal to price makes it possible to draw a demand curve showing—to a reasonable degree of approximation—how the amount demanded will vary with price. A curve based on the previous example of bread consumption is given in Figure 1. This shows that if the family gets 10 slices per day the marginal utility of bread will be nine cents (point A). One may reverse the question and ask how much the family would purchase at any particular price, say three cents. The graph indicates that at this price the quantity would be 30 slices, because only at that quantity is marginal utility equal to the three-cent price (point B). Thus the curve in Figure 1, to a reasonable degree of approximation, may be able to do double duty: it may serve as a marginal-utility curve relating marginal utility to quantity and, at the same time, as a demand curve relating quantity demanded to price.