We implicitly assumed above that utility can be measured. We might imagine utility being measured in monetary units in the sense that the money a consumer pays for a good reflects the satisfaction gained. If this were the case the demand curve for com- modity X would be identical to the positive section of the marginal utility curve. This is shown in Figure 4.2.
In Figure 4.2a the consumer gains a marginal utility of MU1 from consuming the X1 unit. P1 reflects the price the consumer is willing to pay for that unit. In consuming the X2 unit, marginal utility falls to MU2. Price must therefore fall to P2 to persuade the con- sumer to buy that additional unit. The concept of diminishing marginal utility therefore
explains the downward slope of the demand curve.
The negative section of the marginal utility curve would not form part of the demand curve, as the consumer would not buy a good yielding negative utility. The relevance of the negative section might be seen in the sense that a consumer must be paid (or com- pensated) to consume a good yielding negative utility. For example, a person disliking broccoli might be willing to consume the vegetable if paid to do so. (This would be simi- lar in principle to the parent bribing a child to eat their vegetables by the promise of a compensatory treat