One of the most important concepts related to utility functions is marginal utility, the extra utility a consumer receives from a 1-unit increase in consumption.3 Each good in a utility function has its own marginal utility. Using the Junior Mints and Kit Kats utility function, for example, the marginal utility of Junior Mints, MU_J, would be

3 Marginal utility can be calculated for any given utility function.

where ΔJ is the small (1-unit) change in the number of Junior Mints the consumer eats and ΔU(J, K) is the change in utility she gets from doing so. Likewise, the marginal utility of consuming Kit Kats is given by

Later in this chapter, we see that marginal utility is the key to understanding the consumption choices a person makes.