Stigler spends a long time developing utility theory. He claims that abstract utility with its questionable foundation is a useful tool, because of its predictive power. Though the foundations are not great, the results justify it. Stigler spends the rest of chapter four showing off the power of utility theory.
In order to test the predictive power, we obviously need to make some predictions from our model of utility maximization. Stigler sticks to mostly graphical predictions that involve reaching the highest indifference curve that is still within the consumer's budget. The graphs allow economists to do comparative statics. If one aspect changes, how does that affect the consumer's bundle?
One type of change is variations of income. On a graph, this corresponds to a shift of the budget, illustrated below. Normally, when people are richer they consumer more of a good, hence normal goods. Other times, when a consumer is richer, he consumes less of this good. The classic examples are ramen and potatoes. As college graduates start making money, ramen consumption declines. These are inferior goods.
While not a formal prediction, the outline of utility theory's predictive power is starting to form. This model of normal and inferior goods suggests which types of goods will increase with wealth. Inferior goods convey the idea of "buying because it is cheap." This is simply a definition, but it clarifies the thinking. Continue reading