We have now analyzed the optimal consumption choice of a consumer with a given amount of income who faces one particular set of prices—in our Sammy example, $20 of income per week, $4 per pound of clams, and $2 per pound of potatoes.

But the main reason for studying consumer behavior is to go behind the market demand curve—to explain how the utility-maximizing behavior of individual consumers leads to the downward slope of the market demand curve.