The other culprit — the main villain, really — was my insula. This region of the brain is activated when you smell something bad, see a disgusting picture or anticipate a painful shock. It was typically activated in the brains of the other shoppers when they saw a price that seemed too high. I'd like to think of my insula as particularly stoic, the strong, silent type, but he's probably just an oblivious slob.

The lazy insula is a rarer affliction than you’d guess by looking at Americans' indebtedness. Tightwads slightly outnumber spendthrifts, according to surveys by George Loewenstein and his colleagues at Carnegie Mellon, Scott Rick and Cynthia Cryder. These behavioral economists think tightwads aren't any more rational than spendthrifts, because neither group is carefully weighing the long-term benefits of a Foreman grill versus college tuition. Dr. Loewenstein says the brain scans demonstrate that both kinds of shoppers are guided by instant emotions.