PALO ALTO, Calif.

Now that scientists have spotted the pain and pleasure centers in the brain, they’ve moved on to more expensive real estate: the brain’s shopping center. They have been asking the big questions:

What is the difference between a tightwad’s brain and a spendthrift’s brain?

What neurological circuits stop you from buying a George Foreman grill but not a Discovery Channel color-changing mood clock?

Why is there a $2,178.23 balance on my January Visa bill?

This last question isn’t yet fully answered, even after I stared at said Visa bill while lying inside a functional M.R.I. machine at Stanford University. But scientists are closer to solving the mystery. By scanning shoppers’ brains, they think they’ve identified a little voice telling you not to spend your money. Or, in my case, a voice saying, “At this price, you can’t afford not to buy the mood clock!”

For convenience’ sake, economists have traditionally assumed that buyers make rational choices: I think, therefore I shop. You pass up the George Foreman grill because you sagely calculate that the money would be better spent on, say, your child’s college fund. Or at least the mood clock. You choose to forgo one good in exchange for something better.

Even the most rational economists, though, realize that the shopper’s mind is more complicated. The brain’s “impartial spectator,” as Adam Smith warned, has to duel with “the passions.” Last year, after surveying shoppers’ passions, behavioral economists at Carnegie Mellon University developed what they call the Tightwad-Spendthrift scale.