ONE OF ALFRED HITCHCOCK'S most enduring bits of cinematic comedy is the auction scene in the espionage thriller North by Northwest. Cary Grant plays Roger Thornhill, a businessman who has been mistaken for a CIA agent by the ruthless Phillip Vandamm. At a critical juncture, Thornhill is cornered by his enemies inside a Chicago auction house, and the only way he can escape is by drawing attention to himself. When the bidding on an antique reaches $2,250, Thornhill yells out, Fifteen hundred! When the auctioneer gently chides him, he loudly changes his bid: Twelve hundred! When the bidding on a Louis XIV chaise longue reaches $1,200, Thornhill blurts outs, Thirteen dollars! The genteel crowd is outraged, but Thornhill gets precisely what he wants: the auctioneer summons the police, who escort him past Vandamms henchmen to safety.

Clever thinking and good comedy. It is funny for a lot of reasons, and one is that Thornhill violates every psychological rule for how we negotiate price and value with one another. So much of life involves auctions, whether it is buying a used car or making health care choices or even choosing a mate. But, unlike Roger Thornhill, most of us are motivated by the desire for a fair deal, and we employ some sophisticated cognitive tools to weigh offers, fashion responses, and so forthall the to-and-fro in getting to an agreement.

But how does lifes dickering play out in the brain? And is it a trustworthy tool for getting what we want? Psychologists have been studying cognitive bartering for some time, and several basics are well established. For example, an opening bid of any sort is usually perceived as a mental anchor, a starting point for the psychological jockeying to follow. If we perceive an opening bid as fundamentally inaccurate or unfair, we reject it by countering with something in another ballpark altogether. But what about less dramatic counter offers? What makes us settle on a response?

University of Florida marketing professors Chris Janiszewski and Dan Uy suspected that something fundamental might be going on, that some characteristic of the opening bid itself might influence the way the brain thinks about value and shapes bidding behavior. In particular, they wanted to see if the degree of precision of the opening bid might be important to how the brain acts at an auction. Or, to put it in more familiar terms: Are we really fooled when storekeepers price something at $19.95 instead of a round 20 bucks?

Janiszewski and Uy ran a series of tests to explore this idea. The experiments used hypothetical scenarios, in which participants were required to make a variety of educated guesses. For example, they had subjects think about a scenario in which they were buying a high-definition plasma TV and asked them to guesstimate the wholesale cost. The participants were told the retail price, plus the fact that the retailer had a reputation for pricing TVs competitively.

There were three scenarios involving different retail prices: one group of buyers was given a price of $5,000, another was given a price of $4,988, and the third was told $5,012. When all the buyers were asked to estimate the wholesale price, those with the $5,000 price tag in their head guessed much lower than those contemplating the more precise retail prices. That is, they moved farther away from the mental anchor. What is more, those who started with the round number as their mental anchor were much more likely to guess a wholesale price that was also in round numbers. The scientists ran this experiment again and again with different scenarios and always got the same result.

Why would this happen? As Janiszewski and Uy explain in the February issue of Psychological Science, people appear to create mental measuring sticks that run in increments away from any opening bid, and the size of the increments depends on the opening bid. That is, if we see a $20 toaster, we might wonder whether it is worth $19 or $18 or $21; we are thinking in round numbers. But if the starting point is $19.95, the mental measuring stick would look different. We might still think it is wrongly priced, but in our minds we are thinking about nickels and dimes instead of dollars, so a fair comeback might be $19.75 or $19.50.

The psychologists decided to check these lab findings in the real world. They looked at five years of real estate sales in Alachua County, Florida, comparing list prices and actual sale prices of homes. They found that sellers who listed their homes more preciselysay $494,500 as opposed to $500,000consistently got closer to their asking price. Put another way, buyers were less likely to negotiate the price down as far when they encountered a precise asking price. Furthermore, houses listed in round numbers lost more value if they sat on the market for a couple of months. So, bottom line: one way to deal with a buyers market may be to pick an exact list price to begin with.

This isnt all about money, however. Medical information, Janiszewski and Uy note, can also be offered in either precise or general terms: a physician might say that your chance of responding to a medication is good or that your chance of responding is 80 percent. The percentage is more precise, but many studies have shown that patients prefer vague generalities like good, so doctors tend to use them. But remember that life is an auction. In his mind, the patient is dickering with the doctor, so why not negotiate good up to excellent? When treatment choices are on the line, the auction house can indeed be a perilous place.