(FORTUNE Magazine) – "I can still taste the chocolate," says psychologist Dr. G. Clothaire Rapaille of the candy given to him more than 50 years ago, when he was 5, by American soldiers liberating France in their Jeeps. Because he "imprinted" on the vehicle in 1944, Rapaille explains in his book Seven Secrets of Marketing in a Multi-Cultural World, he knows "the secret code" for Jeep--its Jungian archetype as horse, cavalry, savior. This qualified him to tell Chrysler that the Jeep "always has round headlights." It was also why he was hired to help create the PT Cruiser.

In what has become Detroit legend, Rapaille collaborated with Bob Lutz (then Chrysler's top product executive), David Bostwick (its research director), and designer Bryan Nesbitt. Rapaille persuaded customers, lying on mats in darkened rooms, to let their minds drift back to childhood--and reveal what he calls their "reptilian hot buttons." Since April 2000, the PT Cruiser--"Al Capone at the wheel, with a machine gun," says Rapaille--has sold more than 225,000 units.

After years of competing for "best-in-segment rear legroom," Detroit has begun vying for owner self-esteem, sense of security, pride of ownership, and other psychological values. Emotion is the big word in Motown, as in "we want vehicles that make an emotional connection." It's as if the industry had relocated en masse to Marin County.

With his accent and swooping coiffure, Rapaille is the best known and most colorful of a new group of auto psychologists to whom Detroit has turned. Auto companies want to find the next "wow car," as Rapaille calls it, the "segment buster" that reaches across age and income lines, into the subconscious.

Born in occupied France, Rapaille began his career as a psychologist helping autistic children, and now claims he's doing an "extension of the work done by many of the great scholars of the 20th century, including Jung, Laing, Levi-Strauss, and Benedict." He has helped Procter & Gamble focus on aroma as the archetype behind Folger's coffee, inspiring the famous ad in which a mother, awakened by the smell of coffee, realizes her soldier son has come home. Rapaille has also delved into the essence of cheese for Kraft.

He is an equal-opportunity car shrink, willing to work with each of the Big Three. Ford recently called in his company, Archetype Discoveries Worldwide, to help it come up with a new concept for the youth market. "They are still thinking with the cortex" is all he'll say. Rapaille advocates tapping the emotional lower brain instead.

Now that "the homeland is at war," Rapaille is looking into the archetype of "safety and security." "One aspect of the reptilian is survival," he says. "That is why people run out to buy guns--even though you can't shoot down a plane with a gun--or sneakers to run fast, or a big powerful SUV."

At GM he's working again with Bryan Nesbitt and the recently hired Bob Lutz. The archetype of safety and security bodes well for GM's Hummer H-2, due out this summer. It's a smaller, more genteel version of the original adaptation of the military Humvee familiar from CNN and Arnold Schwarzenegger. "But in addition to the survival aspect of the reptilian," he adds, "there is the reproductive aspect. The desire to reproduce--to get the girls."

Auto executives' main problem with psychology is its unpredictability. For every PT Cruiser there is a Pontiac Aztek. Psychology resists easy enumeration--and auto execs live on numbers. So a San Diego company called Strategic Vision has found success with a less squishy approach--it quantifies emotions. Strategic Vision's industry surveys--and competitive ones from more traditional consultants such as AutoPacific and Nextrend--involve interviews and questionnaires probing the souls of more than 100,000 people.

"We all have an ideal image of who we are and who we'd like to be, which products and services help validate and create," says Daniel Gorrell, VP and partner in the company. The idea behind Strategic Vision is that people behave according to a hierarchy of values. Its primary product is the New Vehicle Experience Study of 10,000 customers. Since 1995 it has also published the industrywide Total Quality Index. In contrast with the better-known J.D. Power survey of "things gone wrong," framed as problems per 1,000 cars, the TQI looks at "things gone right."

"No," sneers Gorrell, "we don't have to have them lie down on mats and have low lights."

One of the trends Strategic Vision has tracked is the declining appeal of the SUV. "There was a very high level of emotional delivery with SUVs that almost equaled luxury cars before SUVs got to be ubiquitous," he says. "The emotional opportunity today exists in cross-over vehicles." The latest survey provides surprises: The Hyundai Santa Fe compact SUV, for instance, outdid the Ford Escape.

Industry surveys have also shown that people perceive fewer differences in quality among brands. Because it makes customers feel secure, quality had always been at the top of Strategic Vision's hierarchy of values. Once it's no longer an issue, however, other values--self-esteem, personal fulfillment--come into play. Think of these as the discretionary income of the mind.

Naturally, there's rivalry among the psychological consultants. "We come across them all," Gorrell says. "The vague practitioners, the charlatans who pull rabbits out of hats. Dr. Rapaille is the worst. He has no quantitative at all."

"But now I do have the numbers," Rapaille replies. "The sale numbers of the PT Cruiser."

The one thing the auto shrinks do agree on is that Detroit doesn't get it yet. "They are still producing cortex cars," Dr. Rapaille says disdainfully. "American industry changes only when it is desperate." This may be one of those times: Surveys show that not only do young people today prefer Hondas and Volkswagens to U.S. brands, but that for the first time ever more than half of teens who will be purchasing their first car in the next few years are inclined to buy nondomestic.

Many of the cars inspired by psychology resemble those of the 1950s. For designer J Mays, who was aligned with the psychologically oriented consultant SHR after working for Audi/VW and before joining Ford, his Thunderbird or the Ford 49 concept car "recall optimistic days of the past when people felt more positive about themselves."

The '50s were also the era when psychology last ruled car design. Rapaille most resembles a nearly forgotten figure of the 1950s who held powerful sway in American business, and in the auto industry in particular. Dr. Ernest Dichter trained in psychoanalysis at the University of Vienna before opening the Institute for Motivational Research in the U.S. Dichter, who famously proclaimed that a convertible was like a man's mistress and a sedan was like a wife, was a prime target of Vance Packard's bestselling attacks on sales manipulation, The Hidden Persuaders and The Status Seekers. By 1960 his influence had gone the way of the tail fin.

Whether today's car shrinks will have more durability remains to be seen. Nobody admits to rooting for bean counters, and everyone wants to prefer heart over head. But the Honda Accord is still the bestselling car in America, and the cortex is still the seat of many important functions. Including the check-writing one.

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