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Paco Underhill is a tall man, partly bald, with a neatly trimmed beard and an engaging, almost goofy manner. He wears baggy khakis and shirts open at the collar, and generally looks like the academic he might have been if he hadn't been captivated, 20 years ago, by the ideas of the urban anthropologist William Whyte, who pioneered the use of time-lapse photography as a tool of urban planning, putting cameras in parks and the plazas in front of office buildings in Manhattan, in order to determine what distinguished a public space that worked from one that didn't.

As a Columbia undergraduate, in 1974, Paco heard a lecture on Whyte's work and, he recalls, left the room "walking on air". He immediately read everything Whyte had written. He emptied his bank account to buy cameras and film and make his own home movie, about a pedestrian mall in Poughkeepsie. He took his "little exercise" to Whyte's advocacy group, the Project for Public Spaces, and was offered a job. Soon, however, it dawned on Paco that Whyte's ideas could be taken a step further –- that the same techniques he used to establish why a plaza worked or didn't work could also be used to determine why a store worked or didn't work. Thus was born the field of retail anthropology, and, not long afterward, Paco founded Envirosell, which in just over 15 years, has counselled some of the most familiar names in American retailing, from Levi Strauss to Kinney, Starbucks, McDonald's, Blockbuster, Apple Computer, A.T. &T., and a number of upscale retailers that Paco would rather not name.

What Paco likes are facts. They come tumbling out when he talks, and, because he speaks with a slight hesitation – lingering over the first syllable in, for example, "re-tail" or "de-sign" – he draws you in, and you find yourself truly hanging on his words. "We have reached a historic point in American history," he told me in our very first conversation. "Men, for the first time, have begun to buy their own underwear." He then paused to let the comment sink in, so that I could absorb its implications, before he elaborated: "Which means that we have to totally rethink the way we sell that product." In the parlance of Hollywood scriptwriters, the best endings must be surprising and yet inevitable; and the best of Paco's pronouncements take the same shape. It would never have occurred to me to wonder about the increasingly critical role played by touching – or, as Paco calls it, petting clothes in the course of making the decision to buy them. But then I went to the Gap and to Banana Republic and saw people touching and fondling and, one after another, buying shirts and sweaters laid out on big wooden tables, and what Paco told me – which was no doubt based on what he had seen on his videotapes – made perfect sense: that the reason the Gap and Banana Republic have tables is not merely that sweaters and shirts look better there, or that tables fit into the warm and relaxing residential feeling that the Gap and Banana Republic are trying to create in their stores, but that tables invite – indeed, symbolise – touching. "Where do we eat?" Paco asks. "We eat, we pick up food, on tables."

As a child, Paco had a nearly debilitating stammer, and, he says, "since I was never that comfortable talking, I always relied on my eyes to understand things." That much is obvious from the first moment you meet him: Paco is one of those people who look right at you, soaking up every nuance and detail. It isn't a hostile gaze, because Paco isn't hostile at all. He has a big smile, and he'll call you "chief" and use your first name a lot and generally act as if he knew you well. But that's the awkward thing: he has looked at you so closely that you're sure he does know you well, and you, meanwhile, hardly know him at all.

This kind of asymmetry is even more pronounced when you watch his shopping videos with him, because every movement or gesture means something to Paco – he has spent his adult life deconstructing the shopping experience – but nothing to the outsider, or, at least, not at first. Paco had to keep stopping the video to get me to see things through his eyes before I began to understand. In one sequence, for example, a camera mounted high on the wall outside the changing rooms documented a man and a woman shopping for a pair of pants for what appeared to be their daughter, a girl in her mid-teens. The tapes are soundless, but the basic steps of the shopping dance are so familiar to Paco that, once I'd grasped the general idea, he was able to provide a running commentary on what was being said and thought. There is the girl emerging from the changing room wearing her first pair. There she is glancing at her reflection in the mirror, then turning to see herself from the back. There is the mother looking on. There is the father – or, as fathers are known in the trade, the "wallet carrier" – stepping forward and pulling up the jeans. There's the girl trying on another pair. There's the primp again. The twirl. The mother. The wallet carrier. And then again, with another pair. The full sequence lasted twenty minutes, and at the end came the take-home lesson, for which Paco called in one of his colleagues, Tom Moseman, who had supervised the project.

"This is a very critical moment," Tom, a young, intense man wearing little round glasses, said, and he pulled up a chair next to mine. "She's saying, 'I don't know whether I should wear a belt.' Now here's the salesclerk. The girl says to him, 'I need a belt,' and he says, 'Take mine.' Now there he is taking her back to the full-length mirror."

A moment later, the girl returns, clearly happy with the purchase. She wants the jeans. The wallet carrier turns to her, and then gestures to the salesclerk. The wallet carrier is telling his daughter to give back the belt. The girl gives back the belt. Tom stops the tape. He's leaning forward now, a finger jabbing at the screen. Beside me, Paco is shaking his head. I don't get it – at least, not at first – and so Tom replays that last segment. The wallet carrier tells the girl to give back the belt. She gives back the belt. And then, finally, it dawns on me why this store has an average purchase number of only 1.33. "Don't you see?" Tom said. "She wanted the belt. A great opportunity to make an add-on sale... Lost... Maybe forever!”