The Primp is followed by the Twirl, the Mother and the Wallet- Carrier. The Take-Home is the result, and the Add-On the bonus. In the new language of shopping, this is a girl and her parents being filmed and analysed as they make their purchases. Malcolm Gladwell begins our special issue on modern American lifestyles by investigating the major science of selling. Illustration by Stephen Parkes
Human beings walk the way they drive, which is to say the English tend to keep to the left when they stroll down shopping-mall concourses or city pavements. This is why in a well-designed airport, travellers drifting toward their gate will always find the fast-food restaurants on their right and the gift shops on their left: people will readily cross a lane of pedestrian traffic to satisfy their hunger but rarely to make an impulse buy of a T-shirt or a magazine. This is also why Paco Underhill tells his retail clients to make sure that their window displays are canted - preferably to both sides, but especially to the right - so that the potential shopper approaching the shop on the inside of the pavement, the shopper, that is, with the least impeded view of the store window, can see the display from at least 25 feet away. Of course, a lot depends on how fast the potential shopper is walking. Underhill says that the faster you walk, the more your peripheral vision narrows, so you don't pick up visual cues at the same rate as someone who is just ambling along. People who walk fast also take a surprising amount of time to slow down - just as it takes a good stretch of road to change gears with a stick- shift automobile. Based on his research, Underhill estimates the human downshift period to be about 15 feet, and that's why if you own a shop you never want to be next door to a bank: potential shoppers speed up when they walk past a bank (since there's nothing to look at), and by the time they've slowed down they've walked right past your business. The downshift factor also means that when potential shoppers enter a store, it's going to take them 10 or 15 paces to adjust to the light and refocus and gear down from walking to shopping speed.

Underhill calls that area inside the door the Decompression Zone, and the thing he tells clients over and over again is never, ever put anything of value in that zone - not shopping baskets or tie racks or big promotional displays - because no one is going to see them. Underhill believes that, as a rule of thumb, customer interaction with any product in the Decompression Zone will increase at least 30 per cent once it's moved to the back edge of the zone, and even more if it's placed to the right, because another of the fundamental rules of how humans shop is that upon entering a store the shopper invariably and reflexively turns to the right. Underhill believes in the existence of the Invariant Right because he has actually verified it. He has put cameras in stores trained directly in the doorway, and if you go to his office in Manhattan where the video cassettes from his work over the years are stacked in plastic Tupperware containers practically up to the ceiling, he can show you reel upon reel of grainy entryway video - customers striding in the door, downshifting, refocusing, and then, again and again, making that little half-turn.

Paco Underhill is a tall man in his mid-forties, partially bald, with a neatly trimmed beard and an engaging manner. He dresses in baggy khakis and shirts open at the collar, and generally looks like the academic he might have been had he not been captivated, 20 years ago, by the idea of using time-lapse photography to study shopping behaviour. Today he has a client list that reads like a Who's Who of retailing - working with everyone from Levi Strauss to McDonalds. At a time when retailing around the world is in crisis - when competition is increasing just as shoppers are becoming more and more selective - Underhill's encyclopaedic knowledge of consumer behaviour has made him the man every retailer wants to hire.

Underhill is considered the architect, for example, of what is known in the trade as the butt-brush theory (or, as he calls it more delicately, toucher la derriere), which holds that a woman's likelihood of being converted from a browser to a shopper is inversely proportional to her likelihood of being brushed on her behind while examining merchandise. Touch - or brush or bump or jostle - a woman on the behind when she has stopped to look at an item, and she will bolt. Underhill has made this observation after repeated and close analysis of his videotape library - and now holds it as a commandment: a women's product that requires extensive examination should never, ever be placed in a narrow aisle.

Underhill makes these kinds of pronouncements all the time. 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, out of the blue, in our very first conversation. "Men, for the first time, have begun to buy their own underwear." And then he paused to let the comment sink in, so that I could absorb its implications, before elaborating - "which means that we have to totally rethink the way we sell that product". It would never have occurred to me to wonder about the increasingly critical role played by touching - or, as Underhill calls it, "petting" - in the decision to purchase clothing. But then I went to the Gap and Banana Republic and saw the people touching and fondling and, one after another, buying the shirts and sweaters laid out on those big wooden tables, and what Underhill told me made perfect sense: that the reason Gap and Banana Republic have tables is not merely because sweaters and shirts look better there, or that tables fit into the warm and relaxing residential feel that Gap and Banana Republic are trying to create in their stores, but that tables invite - indeed, symbolize - touching. "Where do we eat?" Underhill asks. "We eat, we pick up food, on tables."

Typically, Underhill will produce for his clients a carefully detailed series of studies filled with product-by-product breakdowns and brightly coloured charts. In one recent case, he was asked by a major clothing retailer to analyse the first of a new chain of stores it planned to open in England, and after he set up his cameras and analysed his data he found all kinds of troubling questions. The purchasers, for example, spent an average of 11 minutes and 33 seconds in the store, and the non-purchasers two minutes and 28 seconds. It wasn't as if the non-purchasers just cruised in and out: in those two minutes and 28 seconds, they went deep into the store and examined an average of 3.29 items. So why didn't they buy? What exactly happened to cause some browsers to buy, and other browsers to walk out the door? Then there was the issue of the number of products examined. The purchasers were looking at an average of 4.67 items, but buying only 1.21 items. This he found deeply disturbing. As the retail market grows more cut-throat, stores have come to realise that it's all but impossible to increase the number of customers coming in, and have concentrated, instead, on getting the customers they do have to buy more. Underhill thinks that if you can sell someone a pair of trousers, you should also be able to sell them a belt, or a pair of socks, or underwear - or even do what the Gap does so brilliantly, and sell them a complete outfit. To Underhill, the figure 1.21 suggested that the store was doing something very wrong and on one of the occasions I visited him in his office he sat me down in front of one of his many VCRs to watch as he looked for the 1.21 culprit.

In one sequence, for example, a camera mounted high on the wall outside the changing rooms at the back documented a man and woman shopping for a pair of trousers for what looked 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 Underhill that he was able to provide a running commentary on what was being said and thought. There was 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, pulling the jeans up from the back. There's the girl disappearing again into the changing room, coming out with another pair. There's the Primp again. The Twirl. The Mother. The Wallet-Carrier. And then again, with another pair, the full sequence taking 20 minutes. The take-home lesson didn't come until the very end and Underhill, at this point, called in one of his colleagues, Tom Moseman, who had supervised the project directly.

"This is a very critical moment," said Moseman, as 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 sales clerk. She 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 sales clerk. The wallet-carrier is telling his daughter to give back the belt. The girl gives back the belt. Moseman stops the tape. He's leaning forward now, finger jabbing at the screen. Beside me, Underhill is shaking his head. I don't get it, at least, not at first, and so Moseman replays just that last segment. The wallet-carrier tells the girl to give back the belt. She gives back the belt. And then finally it begins dawning on me why this store has an average purchase number of only 1.21. "Don't you see?," Moseman says. "She wanted the belt. A great opportunity to make an add-on sale... lost!"

Imagine that you want to open a clothing store. The work of Paco Underhill would suggest that in order to succeed you need to pay slavish attention to the whims of your customers. But it's possible to be much more precise about what that means, in practical terms. Let's start with what's called the shopping gender gap. In the project on the retail store that Underhill showed me, for example, male buyers stayed an average of nine and a half minutes in the store and female buyers stayed just over four minutes more. This is not atypical. "Women have more patience than men," Underhill says. "Men are more distractible. Their tolerance level for confusion or time spent in a store is much shorter than women." If you wanted to, then, you could build a store designed for men. You could make the look a little more masculine - more metal, darker woods. You could turn up the music. You could simplify the store, so men wouldn't get so impatient. James Adams, a prominent American store designer, says he would go "narrow and deep... You wouldn't have 50 different cuts of trousers. You'd have your four basics with lots of colour. You want to stick with the basics. Making sure most of the colour story goes together. That is also a big deal with guys, because they are always screwing the colours up."

You wouldn't want to go so far in the male direction, though, that you alienate women. It's not an accident that stores like the Gap have a single look in both men's and women's sections, and that look - light woods, white wall - is a touch more feminine than masculine. The fact is that women are the shoppers in America and the real money is to be made making retailing styles more feminine, not less. Several years ago, for example, Adams did a project to try and to beef up sales for the Armstrong flooring chain. He found that the sales staff was selling the flooring based on its functional virtues - on the fact that it didn't scuff, that it was long-lasting, that it didn't stain and was easy to clean. It was being sold by men to men, as if it was a car or a stereo. Wrong! "It's a wonder product technologically," says Adams. "But the woman is the decision-maker on flooring and that's not what's she's looking for. This product is about fashion, about colour and design. You don't want to get too caught up in the man's way of thinking."

To appeal to men, retailers do more subtle things. At the Banana Republic Fifth Avenue store in Manhattan, the men's socks are between men's trousers and the cash register (or cash/wrap as it is known in the trade), so the man can grab them easily as he rushes to pay. Women's socks are by the fitting rooms, because women are much more likely to try on trousers first, and then choose an accessory like socks. At the same time, at the men's shirt table the display shirts have matching ties already on them - with the tie rack right next to it. But Banana would never match scarves with women's blouses or jackets. "You don't have to be that direct with women," Banana Republic's president Jeanne Jackson told me. "In fact, the Banana woman is proud of her sense of style. She puts her own looks together." Jackson said she liked the Fifth Avenue store because it's on two floors, so she can separate men's and women's and give men what she calls "clarity of offer", which is the peace of mind that they won't inadvertently end up in, say, women's lingerie. In a one-floor store, most retailers would rather put the menswear up front and the womenswear at the back (that is, if they aren't going to split the sexes left and right), because women won't get spooked walking through menswear while men most assuredly would if they had to navigate womenswear to get to their section.

The next thing you want to do is to encourage the shopper to walk as deep into the store as possible. How did they do that? Well, the trick is to put "destination" items - basics, staples, things that people know you have, and buy a lot of - at the rear of the store. A Gap store, invariably, will have denim on the back wall, which is a classic destination item. Many clothing stores also have the cash-wrap and the fitting rooms in the rear of the store, to compel shoppers to walk back into zone three or four. By the same token, you would never put a destination item at the front. Why bother? In the store's prime real estate - which, given the Underhill's theory of the Decompression Zone and the Invariant Right is to the right of the front entrance 5 to 10 paces in - you always put your hottest and newest merchandise, because that's where the maximum amount of people will see it.

Should we afraid of Paco Underhill? One of the fundamental anxieties of the American consumer, after all, has always been that beneath the pleasure and frivolity of the shopping experience lies an undercurrent of manipulation. The practice of prying into the minds and habits of consumers is now a multi-billion-dollar business - replete with entire armies of consumer research firms and retail anthropologists - and that the fact that some people are now tracking our every shopping move with video cameras seems in many ways the final straw: Underhill's movies are, after all, creepy. They look like the surveillance video taken during convenience store hold-ups - hazy and soundless and slightly warped by the angle of the lens. When you watch them, you find yourself waiting for something bad to happen, for someone to shoplift or pull a gun on a cashier.

The more you watch Underhill's videos, though, the less scary they seem. After an hour or so, it's no longer clear that simply by watching people shop - by analysing their every move - you can learn how to control them. The shopper that emerges from Underhill's videos is not pliable or manipulable. His movies show people filtering in and out of stores, petting and moving on, abandoning their merchandise because check-out lines are too long, or leaving a store empty-handed because they couldn't fit their stroller down between two shirt racks. His shoppers are fickle and headstrong and quite unwilling to buy anything unless conditions are perfect, unless the belt is presented at exactly the right moment. His theories of the butt-brush and petting and the Decompression Zone and the Invariant Right are not about trying to make shoppers conform to the desires of sellers but to make sellers conform to the desires of shoppers. What Underhill is teaching his clients is to adopt a kind of slavish devotion to the shopper's every whim. He is teaching them humility.

Paco Underhill has done some work with supermarket chains, and when you first see his videos of grocery aisles it looks at first as if he really has - at least in this instance - got one up on the shopper. The clip he showed me is of a father shopping with a small child and it is an example of what is known in the trade as "advocacy", which is basically when your four-year-old goes over to the shelf, grabs the bag of cookies that the store has conveniently put on the bottom shelf, and demands that it be purchased.

In the clip, the father takes what the child offers him. "Generally, dads are not as good as moms at saying `no'," Underhill says, as we watch the little boy approach his dad. "Men tend to be more impulse-driven than women in grocery stores. We know, simply by watching them shop, that they can be marching down the aisle, and something will catch their eye, and they will stop and buy." The kind of weakness on the part of fathers would seem to give the supermarket an advantage in the cookie-selling wars, particularly as more and more men go grocery shopping with their children. Except that then Underhill lets drop a small hint about a study he just did, at which point it becomes abundantly clear that even knowing everything he does, even knowing about the Invariant Right, and the Decompression Zone, and respecting the female derriere, and putting destination items at the back and treating male shoppers like small children, is not always enough to capture the elusive consumer. What Underhill found out was that grocery shoppers were already one step ahead, that families are no longer shopping the cookie aisle.

1997 Malcolm Gladwell. All rights reserved. Originally published in a slightly different version in `The New Yorker Magazine', Inc