A peek inside stores’ shopping data highlights regional preferences for certain colors and brands; Victoria’s real secret.

Luxury-spending data can tell us a lot about the state of the nation — and our own neighborhoods.

Take Detroit — not the city where one might expect to see the strongest recovery. Yet when American Express Co. (NYSE: AXP – News) looked at luxury spending in top and midsize cities around the country, Detroit led the list, with growth of 18% in the first quarter of 2010 from the year-earlier quarter. Lo and behold, Ford (NYSE: F – News) stock is up, too, suggesting that Detroit’s local investors are feeling more optimistic than they were when auto executives were driving hybrids down to Washington to beg for bailouts.

New York City, on the other hand, is still cutting back. Luxury spending was down 7.7% in the quarter. Atlanta was down the most at 18.2%.

By tracking customers’ spending habits, retailers get a bird’s-eye view of tastes as they ebb and flow. Online retailers, in particular, see every click we make. They know which brands we’ve peeked at, how long we pondered, and what we actually purchased. They know the time of day and the days of the week that we shop. They know — and record — our color choices, sizes and tastes so that they can recommend clothes that are in tune with our yearnings.

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Our banks have nearly as much information about our purchasing habits. “We know where the customers live and we can track their behavior back to where they live,” says Ed Jay, senior vice president of American Express’ Business Insights unit, which mines its credit-card data for consumer trends and sells reports to clients. American Express (NYSE: AXP – News) says it doesn’t provide data on individual consumers.

This might seem slightly creepy to pre-Facebook generations who imagine their tastes and habits are private. But all this clicking amounts to a heap of insight into what people are spending on and even what they’re thinking about.

Some of the data confirm regional stereotypes. Southerners bought more white, green, and pink than other regions’ residents, for instance, according to data from private-sale site Hautelook.com, which caters to young, urban professional women. Now I know, too, why I feel like such a loner wearing brown in Los Angeles, where black, white and gray are preferred.

Retailers’ data also bust a few commonly held beliefs. Though Dallas has a flashy, big-spending image, the average woman there spends less on fashion than one in notoriously frumpy Washington, D.C., according to fashion website ShopItToMe.com.

The data also offer a window on populations moving among trendy neighborhoods. ShopItToMe, which notifies members of sales on their favorite brands, observed on its blog recently that New York’s “most conservative” dressers reside on the Upper West Side, which has a reputation for being culturally liberal. Of the site’s more than 600 brands, Upper West Side residents’ favorite is Gap Inc.’s (NYSE: GPS – News) Banana Republic. Meanwhile, residents across town on the Upper East Side favor flashier, more expensive apparel, such as Jimmy Choo shoes.

The website also looks at city-by-city data. The most popular brand in every ShopItToMe city, including Cleveland, St. Louis, Boston, New York and Los Angeles? Victoria’s Secret. Everyone needs underwear.

And despite the fashion press’s obsession with J. Crew, the company is among the top five brands only in New York City and Boston.

Other expectations for the nation only seem to be born out by fashion data — until one looks deeper. When ShopItToMe looked at clothing sizes, the results seemed to confirm what folks say — that women are thinner on the coasts. In New York and Los Angeles, 14% of women selected size 0 tops, compared with only 5% nationwide. ShopItToMe looked at a random sample of 86,225 women who registered between June 2009 and June 2010.

But that’s not necessarily because of a predominance of tall model types. Petite clothing and small shoe sizes were also popular on the coasts, raising the possibility that the women there are just smaller.

Even boredom with fashion appears in demographic data. When the Affluence Collaborative, a research group, asked luxury shoppers to look at a list and choose brands they think are boring, they found that tastes differed by gender. Male luxury consumers with incomes between $75,000 and $199,000 — the biggest group surveyed — said they were bored by Saks Fifth Avenue. Women in the category were bored by Best Buy (NYSE: BBY – News).

United on one front, they were all bored by Dom Perignon.

Fashion trends, of course, don’t exist in a vacuum. As a trend analyst for the Center for Culinary Development, San Francisco-based foods consultant Kara Nielsen sees regional tastes that parallel fashion preferences. Midwesterners favor sophisticated “high-end dining,” but “they’re not swayed so much by trendy things.” The same goes for fashion. Chicagoans are big buyers of Christian Dior, for instance, but according to Hautelook, they also favor North Face and other practical, well-established apparel brands.

Los Angelinos, on the other hand, like things casual and locally grown — in food and denim. And Brooklyn, New York, is increasingly a center for indie fashion, with designers whipping up jewelry in their apartments, selling them at the “Brooklyn Flea” market and wearing home-sewn clothes. In food, says Ms. Nielsen, Brooklyn residents are likewise leading the trends for the area. “They’re making their own pickles, butchering their own meat,” she says. “It’s what I call ‘party like it’s 1899.’ ”

As for Detroit’s spending spree, it comes as little surprise to Karen Daskas, owner of the Tender Birmingham store in a wealthy Detroit suburb. Tender was left with oodles of unsold runway fashions in 2009, when customers told Ms. Daskas “it doesn’t look nice” to shop. But Ms. Daskas now says she has now sold so much that the store is unusually empty and awaiting fall shipments. “We don’t even have three roller-racks of clothes,” she says.

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