NEW YORK – It’s a retail maxim followed by everyone: Get shoppers in the door and then sell them stuff they didn’t plan on buying.

But impulse purchases require people to browse the aisles, and that’s happening less and less these days because Web-savvy consumers often already know what they plan to buy and simply pick it up and leave, said Bill Martin, the chief executive officer of ShopperTrak.

These mission shoppers visit fewer stores — three per trip, down from five pre-recession, according to ShopperTrak. As a result, foot traffic may fall 2.2 percent during the holiday shopping marathon, says the Chicago-based research firm.

“There’s been a fundamental change in buying behavior,” Martin said.

The shift will hit some chains harder. Electronics stores such as Best Buy are most at risk because it’s so easy to research gadgets online first. Apparel chains like Limited Brands may fare better because even mission shoppers have to stick around long enough to try on clothes.

While retail sales may rise an estimated 2.8 percent this holiday, according to the National Retail Federation, they’d be stronger if more mission shoppers browsed, Martin said.

Retailers are being forced to adapt at a time when they’re closing stores and trying to attract consumers made cautious by a 9.1 percent jobless rate. The rising competitive pressures may accelerate an ongoing shakeout, said David Maddocks, who runs an eponymous consulting firm in Portland, Ore.

“Most retailers are geared for people to come buy,” said Maddocks, a former chief marketing officer at Converse. “Retail has to start gearing itself to sell, to get more out of every person that walks through the door.”

Martin has been analyzing shopping patterns for the past 15 years. Typically, traffic and sales growth rise or fall in tandem. Last year he spotted something new. Even as retail sales grew 3.5 percent, according to the NRF, traffic declined 0.5 percent, he said.

When the trend continued this year, Martin didn’t know how to explain to clients what was happening, so he asked his researchers to dig for answers. They discovered Americans were turning into what he calls “surgical shoppers.”

Men, the original mission shoppers, adopted e-commerce first because it saved time. Faster Web connections and a better shopping experience drew in more women. This has accelerated change in shopping behavior because women outspend men and retailers have long focused their efforts on them.

Retailers pay close attention to the percentage of people who actually buy something after walking in the door. As Americans make fewer trips to the mall, each shopper becomes more valuable and persuading them to buy more shirts, power drills and bedsheets becomes all the more crucial to retailers, says ShopperTrak’s Martin.

Jordan’s Furniture, a chain in eastern Massachusetts and southern New Hampshire owned by Warren Buffett’s Berkshire Hathaway Inc., used 3-D movies, ice cream parlors, laser shows and trapeze lessons to boost traffic.

“They are doing everything they can to get customers to stop and smell the roses,” said Mike Gatti of the NRF.