Add in the online-coupon phenomenon, which feeds the psychological hunger for finding impossible bargains, and you've got a recipe for trouble, said Theresa Williams, a marketing professor at Indiana University.

``These are people who should know better and have enough stuff already,'' Williams said. ``What's going to be next year, everybody getting Tasered?''

Across the country on Thursday and Friday, there were signs that tensions had ratcheted up a notch or two, with violence resulting in several instances.

A woman turned herself in to police after allegedly pepper-spraying 20 other customers at a Los Angeles-area Walmart on Thursday in what investigators said was an attempt to get at a crate of Xbox video game consoles. In Kinston, North Carolina, a security guard also pepper-sprayed customers seeking electronics before the start of a midnight sale.

In New York, crowds reportedly looted a clothing store in lower Manhattan's Soho district. In Arizona, at a Walmart near Phoenix, a man was bloodied while being subdued by police officer on suspicion of shoplifting a video game. There was a shooting outside a store in San Leandro, California, shots fired at a mall in Fayetteville, North Carolina, and a stabbing outside a store in Sacramento, New York.

``The difference this year is that instead of a nice sweater you need a bullet proof vest and goggles,'' said Betty Thomas, 52, who was shopping Saturday with her sisters and a niece at Crabtree Valley Mall in Raleigh, North Carolina

The wave of violence revived memories of the 2008 Black Friday stampede that killed an employee and put a pregnant woman in the hospital at a Walmart on New York's Long Island. Walmart spokesman Greg Rossiter said Black Friday 2011 was safe at most of its nearly 4,000 U.S. stores despite ``a few unfortunate incidents.''

Black Friday _ named that because it puts retailers ``in the black'' _ has become more intense as companies compete for customers in a weak economy, said Jacob Jacoby, an expert on consumer behavior at New York University.

The idea of luring in customers with a few ``doorbuster'' deals has long been a staple of the post-Thanksgiving sales. But now stores are opening earlier, and those deals are getting more extreme, he said.

``There's an awful lot of psychology going on here,'' Jacoby said. ``There's the notion of scarcity _ when something's scarce it's more valued. And a resource that can be very scarce is time: If you don't get there in time, it's going to be gone.''

There's also a new factor, Williams said: the rise of coupon websites like Groupon and LivingSocial, the online equivalents of doorbusters, which usually deliver a single, one-day offer with savings of up to 80 percent on museum tickets, photo portraits, yoga classes and the like.

The services encourage impulse buying and an obsession with bargains, Williams said, while also getting businesses hooked on quick infusions of customers.

``The whole notion of getting a deal, that's all we've seen for the last two years,'' Williams said. ``It's about stimulating consumers' quick reactions. How do we get their attention quickly? How do we create cash flow for today?''

To grab customers first, some stores are opening late on Thanksgiving Day, turning bargain-hunting from an early-morning activity into an all-night slog, said Ed Fox, a marketing professor at Southern Methodist University in Dallas. Midnight shopping puts everyone on edge and also makes shoppers targets for muggers, he said.

In fact, robbery appeared to be the motive behind the shooting in San Leandro, about 15 miles (24 kilometers) east of San Francisco. Police said robbers shot a victim as he was walking to a car with his purchases around 1:45 a.m. on Friday.

``There are so many hours now where people are shopping in the darkness that it provides cover for people who are going to try to steal or rob those who are out in numbers,'' Fox said.

The violence has prompted some analysts to wonder if the sales are worth it, and what solutions might work.

In a New York Times column this week, economist Robert Frank proposed slapping a 6 percent sales tax on purchases between 6 p.m. on Thanksgiving and 6 a.m. on Friday in an attempt to stop the ``arms race'' of earlier and earlier sales.

Small retailers, meanwhile, are pushing so-called Small Business Saturday to woo customers who are turned off by the Black Friday crush. President Barack Obama even joined in, going book shopping on Saturday at a small bookstore a few blocks from the White House.

``A lot of retailers, independent retailers, are making the conscious decision to not work those crazy hours,'' said Patricia Norins, a retail consultant for American Express.

Next up is Cyber Monday, when online retailers put their wares on sale. But on Saturday many shoppers said they still prefer buying at the big stores, despite the frenzy.

Thomas said she likes the time with her sisters and the hustle of the mall too much to stay home and just shop online.

To her, the more pressing problem was that the Thanksgiving weekend sales didn't seem very good.

``If I'm going to get shot, at least let me get a good deal,'' Thomas said.