Customers rush Potomac Mills Mall at the start of last year's Black Friday shopping. (Richard A. Lipski/For The Post)

When Jane Boyd Thomas and Cara Peters, two professors at South Carolina’s Winthrop University, interviewed subjects for a study on Black Friday, almost all the shoppers said they started their days before 9 a.m., and most spent at least three hours hunting down bargains. But none matched the tenacity of one woman, a 40-year-old named Tracy, who had been a Black Friday shopper for 18 years. Starting her day at midnight on Thanksgiving, she spent the next 16 hours shopping.

“For the person who’s been doing this for decades, this is as much of their Thanksgiving tradition as having turkey,” Thomas said. “That’s why they’re going to endure lines and probably even thrive in the lines.”

The National Retail Federation estimates 152 million people will shop between Friday and Sunday after Thanksgiving, up from the 138 million last year. That means nearly half of Americans will lose sleep, crush into stores and wait in eternal lines in order to take part in holiday shopping. But far from being mass synchronized temporary insanity, the Black Friday ritual has distinct psychological underpinnings.

Other than the chance to get a smart phone for a cent, here’s a look at why we all shop on the same day:

1) The crowds make us happy

First, a disclaimer: The vast majority of the subjects in these studies are women. Not only do women do the overwhelming majority of holiday gift-buying, Thomas said, the stereotypes about men and shopping held true for the researchers.

“The only time men were present, they were sort of just tagging along,” Thomas said.

Sang-Eun Byun, an assistant professor of consumer affairs at Auburn University in Alabama, surveyed hundreds of shoppers at Zara and H&M and found that the limited availability of goods in those stores excited the customers. Even though it wasn’t Black Friday, she said her findings hold true for any shopping situation in which high-value goods are scarce.

Ordinarily, Byun said, shoppers are turned off by crowds. But when crowds create a sense of competition — such as when hundreds of shoppers are rushing to collect marked-down goods — they generate a different feeling entirely. Competition creates what’s called hedonic shopping value, or a sense of enjoyment from the mere process of buying goods.

“At certain levels, consumers enjoy arousal and challenges during the shopping process,” Byun said. “They enjoy something that’s harder to get, and it makes them feel playful and excited.”

Byun said retailers can extend this feeling past the holiday shopping season by simply limiting the availability of sought-after products.

“Even small retailers can exercise this scarcity principle,” Byun said. “They can create a promotional strategy that has a high value for a limited time.”

Hence those two words ubiquitous to every car commercial: “Act now.”

2) Those who plan, push

Thomas’s research found that most women scoured newspaper, Internet and TV ads for the best Black Friday deals and planned their days extensively with their co-shoppers ahead of time. They then strategically mapped out their routes to hit the best deals first.

Surprisingly, thorough planning was also correlated with another common Black Friday phenomenon: rowdiness among customers. Sharron Lennon, a fashion studies professor at the University of Delaware, found that people who committed acts of “consumer misbehavior,” like pushing, shouting or grabbing merchandise from another person’s cart, were also more likely to have planned their Black Friday expedition.

“I think it’s because it sets up their expectations, and when they actually get in the stores and find out there are not very many of the promoted items still available, they might be disappointed,” Lennon said. “People who have their goals blocked are more prone to act out on Black Friday.”

According to another of Lennon’s studies, 18 out of 25 Black Friday retailers interviewed reported seeing some sort of “consumer misbehavior” on the day.

3) We love the hunt

Black Friday is “hunting for women,” said Leisa Reinecke Flynn, professor of marketing and fashion merchandising at the University of Southern Mississippi. “It’s so much like deer hunting it’s hard to tell the two apart.”

Both activities hinge on long-standing traditions and involve pursuing a goal as a group. Whether the group actually hits its target is secondary to the fun of the chase.

Does Flynn ever shop on Black Friday?

“I wouldn’t go anywhere on Black Friday,” she said. “But then again I’m not a hunter.”

Thomas likens the process to a marathon, in that a long-distance runner is energized by the grueling trek in much the same way a Black Friday shopper thrives on long lines and frenzied grabs at cashmere sweaters.

“They’re pumped much like a marathoner is pumped by the day of the race,” she said.

Thomas also found that her subjects loved to swap stories and show off their prizes at the end of the day. “It’s ‘mission accomplished,’” she said. “You brag about your great deal, or about how you got the last one.”

4) It’s about togetherness

Thomas and Peters’s research found that Black Friday shopping combines elements of both traditional shopping and holiday rituals. Shoppers planned extensively for Black Friday — as they would for a holiday meal — and relish the day in part because it allows them to spend time with close friends and family.

Thomas notes that Black Friday shopping rituals fall in line with the theory of the “collective self,” the idea that shopper’s identity is shaped by the other shoppers around her. Thomas’s research shows that collectivists like to shop for recreational purposes and are not confused by having too many choices.

Tracy, the 16-hour uber-shopper, told the researchers:

“While we’re waiting in line we usually tell funny stories or help each other get our thoughts together. So it doesn’t really feel like you’re waiting in line forever because you’re not by yourself. It’s about being together.”

It also helps ensure robust sales for retailers for years to come. As the study notes, “sharing the shopping ritual with family members and indoctrinating children helps to ensure that the ritual is continued in the next generation.”

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