Christmas Day, everything is twinkling lights and shiny packages under the tree. We thank Aunt Ida and assure her that we love the cable-knit reindeer sweater and matching socks. We actually believe it is the thought that counts.

Today, reality will set in. Today, we shall return.

Stores are expecting nearly 9% of holiday purchases to be returned, according to a recent survey by the National Retail Federation, compared with an annual rate of about 7%. Many retailers plan to assign staff to do nothing but handle returns today and will set up special lines to handle customers with their bags of unwanted gifts.

“Retailers know that returns are part of doing business,” a vice president of merchandising and retail operations for NRF, Dan Butler, said. “They plan for it.”

But the return is, nevertheless,”one of the most difficult parts of the business,” the author of “Why We Buy: The Science of Shopping,” Paco Underhill, said. It’s hard on both parties involved. Stores have to take back unwanted or defective merchandise that often cannot be resold at full price. They also must balance their desire to keep customers happy with the possibility of the return being a ruse by increasingly active retail-crime gangs. Consumers, meanwhile, have a time-consuming trip to the store for a transaction that is often an emotional minefield of guilt or disappointment.

From the retail point of view, at least, things weren’t always this way. Caveat emptor — buyer beware — was the rule, and customers bore the burden of ensuring that the products they bought were desirable and functional.

Not until Marshall Field founded his famed namesake department store in Chicago in the 1880s did that mind-set begin to change. “Give the lady what she wants” was his motto. And one thing the lady wanted was the ability to refund or exchange purchases that were flawed or didn’t meet her needs, no questions asked.

The concept sparked a revolution. Soon, other retailers began to adopt it, and the liberal return policy that American consumers have taken for granted for most of the 20th century was born.

Recently, however, the pendulum has begun swinging in the other direction as retailers have started tallying their losses from less-than-scrupulous shoppers taking advantage of those policies. The industry expects stores to lose $3.5 billion in “return fraud” this holiday season.

Mr. Butler said some shoppers are “wardrobing” themselves, purchasing items for one-time use — say, prom night or the company holiday party — then returning them. Others bring back stolen merchandise without a receipt and pocket the refund.

More worrisome is the arrival of gangs targeting stores on a large scale. Mr. Butler told of one strike a few years ago in which one thief scanned a valid receipt for $500 in store credit from a national retailing chain into his computer and digitally altered the amount to $1,200. Gang members printed copies and presented them in 16 stores across 12 states within an hour of one another. In response, many retailers changed their return policies. This season, about a quarter told the NRF they are becoming more stringent, particularly for customers without receipts.

“We’ve had 50-plus years of very liberal return policies from many retailers,” Mr. Butler said. “As more and more fraud happens, retailers have had to have policies that are much more exact and specific, especially for areas where the products are in high demand.”

The result is that stores’ procedures vary widely, often creating confusion and unrealistic expectations among shoppers that is exacerbated during the hectic days after Christmas.

“I think that retailers really need to understand that this is not just about volume” of merchandise returned, a principal at management consulting firm Katzenbach Partners, Traci Entel, said. “It’s a very stressful and emotional time for customers.”

For many shoppers, there are complex machinations behind even the most straightforward returns. Shouldn’t your husband realize by now that he has given you the same body butter gift set from Bath & Body Works three years in a row? Shouldn’t your big sister have known that rather than a couple of self-help books and Shania Twain’s Greatest Hits CD, what you really wanted was some cute sweatpants from Victoria’s Secret? And did you really hurt your mother’s feelings when you bought her a size 14 blouse and she needs a 10?

Mr. Underhill said the psychology behind returns runs deep. Bringing back a gift means someone misunderstood us. Acknowledging that can be difficult — both for the giver and the recipient.

“Christmas is about buying the icons of our relationship to somebody else. We’re often trying to buy gifts that tell something that we don’t have the courage to tell them to their face: ‘I love you, I’m proud of you, I support you,'” Mr. Underhill said. “The return is the emotional failure of somebody to have truly thought through what somebody’s needs are.”

Shoppers walk into stores in the days after Christmas carrying enormous amounts of baggage. They also often bring a sense of entitlement — sometimes rightfully so, sometimes not — cultivated over the generations since Field popularized his philosophy of customer service.

Consumers increasingly have the upper hand in their relationships with retailers, Mr. Underhill said. Shoppers are savvier than ever before, researching price and product quality on the Internet before setting foot in a store. They are used to retailers competing for their money and they eschew brand loyalty in their quest for the best values.

“There are many shoppers that feel totally empowered,” Mr. Underhill said. “They want the lowest price with the best possible service. They recognize that they have so many places to shop and that the merchant is desperate for their business. They feel that they have some leverage.”

Dissatisfied consumers frequently take out their anger on retailers. In a survey by Katzenbach Partners, 62% of shoppers said they would not go back to a store where they received poor customer service — a term that is synonymous with returns during the holiday season, Ms. Entel said.

Harold Li, 41, of Frederick, Md., is in that group. He recently mailed back a prepaid cell phone he bought from a small online retailer only to be told there was no record of his return, he said. The company has refused to reimburse him or replace the phone. Mr. Li said he is so livid that he is considering filing complaints with several consumer protection agencies.

“I never encountered any merchant like that,” he said. “I don’t remember anything as bad as this.”

Ms. Entel recommended that retailers look at returns not as a burden but as an opportunity to fix what went wrong. She said it often doesn’t take much effort.

“Customers don’t expect to be wowed during a return process,” Ms. Entel said. “They expect to get the basics right.”