Once upon a time, Cyber Monday was just another ambitious shopping holiday.

This year, Cyber Monday is expected to set a new online sales record of $9.4 billion, up nearly 19% on last year, according to data from Adobe Analytics ADBE, +2.58% . In 2019, more than half of our holiday shopping takes place online. And internet holiday sales are booming, too: Deloitte estimates they’ll jump 14% to 18% this year alone.

There’s so much to like about online shopping. You can do it quickly and easily from your home or office. You don’t have to drive anywhere. You don’t have to face any crowds.

But the story isn’t all eggnog and holly.

E-commerce sites are using a growing number of sophisticated tricks to part you from your money, experts say. A Federal Trade Commission spokesman said it was empowered by law “to prevent unfair methods of competition and unfair or deceptive acts or practices in or affecting commerce.”

A recent study conducted by Princeton economists found that around 11% of popular shopping websites use these “dark patterns,” tricks designed to get you to spend more than you wanted. It looked at over 50,000 product pages and more than 10,000 online stores,

Do you think you’re too clever to be swayed by such tricks? You might want to think again. Lior Jacob Strahilevitz, a law professor at the University of Chicago, and Jamie Liguri, who recently graduated from the law school and now clerks for U.S. District Judge Brenda Sannes in New York, recently revealed the results of an experiment they conducted into online-sales manipulation.

In the latest study on online retailers, nearly 2,000 participants took part, and were exposed to various sales techniques. Bottom line: Mild manipulation more than doubled the number who ended up buying a product. Aggressive manipulation nearly quadrupled the number.

Of those exposed to the online hard sell, 42% ended up forking over their money. But when people were given a softer sell, just 11% decided to buy. And just to prove how powerful this is, the researchers tried tripling the cost of the product in some of the experiments. It made no difference.

Watch out for the tactics used by some online retailers this holiday season:

1. 11th-hour fees

Check that shopping basket a second time before you click “buy.” Some hit you with extra fees — like hotel “resort fees,” or unwanted insurance — on top of what you expected. Or they may sneak you into subscription or an automatic renewal plan you don’t want. You may turn down a discounted 12-month subscription, only to find your more expensive monthly plan comes with “automatic renewal” hidden in the fine print.

2. ‘You might like this!’

Fishermen dangle bait in the water to get the fish to swallow the hook. Some online retailers operate on the same lines. When you buy product A, they use that opportunity to nudge you into buying something else as well, like a subscription to a service. “Buy this? You might like these!” Then you are presented with a list of tempting items from the same designer or category. It’s known as “cross-selling.” Be sure to check the fine print, and work out exactly how much it’s all going to cost.

3. Sleight of hand

Like magicians, websites can dazzle and distract. They may use bigger print or brighter colors for the more expensive option. They can “pre-select” their preferred option. Or they can use trick questions. “Are you sure you want to cancel your membership? Continue / Cancel.” In one case, the report found that clicking “cancel” meant you wanted to continue your membership, and clicking “continue” means you wanted to cancel.

4. ‘One one item left!’

Some websites will try to hustle you into making a purchase by saying the offer is about to expire. Others will tell you they’re about to run out of the product. The message: Hurry, hurry, hurry! It’s one of the oldest high-pressure sales techniques. Tune it out. Studies have found that we are more likely to make bad purchases when we’re under pressure.

5. Good luck canceling

Some websites deliberately try to make it as hard and time-consuming as possible for you to act prudently. They’ll hide final prices behind multiple web pages, so that it becomes hard to compare their prices with their competitors. Or they’ll make it easy as possible to sign up for a service, but as hard as they can to cancel.

6. They know what you’ve browsed

It costs a company nothing to have software ask you repeatedly if you want to buy something, even after you’ve already turned it down. It costs you time and effort to keep clicking “no.” And they only need you to change your mind once. Watch out that online retailers don’t nag you into giving in. Did you click on an item on eBay EBAY, +1.35% ? You may receive an email to say it’s now discounted. Think twice before being tempted by emails that say the produce you looked at is suddenly on sale.

7. Bogus popularity

Who cares if a bunch of total strangers say they liked this product, or if 35 “other people” are “looking at this right now?” Chances are, you do. So-called “social proof” has always been a powerful sales technique. But there is no way of knowing if the reviews are real, or if those 35 other people actually exist. Don’t let yourself get hustled by an anonymous online crowd.

The Internet Merchants Association, eBay and Amazon AMZN, +2.49% were not immediately available for comment. For their part, online retailers have long said they’re merely doing what brick and mortar retailers have been doing for years. They are also marketing their wares and refining their advertising in a way that physical retailers can’t ordinarily do, arguably reducing the amount of irrelevant spam.

Case in point: Many brick and mortar retailers pile on the accessories for products, and have cheap treats at the entrance to get consumers to “open their wallets” and sweet treats at the cash register to reward them for their shopping with tempting candy. Supermarkets often stock items that you don’t need at eye level, while you may have to reach for those everyday necessities.

They also offer reusable shopping bags or large trolleys. The bags sometimes even bear the company’s logo, providing free advertising for the store. (IKEA’s blue bags are instantly recognizable even without a logo.) But mainly the bags create a void that needs to be filled, consumer psychologists say.