You’re on the checkout page of a website when something appears in your shopping basket that you didn’t put there — say, you’re buying a laptop, but insurance sneaks in, too, unless you uncheck a box. Or you think you’re signing up for a one-time deal or even a free trial, but you’ve unintentionally committed to a subscription with recurring fees.

Or maybe your relaxed online browsing session takes on an air of urgency because a countdown timer tells you that a hot deal is about to expire, or the site gives you an “almost out of stock” warning.

Not all of these “nudges” are flat-out deceptive. But all are instances of dark patterns, or website designs meant to steer, coerce or deceive people into making unintended or potentially disadvantageous decisions.

Dark patterns have been a topic of discussion among techies, behavioral scientists and marketers since 2010, when British cognitive scientist Harry Brignull coined the term. His website, www.darkpatterns.org, serves as an information clearinghouse and a dark pattern Hall of Shame. A new study from Princeton University researchers documents how ubiquitous dark patterns can be and explains why we fall for them.

Using a web crawler, the Princeton researchers investigated more than 11,000 shopping websites. They found 1,841 dark patterns on more than 11% of the sites, or more than 1,200 of them. The researchers identified 15 patterns in seven broad categories, including sneaking, urgency and misdirection. The more popular the website, the more likely it was to feature dark patterns.

The most egregious examples, deemed “deceptive” by the researchers, were relatively rare: only 234 instances on 183 websites. The authors note, however, that their limited study no doubt understates the prevalence of dark patterns.

We fall for the trickery because of innate cognitive biases. We might go ahead with a transaction even after a previously undisclosed charge gets added at the last minute because of a bias known as the sunk-cost fallacy — the feeling that we’ve invested too much time, energy or money to turn back. And something known as the framing effect makes confirm shaming a go-to dark pattern for some sites. You’ve seen it when a shopping site promises a discount in exchange for your e-mail address. If you don’t want to give it out, you’re forced to click on something like “No thanks, I like paying full price.”

Once aware of dark patterns, we tend to adapt. Some 65% of British consumers in a recent survey pegged instances of dark patterns on a hotel booking site as the sales pressure they are. But the frontiers keep expanding.

You’ll find dark patterns on mobile apps, video games and social media platforms, too. Congress has taken notice: A bill introduced in the Senate in April would empower the Federal Trade Commission to crack down on dark patterns.

Anne Kates Smith is executive editor at Kiplinger’s Personal Finance magazine. Send your questions and comments to moneypower@kiplinger.com. And for more information on this topic, visit Kiplinger.com.