There’s the “sneak into basket” technique, where a retailer automatically adds products — like a magazine subscription or travel insurance — to consumers’ shopping carts and makes it hard for them to remove the unwanted items. There’s the “roach motel” or “walled-garden” technique, in which sites offer fast-and-easy sign-up processes but make it much more cumbersome for consumers to close accounts.

There’s also “misdirection,” in which prominent marketing come-ons may distract users from seeing check boxes that by default, say, sign them up for a newsletter or membership, spam their contacts or alter their home pages.

And then there’s scarcity inflation: “Only two hotel rooms left at this price!”

“If it’s real scarcity, that’s fine,” Mr. Brignull says. “If it’s fake scarcity, that’s taking advantage of people’s natural biases. It’s a blurry line.”

Persuasive design is a longstanding practice, not just in marketing but in health care and philanthropy. Countries that nudge their citizens to become organ donors — by requiring them to opt out if they don’t want to donate their body parts — have a higher rate of participation than the United States, where people can choose to sign up for organ donation when they obtain driver’s licenses or ID cards.

But the same techniques that encourage citizens to do good may also be used to exploit consumers’ cognitive biases. User-experience designers and marketers are well aware that many people are so eager to start using a new service or complete a task, or are so loath to lose a perceived deal, that they will often click one “Next” button after another as if on autopilot — without necessarily understanding the terms they have agreed to along the way.