Choices and persuasion
Decoy effect
Also called Asymmetric dominance
Adding an option that's clearly worse than one of the others makes that one look better, and shifts people towards it.
People find it hard to judge an option on its own and easy to judge it against something similar. A third option that's nearly the same as one choice but plainly worse makes that choice look like a bargain.
It's common in pricing: a middle plan that costs almost as much as the top one, so the top one looks generous. Know it's there when you read your own pricing page, and when you test it.
In practice
A subscription offered as online only for $59, print only for $125, or print and online for $125: almost nobody picks print only, but its presence pushes people to the combined plan.
Common mistake
Building plans nobody should want purely to steer people. If a plan exists only as a decoy, customers who notice stop trusting the rest of the page.
Where it comes from
Described by Joel Huber, John Payne and Christopher Puto in 1982. Dan Ariely's subscription example in Predictably Irrational (2008) made it widely known.