Definition
The decoy effect is a psychological pattern: when you're torn between two options and a third option that is worse than one of them in every way (the decoy) shows up, that one option suddenly looks much more attractive and gets chosen more often. Sellers use this on purpose, placing a decoy right next to the product they really want to sell. In technical terms, it is also called the "asymmetric dominance effect."
Why do we pick the large popcorn instead of the medium?
Think about ordering popcorn at the movie theater snack counter. If the small is 5,000 won and the large is 7,000 won, you'd probably wonder, "Is it worth paying 2,000 won more for the big one?"
But then a 6,500 won medium slips into the lineup, and everything changes. The moment you see the medium, a thought pops up: "For just 500 won more, I can get a much bigger one?" The large starts to feel like an overwhelmingly good deal.
The medium popcorn was never meant to sell in large numbers. It's just a decoy tossed in to make the large look great. Shoppers think they made a smart choice on their own, but they were steered, quite naturally, by a setup the seller built.
A bit more precisely: how asymmetric dominance works
Behavioral economics calls this decoy an "asymmetrically dominated option." Put simply, the decoy is worse than one particular product in every way, yet it's designed so that it is hard to compare with the other product.
In the popcorn example, the medium holds much less than the large but costs only 500 won less, so it effectively loses. A strict decoy is the same price or even more expensive, so it loses on every front. But even when it only nearly loses like this, a similar effect shows up. When a clearly inferior comparison appears right in front of you next to one option, your brain stops doing the complicated math and instantly sees the better of the two as the superior product.
The human brain doesn't like spending a lot of energy working out absolute value. Instead, it prefers to simply compare the two easiest things in front of it. The decoy does exactly this job: it hands your brain an easy yardstick.
How to spot and avoid everyday decoys
The decoy effect isn't limited to movie theater popcorn. It's used all the time with subscription services and in electronics stores. For example, if an e-book subscription is listed as "mobile only 9,900 won," "PC only 14,900 won," and "all-access pass 15,000 won," it's easy to lean toward the all-access pass.
To avoid falling into this kind of marketing trap, ask yourself a question before you choose: "What amount or feature did I want in the first place?" The key is to look back at your original need.
If what you needed was one small cup of popcorn, then getting the large for a 500 won difference isn't a bargain. It's just 2,000 won of needless spending. It helps to practice anchoring your comparison on your own original purpose, not on the options the seller laid out.
Common misconceptions
The decoy effect is only used to trick people into buying bad, low-quality products.
It isn't fraud about quality. It's a psychological design that places the prices and terms of three perfectly good options in a way that steers shoppers toward one particular product.
Where you meet it
At the movie theater, you choose the large popcorn from a small (5,000 won), a medium (6,500 won), and a large (7,000 won).
A newspaper lists "web subscription 50,000 won," "print paper 120,000 won," and "web + print bundle 125,000 won" to steer readers toward the bundle.
In one sentence
It's a psychological pricing strategy that slips in an unattractive third option to make a particular product look more reasonable.