Imagine choosing between two drinks.
Small: $3.
Large: $6.
You are not sure which one is the better deal.
Then a third option appears.
Medium: $5.75.
Suddenly the large drink looks excellent.
For only twenty-five cents more than the medium, you get the biggest size.
This is a classic example of the decoy effect.
We judge choices comparatively
People do not evaluate every option in isolation.
We compare.
When a third choice appears, it changes the comparison among the original options.
The decoy is designed to be clearly worse
A strategically chosen decoy is usually similar to one option but inferior in an obvious way.
The $5.75 medium makes the $6 large look strong.
The large now dominates the comparison.
Does anyone need to buy the decoy?
No.
The decoy can influence decisions even if almost nobody chooses it.
Its job is not necessarily to sell.
Its job can be to reshape how the other options look.
Where can this show up?
Restaurant sizes.
Subscription plans.
Electronics.
Insurance packages.
Membership levels.
Any menu of options where one choice can make another look clearly superior may create a decoy effect.
Is this the same as anchoring?
Not exactly.
Anchoring uses a reference number to influence judgment.
The decoy effect changes the structure of the choices themselves.
Both rely on comparison, but they do it differently.
Are people helpless against it?
No.
A decoy does not force a decision.
You can reduce the effect by deciding what features and price matter to you before comparing the options.
Ask:
“Would I still want the large if the medium were not here?”
Sometimes one option’s main job is to stand there looking bad enough to make another option look good.
A related piece of human behavior appears in Why does an expensive option make another price seem reasonable?, where the brain handles another everyday judgment or response.
And that’s the short explanation.
Related explanations: Why does an expensive option make another price seem reasonable?, Why does the first price we see affect what seems reasonable?, and Why can having more choices make choosing harder?