A city planner was evaluating two park designs. Design 1 included a playground and a walking trail for two million dollars. Design 2 included those features plus a small lake for three million. The choice was genuinely difficult - the first was well within budget, the second offered more but at meaningful additional cost.
Then a third proposal arrived. Design 3 offered the lake but omitted the trail, at two million eight hundred thousand dollars. It had fewer features than Design 2 and cost more than Design 1. It was dominated by Design 2 in every relevant dimension.
The planner approved Design 2.
Design 3 had never been a real contender. But it changed the outcome.
What It Is
The decoy effect is the tendency for a clearly inferior third option - the decoy - to shift people's preference toward one of the original options that dominates the decoy.
When two options are presented, choosing between them requires weighing real trade-offs: one is cheaper, one has more features. The decoy introduces a third option that is worse than one of the originals in every way - making the comparison with that original straightforwardly favourable. Adding a dominated third option tends to increase the likelihood of choosing the option that dominates the decoy.
The decoy does not need to be attractive. It needs to make one of the other options look comparatively excellent.
The Mechanism
The decoy shifts perceived trade-offs without changing the actual values of the original options.
Before the decoy appears, choosing between two options requires tolerating genuine uncertainty: this option is cheaper, but that one has more. After the decoy appears, one of the options can now be evaluated as clearly better than at least one alternative - and the ease of that comparison makes it feel like the right choice overall.
The decoy tends to make the dominating option appear more attractive relative to the decoy, simplifying the decision. In a direct comparison with the decoy, the dominating option wins cleanly. That win feels more salient than the messier comparison with the other original option, and it biases the overall judgment.
Where It Shows Up
Pricing and subscriptions. A classic demonstration involves offering three subscription tiers where the middle tier is dominated by the top tier (same or higher price, fewer features). The dominated middle tier serves as a decoy, driving choices toward the top tier that would otherwise seem expensive compared to the basic option.
Product line architecture. Retailers and manufacturers often include a product configuration that is deliberately positioned to make a more profitable option look like better value. The configuration does not sell; it frames.
Institutional decision-making. Design evaluations, policy proposals, and budget decisions often involve a third option that appears to be a serious contender but functions to make one of the other options feel like the obvious choice.
What the Decoy Effect Is Not
The effect does not require the decoy to be identical to either original option - it needs to be inferior in all relevant dimensions to one and only partially inferior to the other. It does not operate only across price; it works across any attribute dimension - quality, features, risk, time.
Adding any third option does not produce the effect. The third option must be specifically dominated to create the shift. A genuinely competitive third option changes the decision framework entirely, rather than nudging within it.