We compare things to each other, but we also tend to focus on things that are easily comparable and avoid those that are not. For example, if you have two traits, A & B, and you have three objects, two of which vary along the dimension of A (an “A” and an inferior “A-”). People will tend to pick the “A” version without comparing the version that varies positively along the B trait. In essence, introducing (-A), the decoy, creates a simple relative comparison with (A), and hence makes (A) look better, not just relative to (-A), but overall as well. As a consequence, the inclusion of (-A) in the set, even if no one ever selects it, makes people more likely to select A1.
1. Ariely, D. D. Predictably Irrational, Revised and Expanded Edition: The Hidden Forces That Shape Our Decisions. (Harper Perennial, New York, NY, 2010).