Also called the 'asymmetric dominance effect' (don't ask me why), this is when you are unwittingly manipulated by the choices presented by others.
You may have wanted to hire John because he was substantially smarter than Bill, but the boss liked Bill so he made you interview someone just as smart as John, but who also has a stutter or bad breath. Pretty soon you're thinking, Bill actually looks pretty good, come to think of it, I agree with the boss.
The decoy effect is how estate agents get you to buy the more expensive flat by offering you a third, inferior, alternative. It's an essential tool for politicians (throw in a dud and you change your vote) and retailers wanting you to buy the most expensive item.
Next time your company is asked to be part of a beauty contest, watch out. You may be the decoy.
Contact Helen at firstname.lastname@example.org