The Decoy Effect

Shankar Vedantam has written an article in  Washington Post about the decoy effect.

What the decoy effect basically shows is that when people cannot decide between two front-runners, they use the third candidate as a sort of measuring stick. If one front-runner looks much better than the third candidate, people gravitate toward that front-runner. Third candidates, in other words, can make a complicated decision feel simple.

The Wikipedia page gives an interesting example of MP3 players to illustrate the effect, while the article in the Washington Post sites the examples of choosing a restaurant, and a politican.