Given a choice between two options, you influence the result by adding a third, inferior, alternative
Shankar Vedantam wrote a Washington Post article and also appeared on NPR to discuss The Decoy Effect: Given a choice between two options, introducing a third, clearly inferior, option can influence your original decision. You won’t pick the third option, but a clever choice of the bad third option can sway the decision toward either of the other two.
Rationally, an inferior third option should have no effect on your choice between the two other options, but psychologists (and marketing majors) have discovered that human beings are not rational decision makers. (Anyone who has interacted with a two-year-old child is already well aware of this.)