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.)