The assessment of any given option in a set of options does not occur within a vacuum. The choices available have an impact because our preferences are not absolute, they are relative. One can manipulate a decision maker’s selection from two options with the insertion of a third ‘decoy’ option. This decoy serves as a similar but importantly less attractive alternative to the target option (that which we want the agent to select).

This third option must be “asymmetrically dominated” which means it must be inferior in all senses to the target option, but superior in some dimensions and inferior in others as compared to the other option. An example would be a pack of 6 2-ply toilet rolls at £4 and a pack of 8 3-ply toilet rules at £6. The addition of a decoy pack of 6 3-ply toilet rolls at £6 could be the necessary clincher to get a customer to now see the 8 higher quality rolls as good value for the money.

This effect can in some sense be seen in politics. In the US electoral system much is made of the danger of an independent stealing votes from a more viable democratic or republican candidate. Within the primaries though there can be dynamic wherein of one of two similar candidates (A and B) would jump ahead with the presence of a third candidate (C). If C were similar but entirely inferior to A, and similar but not clearly inferior or superior to B, candidate A may well come out looking like the best option of the 3.