The Decoy Effect, a psychological trick that can influence what you buy and who you vote for

The Decoy Effect is a simple but powerful trick that marketers use to influence you to buy something that is bigger or more expensive than you need or want. I fall for this every time I go to the movies and think I'm going to buy the medium popcorn but end up getting the large because it costs just a few cents more. The medium popcorn is the decoy that nudges you to buy the large. But social psychologists have also studied the Decoy Effect outside retail environments. From the BBC News:

The decoy effect might also influence our voting in elections, and recruitment decisions. In these kinds of situations, the "decoy" may appear by accident rather than having been deliberately placed in the selection, but if you do come across two candidates who are similar, but one is slightly superior to the other, it will heighten your regard for them compared to the other competitors...

On a more positive note, scientists in the UK have also started to consider whether the decoy effect might be used to encourage people to make healthier life choices. Christian Von Wagner, a reader in behavioural science and health at University College London, for instance, recently explored people's intentions to undergo a vital - but unpleasant - screening for colorectal cancer. He found that given the choice between arranging an appointment for the screening or not having the procedure at all, many people chose not to go. But if he also presented them with a third option - an appointment at a less convenient hospital with a longer waiting time, ie, the decoy - the uptake was greater.

For more on the Decoy Effect and other fascinating cognitive biases, I highly recommend behavioral economist Dan Ariely's fantastic book Predictably Irrational, Revised and Expanded Edition: The Hidden Forces That Shape Our Decisions.