Scarcity principle
http://www.nngroup.com/articles/scarcity-principle-ux/
The Scarcity Principle is 1 of 6 influencing principles coined by Dr. Robert Cialdini, a professor at Arizona State University famous for his 1984 book “Influence: The Psychology of Persuasion.” Cialdini’s book is a study of the psychology of compliance. As a psychology professor, he and his students conducted numerous research studies to identify and prove the 6 influencing principles discussed in his book.
Scarcity is largely effective because of a cognitive bias known as loss aversion, first demonstrated in research conducted by Daniel Kahneman and Amos Tversky. Kahneman and Tversky determined that people put more subjective value on loss than on gain and thus strongly prefer to avoid losses instead of acquiring gains. In other words, the pain of losing $100 is perceived as bigger (in fact, twice as big according to Kahneman and Tversky) than the satisfaction of gaining $100.
The strong tendency to avoid losses explains why scarcity is so effective: we feel that if we don’t act upon the scarce product or information, we lose it. For example, if a person is shopping for airline tickets and finds a flight that meets her criteria, but the description indicates this is the last ticket at that price, the person might buy the ticket for fear of losing out even if she wasn’t quite ready to book yet. (The agony of losing the current $100 discount will only be overcome if the user could save $200 on some future deal, and that’s sufficiently unlikely to happen that many users prefer to act to lock in the savings.)
Scarcity is a particularly effective persuasion tool because what, specifically, is scarce can take several forms: quantity, time, or information. Knishinsky ran an experiment that proved the additive effects of these factors upon persuasion. In his study, salesmen were able to double order sizes when they told wholesale beef buyers that the supply would be scarce in upcoming months. The most incredible finding of this study is that they were able to increase order sizes 6 times when they altered their pitch to not only indicate scarcity of supply, but that this information was a secret and only being divulged to the buyer. The double-scarcity pitch (low supply, secret information) was 3 times more compelling than the single-scarcity pitch (low supply)!
Robert B. Cialdini, Influence: Science and Practice. Pearson Education Inc., 2009.
Daniel Kahneman, Thinking, Fast and Slow. Farrar, Straus and Giroux, New York, 2011.