The Scarcity principle was discovered by scientists Worchel, Lee and Adewole in 1975. They conducted an experiment that simply used a jar full of cookies and another that was almost empty; they found that people overwhelmingly tended to desire a cookie from the jar that was almost empty simply because of the scarcity effect. This effect also generates an urgency to have the scarce product before it is gone entirely.
This principle is explained by the idea that the more difficult or urgent it is to acquire an item, or the more easily it might be lost, the more value that item has in our minds. Scarcity is associated in our brains with something positive, luxurious and exclusive as we automatically assume that it is scarce because everyone wants or has already bought this product and therefore it must be a good product. In other words, scarce objects arouse our interests and so immediately become more desirable than a product that is readily available.
Many brands make use of this through the way they market their products: by offering limited edition products, flash sales or only producing something in limited supply, brands are able to stimulate demand. For example, just one day after Apple launched the last iPhone, stocks had already sold out, which in turn made even more people want to buy it because it generated the scarcity effect and automatically branded it as a luxury item that people were willing to queue for hours to have.