In Hooked, Nir Eyal explores the relationship between human psychology and product design. He argues that the most successful products are those that cater to intrinsic human needs, instincts, and biases. Throughout his work, Eyal refers to scientific studies which reveal different phenomena in human psychology, and how these aspects of cognition can be successfully exploited by advertising or product design.

For instance, Eyal describes how in 1975, researchers investigated how the quantity of an item influenced people’s perception of its worth. They found that people tended to value scarce items as more valuable than those which were plentiful. He explains: “The appearance of scarcity affected their perception of value […] The group left with only two cookies rated them to be more valuable, while those experiencing sudden abundance by going from two to ten actually valued the cookies less” (85).

Eyal refers to this as the “scarcity effect.” He argues that this heuristic is also highly relevant to people’s online behavior. Online shopping sites often employ a tactic that depends on the scarcity effect: They announce that there are few items left in stock, thereby making that product seem more appealing and valuable to the consumer. Eyal uses his own experience of shopping on Amazon as an example of this tactic at work.