In what year was George Washington elected President? What is the freezing point of vodka? Few people know the answers to these questions, but most can arrive at a reasonable estimate by tinkering with a value they know is wrong. Most know that the United States declared its independence in 1776, so Washington must have been elected sometime after that. And most people also know that alcohol freezes at a lower temperature than water, so vodka must freeze at something colder than 32°F. To answer questions like these, in other words, people may spontaneously anchor on information that readily comes to mind and adjust their responses in a direction that seems appropriate, using what Tversky and Kahneman (1974) called the anchoring and adjustment heuristic. Although this heuristic is often helpful, adjustments tend to be insufficient, leaving people's final estimates biased toward the initial anchor value.

To examine this heuristic, Tversky and Kahneman (1974) developed a paradigm in which participants are given an irrelevant number and asked if the answer to a question is greater or less than that value. After this comparative assessment, participants provide an absolute answer. Countless experiments have shown that people's absolute answers are influenced by the initial comparison with the irrelevant anchor. People estimate that Gandhi lived to be roughly 67 years old, for example, if they first decided whether he died before or after the age of 140, but only 50 years old if they first decided whether he died before or after the age of 9 (Strack & Mussweiler, 1997).