Daniel Kahneman, answered from the texts and cited to the page.
The anchoring effect is one of the most reliable findings in experimental psychology, and the story of how Amos and I understood it — and disagreed about it — is worth telling carefully. The basic phenomenon: when people are asked to estimate an unknown quantity, any number they have recently considered pulls their estimate toward it.1 We demonstrated this most starkly with a wheel of fortune.
Subjects watched it spin and land on either 10 or 65, then estimated the percentage of African nations in the UN. Those who saw 10 gave an average estimate of 25%; those who saw 65 gave 45%.2 The wheel was plainly uninformative — there was no way to describe its influence as reasonable — and yet the influence was there. That experiment appeared in our 1974 Science paper and remains one of the best-known results we reported.3
The adjustment mechanism is the one Amos favored as an explanation: you start from an initial value — whether given to you or produced by a partial computation — and you move from it until you feel you have reached a plausible answer.4 The trouble is that people stop too soon, still inside the range of uncertainty rather than at its edge, so the final estimate remains biased toward where they started.5
A driver coming off the highway onto city streets adjusts down from highway speed, but not enough; a teenager turning down loud music to satisfy a parent adjusts down from a high anchor, and the parent's complaint that the effort is insufficient is, statistically speaking, correct.6 The multiplication experiment from the Science paper shows the same mechanism operating on incomplete computation.7
High school students had five seconds to estimate either 8×7×6×5×4×3×2×1 or the same sequence ascending. They performed a few steps and extrapolated. Because the descending sequence yields a higher product in its first steps, those students produced a higher estimate — a median of 2,250 against 512 for the ascending sequence. The correct answer is 40,320.
Both groups underestimated badly, and the direction of the bias followed the anchor produced by the partial calculation.8 Amos and I never settled our disagreement about the underlying psychology, and it was resolved only after his death by researchers who had worked closely with him — Eldar Shafir and Tom Gilovich among them.9 The resolution was that we were both right.
Anchoring operates through two distinct mechanisms: the deliberate adjust-and-anchor process Amos favored, which is a System 2 operation, and a priming effect that operates automatically through System 1.10 The wheel-of-fortune result is best understood as the latter; the adjustment account fits cases where people consciously work outward from a starting point and stop too soon.
It occurs when people consider a particular value for an unknown quantity before estimating that quantity. What happens is one of the most reliable and robust results of experimental psychology: the estimates stay close to the number that people considered.Thinking, Fast and Slow
The average estimates of those who saw 10 and 65 were 25% and 45%, respectively.Thinking, Fast and Slow
Amos and I published the experiment in our Science paper, and it is one of the best known of the findings we reported there.Thinking, Fast and Slow
In many situations, people make estimates by starting from an initial value that is adjusted to yield the final answer. The initial value, or starting point, may be suggested by the formulation of the problem, or it may be the result of a partial computation.Judgment under Uncertainty, the Kahneman and Tversky Chapters, p. 13
In either case, adjustments are typically insufficient (Slovic & Lichtenstein, 1971). That is, different starting points yield different estimates, which are biased toward the initial values.Judgment under Uncertainty, the Kahneman and Tversky Chapters, p. 13
Insufficient adjustment neatly explains why you are likely to drive too fast when you come off the highway onto city streets... a 'well-intentioned child who turns down exceptionally loud music to meet a parent's demand that it be played at a reasonable volume may fail to adjust sufficiently from a high anchor.'Thinking, Fast and Slow
Two groups of high school students estimated, within 5 seconds, a numerical expression that was written on the blackboard. One group estimated the product 8×7×6×5×4×3×2×1 while another group estimated the product 1×2×3×4×5×6×7×8.Judgment under Uncertainty, Heuristics and Biases, p. 6
The median estimate for the ascending sequence 512, while the median estimate for the descending sequence was 2,250. The correct answer is 40,320.Judgment under Uncertainty, Heuristics and Biases, p. 6
Decades after our disagreement, and years after Amos's death, convincing evidence of such a process was offered independently by two psychologists who had worked closely with Amos early in their careers: Eldar Shafir and Tom Gilovich together with their own students.Thinking, Fast and Slow
Two different mechanisms produce anchoring effects—one for each system. There is a form of anchoring that occurs in a deliberate process of adjustment, an operation of System 2. And there is anchoring that occurs by a priming effect, an automatic manifestation of System 1.Thinking, Fast and Slow