The Halo Effect
The halo effect is the spread of a favourable or unfavourable global impression to attributes that were never observed. Like the look of someone and you will credit them with generosity, competence and taste you have no evidence about. The name carries two senses and this page keeps them apart: the perception-psychology sense, demonstrated by Solomon Asch and used by Daniel Kahneman, and Phil Rosenzweig's business sense, which is a methodological objection to an entire publishing genre. The mechanism is not disputed. Most of the numbers attached to it here are unverified, which is why the page is marked mixed.
What it is
The perception sense first. It is not primarily an error of laziness. It is a preference for a consistent story, and the missing evidence about the unobserved attributes never announces itself as missing. That is why the effect sits directly underneath what Kahneman calls WYSIATI, the assumption that what you can see is all there is.
Attribution has to be handled carefully and this page handles it by declaring what it does not know. Kahneman names no originator anywhere in Thinking, Fast and Slow. He observes only that the term has been in use in psychology for roughly a century, which places it without crediting anybody. The standard attribution outside that book is to Edward Thorndike. That is recorded here as background and has not been verified from a primary source in this vault, so it should not be leaned on. This publication has caught the same failure ten or more times, where a finding is credited to the best-known name nearby rather than to whoever did the work, and leaving a field empty is cheaper than filling it from memory.
The demonstration in the book is Solomon Asch's Alan and Ben, and its elegance is that nothing changes except the order. Six adjectives, presented for Alan as intelligent, industrious, impulsive, critical, stubborn, envious, and in reverse for Ben. Same six words, same person on paper. Alan reads as a capable and forceful character whose stubbornness is the price of his strengths. Ben reads as someone whose envy and stubbornness poison whatever ability he has. The early adjectives set the global impression, and the later ones are absorbed into it and reinterpreted rather than allowed to disturb it.
There is a sharper variation. Participants shown the first three adjectives and the last three as descriptions of two different people mostly judged it impossible that all six could describe one person. They could not reassemble what an ordering had split. The book gives no figure for this variation and names nobody for it, so its strength cannot be stated from this source.