The horn effect, closely related to the halo effect, is a form of cognitive bias that causes one's perception of another to be unduly influenced by a single negative trait. An example of the horn effect may be that an observer is more likely to assume a physically unattractive person is morally inferior to an attractive person, despite the lack of relationship between morality and physical appearance.
The term is derived from the word "horn" and refers to the devil's horns. This is in contrast to the word halo and the halo effect, based on the concept of a saint's halo.
In a 1920 study published by Thorndike that focused on the halo effect, it was noted that "ratings were apparently affected by a marked tendency to think of the person in general as rather good or rather interior [sic][a] and to color the judgments of the qualities by this general feeling".[b]
It is sometimes called the horns effect, reverse-halo effect, or devil effect.
The horn effect occurs when "individuals believe that negative traits are connected to each other." It is a phenomenon in which an observer's judgment of a person is adversely affected by the presence of (for the observer) an unfavorable aspect of this person.