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The halo effect or halo error is a cognitive bias in which one's judgments of a person’s character can be influenced by one's overall impression of him or her. It can be found in a range of situations from the courtroom to the classroom and in everyday interactions. The halo effect was given its name by psychologist Edward Thorndike; subsequent researchers have studied it in relation to attractiveness and its bearing on the judicial and educational systems.
Edward Thorndike, known for his contributions to educational psychology, coined the term "halo effect" and was the first to support it with empirical research. He gave the phenomenon its name in his 1920 article “The Constant Error in Psychological Ratings.” He had noted in a previous study made in 1915 that estimates of traits in the same person were very highly and evenly correlated. In “Constant Error,” Thorndike set out to replicate the study in hopes of pinning down the bias that he thought was present in these ratings.
In "The Constant Error in Psychological Ratings," Thorndike asked two commanding officers to evaluate their soldiers in terms of physical qualities (neatness, voice, physique, bearing, and energy), intellect, leadership skills, and personal qualities (including dependability, loyalty, responsibility, selflessness, and cooperation). His goal was to see how the ratings of one characteristic affected other characteristics.
Thorndike's experiment showed how there was too great a correlation in the commanding officers' responses. In his review he stated, "The correlations were too high and too even. For example, for the three raters next studied the average correlation for physique with intelligence is .31; for physique with leadership, .39; and for physique with character, .28." The ratings of one of the special qualities of an officer tend to start a trend in the rating results. If an officer had a particular "negative" attribute given off to the commanding officer, it would correlate in the rest of that soldier's results.
The correlation in the halo effect experiment was concluded to be a halo error. The halo error showed that the officers relied mainly on general perception of certain characteristics that determined the results of their answers.
The halo effect is not exclusively limited to individual traits or an individual's overall appearance. A person’s attractiveness has also been found to produce a halo effect.
Dion and Berscheid (1972) conducted a study on the relationship between attractiveness and the halo effect. Sixty students from University of Minnesota took part in the experiment, half being male and half being female. Each subject was given three different photos to examine: one of an attractive individual, one of an individual of average attractiveness, and one of an unattractive individual.
The participants judged the photos’ subjects along 27 different personality traits (including altruism, conventionality, self-assertiveness, stability, emotionality, trustworthiness, extraversion, kindness, and sexual promiscuity). Participants were then asked to predict the overall happiness the photos' subjects would feel for the rest of their lives, including marital happiness (least likely to get divorced), parental happiness (most likely to be a good parent), social and professional happiness (most likely to experience life fulfillment), and overall happiness. Finally, participants were asked if the subjects would hold a job of high status, medium status, or low status.
Results showed that participants overwhelmingly believed more attractive subjects have more socially desirable personality traits than either averagely attractive or unattractive subjects. Participants also believed that attractive individuals would lead happier lives in general, have happier marriages, be better parents, and have more career success than the others. Also, results showed that attractive people were believed to be more likely to hold secure, prestigious jobs compared to unattractive individuals.
Landy and Sigall’s 1974 study demonstrated the halo effect on judgments of intelligence and competence on academic tasks. Sixty male undergraduate students rated the quality of essays which included both well and poorly written samples. One third were presented with a photo of an attractive female as author, another third with that of an unattractive female as author, and the last third were shown neither.
Participants gave significantly better writing evaluations for the more attractive author. On a scale of 1–9, the well-written essay by the attractive author received an average of 6.7 while the unattractive author received a 5.9 (with a 6.6 as a control). The gap was larger on the poor essay: the attractive author received an average of 5.2, the control a 4.7, and the unattractive a 2.7, suggesting readers are generally more willing to give physically attractive people the benefit of the doubt when performance is below standard than others.
In Moore, Filippou, and Perret’s 2011 study, researchers sought to determine if residual cues to intelligence and personality existed in male and female faces by attempting to control for the attractiveness halo effect. They manipulated the perceived intelligence of photographs of individuals, finding that faces manipulated to look high in perceived intelligences were also rated as more attractive. It was also found that the faces high in perceived intelligence were also rated highly on perceived friendliness and sense of humor.
Multiple studies of the halo effect in jury outcomes have shown attractive individuals both receive lesser sentences and are less likely to be convicted than unattractive ones.
Efran (1974) found subjects were more lenient in sentencing attractive individuals than unattractive ones even when exactly the same crime was committed. This has been attributed to people with a high level of attractiveness being seen as more likely to have brighter futures in society thanks to socially desirable traits they are believed to possess.
Monahan (1941) studied social workers accustomed to interacting with people from all types of backgrounds, finding the majority found it very difficult to believe beautiful people are guilty of a crime.
The relation of a crime to attractiveness is also subject to the halo effect. A study presented two hypothetical crimes: a burglary and a swindle. The burglary involved a woman illegally obtaining a key and stealing $2,200; the swindle involved a woman manipulating a man to invest $2,000 in a fabricated business. The results showed that when the offense was not related to attractiveness (as in the burglary) the unattractive defendant was punished more severely than the attractive one. However, when the offense was related to attractiveness (the swindle), the attractive defendant was punished more severely than the unattractive one. It is imputed participants may have believed the attractive person more likely to manipulate someone using their looks.
Abikoff found the halo effect is also present in the classroom. In this study, both regular and special education elementary school teachers watched videotapes of what they believed to be children in regular 4th-grade classrooms. In reality, the children were actors, depicting behaviors present in attention deficit hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), or standard behavior. The teachers were asked to rate the frequency of hyperactive behaviors observed in the children. Teachers rated hyperactive behaviors accurately for children with ADHD; however, the ratings of hyperactivity and other behaviors associated with ADHD were rated much higher for the children with ODD-like behaviors, showing a halo effect for children with oppositional defiant disorder.
Foster and Ysseldyke (1976) also found the halo effect present in teachers’ evaluations of children. Regular and special education elementary school teachers watched videos of a normal child whom they were told was either emotionally disturbed, possessing a learning disorder, mentally retarded, or "normal". The teachers then completed referral forms based on the child's behavior. The results showed that teachers held negative expectancies toward emotionally disturbed children, maintaining these expectancies even when presented with normal behavior. In addition, the mentally retarded label showed a greater degree of negative bias than the emotionally disturbed or learning disabled. 
The term "halo effect" has been applied to human rights organizations that have used their status to move away from their stated goals. Political scientist Gerald Steinberg has claimed that non-governmental organizations (NGOs) take advantage of the "halo effect" and are "given the status of impartial moral watchdogs" by governments and the media.
In brand marketing, a halo effect is one where the perceived positive features of a particular item extend to a broader brand. One famous example is how the popularity of Apple’s iPod has generated enthusiasm for its other products. The effect is also exploited in the automotive industry, where a manufacturer may produce an exceptional halo vehicle in order to promote sales of an entire marque. Modern cars often described as halo vehicles include the Dodge Viper, Ford GT, and Acura NSX.
Some researchers allege that the halo effect is not as pervasive as once believed. Kaplan’s 1978 study yielded much of the same results as are seen in other studies focusing on the halo effect—attractive individuals were rated high in qualities such as creativity, intelligence, and sensitivity than unattractive individuals. In addition these results, Kaplan found that women were influenced by the halo effect on attractiveness only when presented with members of the opposite sex. When presented with an attractive member of the same sex, women actually tended to rate the individual lower on socially desirable qualities.
Criticisms have also pointed out that jealousy of an attractive individual could be a major factor in evaluation of that person. A study by Dermer and Thiel has shown this to be more prevalent among females than males, with females describing physically attractive women as having socially undesirable traits.
The devil effect, also known as the reverse halo effect, is when people allow an undesirable trait to influence their evaluation of other traits, such as in Nisbett and Wilson's study on likeable versus unlikeable lecturers. The devil effect can work outside the scope of personality traits and is expressed by both children and adults. The Guardian wrote of the devil effect in relation to Hugo Chavez: "Some leaders can become so demonised that it's impossible to assess their achievements and failures in a balanced way."