Defining the halo effect and the history of its study
The halo effect is a cognitive bias first identified by Edward Thorndike in 1920 during research on military performance ratings. It refers to the phenomenon whereby an evaluation of one salient characteristic of a person spills over into evaluations of other, unrelated characteristics. The name comes from the ring of light drawn above the heads of saints in religious painting, capturing the image of one good quality illuminating the whole. In Thorndike's study, officers rating their subordinates showed a tendency to award soldiers of superior physical appearance higher marks for intelligence, leadership, and character as well. The effect works in the negative direction too, in which case it is called the horn effect: a single unfavorable characteristic can drag down the evaluation of everything else. This bias operates so automatically that deliberate effort cannot eliminate it entirely, and it colors the judgments of trained experts along with everyone else's.
The attractiveness halo and romantic attraction
The halo effect shows itself most clearly in the domain of physical attractiveness. The classic study by Dion, Berscheid, and Walster in 1972, known by the phrase "what is beautiful is good," demonstrated that physically attractive people are rated more highly on unrelated characteristics such as intelligence, sociability, morality, and happiness. In romantic contexts, the attractiveness halo strongly influences the earliest stages of choosing a partner. An attractive person is assumed to have a good character as well, and to be enjoyable company, so liking forms before anything is actually known about who they are. There is little to suggest, however, that physical attractiveness tells you much about actual personality traits as measured by the Big Five. In other words, the expectations the halo effect generates are in most cases disappointed. As a relationship lengthens, actual personality traits and patterns of communication are generally thought to come to the fore, with physical attractiveness receding into the background.
Living with the halo effect on a first impression
Because of the halo effect, natural human judgment on its own has a hard time gauging genuine compatibility of character. Physical attractiveness, or the brightness of a first impression, lifts the assessment of qualities that ought to have nothing to do with it. A compatibility assessment, which separates out substantive factors one at a time, personality traits, values, styles of communication, is less easily swayed in this way. Even so, the halo effect finds its way into how a result is read. When one factor comes back showing a high degree of compatibility, its glow can make mismatches on the other factors look smaller than they are. A result is therefore worth reading factor by factor, keeping track of what fits and what does not, rather than remembered as a single overall verdict. And the halo effect is the automatic kind of process that does not disappear simply because you know about it. A good first impression is a doorway into getting to know someone; it is not an answer about compatibility itself. Whether the expectation it created was accurate is something you find out only through actual interaction, accumulated over time.