"Don't I know you?" may be more than an old pickup line. The perception that strangers are familiar may be the result of a mental shortcut gone awry.

Benoit Monin, assistant professor of psychology at Stanford University, showed college students 80 photos of faces, then asked them which ones they recognized from among the 40 they'd seen in an earlier session. The more attractive the photo (as rated by another group of students) the more likely it was to be recognized—regardless of whether the face had been seen before.

Attractive people are often given credit—known as the halo effect—for other qualities such as high intelligence and better job performance. But the attractive-is-familiar phenomenon is not simply an instance of the halo effect, says Monin: "The face's attractiveness actually changes your perception of your past," in this case, the perception of whether you've seen the face before. The shortcut may lead to errors, but it may also help us manage our busy lives, says Monin. "We tend to like familiar things, so it makes perfect sense that over time we would use liking as a clue to familiarity."

Psychologists have explored the possibility that attractive stimuli tend to look familiar because they are typical or representative of a category. But Monin's research, published in the Journal of Personality and Social Psychology, goes a step further. In what he calls the "warm-glow heuristic," people consider their affinity for a specific person or place as an indicator of familiarity. As with other mental shortcuts, people resort to this heuristic when they lack enough data on which to base their decisions.

The warm-glow heuristic is not only a reaction to physical beauty. In another study, Monin first showed participants a series of words. In a second session, he showed them an entirely new set of words and asked which words were familiar from the earlier, bogus session. Subjects were more likely to think they'd seen positive words—such as "charm" and "glory"—than either negative or neutral words that appear with the same frequency in English.

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