In fractions of a second, people make judgments about whether someone is nice or mean, trustworthy of a crook, competent or inept. Although not everyone's instincts are exactly the same, there is such great overlap that researchers can create computer-generated images of what a nice, competent, or trustworthy person looks like. Using such images, scientists have now designed a software program that can "tell" whether a face appears mean or threatening, just like humans. The program was able to make judgments about celebrities' appearances much in line with popular perceptions of those celebrities.
Snap judgments based on appearance have real-world implications. Politicians who appear competent are more likely to win elections. Dominant-looking soldiers rise quickly through the ranks. Baby-faced men are more likely to be found not guilty in court. Studies have found that even people with prosopagnosia (the inability to recognize familiar faces) can still discern trustworthiness. These instincts are powerful.
But just because they are powerful does not mean they are right. The evidence is mixed: A 1966 study found that people ranked strangers similarly to how those strangers scored on personality tests. But a similar experiment with tighter controls found support only for the traits of extroversion and conscientiousness. If there is a real correlation between appearance and personality, it's possible the explanation could be chemical (higher levels of testosterone have been shown to increase both dominance and the width of men's faces) or social -- the result of a self-fulfilling prophecy (that is, when people treat you as a nice person because you have a nice-person appearance, you learn to act as a nice person).