Depends on how old they are:
The authors investigated accuracy of judging intelligence from facial photos of strangers across the lifespan, facial qualities contributing to accuracy, and developmental paths producing correlations between facial qualities and IQ scores. Judgments were more accurate than chance in childhood and puberty, marginally more accurate in middle adulthood, but not more accurate than chance in adolescence or late adulthood. Reliance on the valid cue of facial attractiveness could explain judges’ accuracy. Multiple developmental paths contributed to relationships between facial attractiveness and IQ: biological, environmental, influences of intelligence on attractiveness, influences of attractiveness on intelligence. The findings provide a caveat to evolutionary psychologists’ assumption that relationships between attractiveness and intelligence or other traits reflect an influence of “good genes” on both, as well as to social and developmental psychologists’ assumption that such relationships reflect self-fulfilling prophecy effects. Each of these mechanisms failed to explain some observed correlations.
Source: “Looking Smart and Looking Good: Facial Cues to Intelligence and their Origins” from Personality and Social Psychology Bulletin
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