Via Barker:

Several studies have demonstrated some accuracy in personality attribution using only visual appearance. Using composite images of those scoring high and low on a particular trait, the current study shows that judges perform better than chance in guessing others’ personality, particularly for the traits conscientiousness and extraversion. This study also shows that attractiveness, masculinity and age may all provide cues to assess personality accurately and that accuracy is affected by the sex of both of those judging and being judged. Individuals do perform better than chance at guessing another’s personality from only facial information, providing some support for the popular belief that it is possible to assess accurately personality from faces.

Source: Using composite images to assess accuracy in personality attribution to faces” from British Journal of Psychology

For most aspects of personality I would bet that the reason is simply that your face causes your personality.  Attractive people are confident and extroverted, unattractive people less so.  For other aspects its the other way around.  People who are conscientious comb their hair, etc.

But I am often firmly convinced that I can tell how smart a person is just by looking at them.  And although I can see a few potential causalities, they seem flimsy to me:

  1. You invest more in appearance if it is a substitute for intelligence.  (But can’t they be complementary?  I cling to the belief that my wife sees them so.)
  2. Investments in appearance are short-term.  Smarter people are more patient than that.  (Tell that to Fabio.)
  3. Smart people exude confidence.  (Do they? Dumb people may be even more confident.)

Anyway, I don’t usually think that attractiveness is the key signal (not that I can say what is the key signal.)  But I can imagine that I am consistenly wrong in my judgments of how smart a person is.  If you assume a person is smart then everything they say tends to sound smart.  (Why else would you still be reading this?)