The implicit personality theory explains how assumptions are quickly made about a person just introduced.  The assumptions that are initially made are beliefs due to an impression received on the basis of the limited information available.  We form such impressions about others and construct our interpretations of their personalities so that we may predict how that person will behave towards us.  People construct a well-organized pattern of what they consider to be important about another person on such little information, which proves powerful in their perception of the other person (LeFleur, 2005).

For more information, read Craig Anderson's study: Implicit Personality Theories and Empirical Data: Biased Assimilation, Belief Perseverance and Change, and Covariation Detection Sensitivity.