A vivid example of a north korean's behavior has an unusually strong influence on people's judgments of all north koreans primarily because people

Because people "estimate the frequency of group characteristics in terms of the memorability of these characteristics".

Each gathering has an arrangement of standards and characteristics: a set of accepted rules about what is satisfactory conduct. They may apply to everybody in the gathering or to specific individuals as it were. Some characteristics will be entirely clung to while others allow an extensive variety of conduct.