Pluralistic ignorance
In social psychology, pluralistic ignorance is a situation in which a majority of group members privately reject a norm, but go along with it because they incorrectly assume that most others accept it. This is also described as "no one believes, but everyone thinks that everyone believes". In short, pluralistic ignorance is a bias about a social group, held by the members of that social group.
Knightian uncertainty
The Ellsberg paradox is based on the difference between these two types of imperfect knowledge, and the problems it poses for utility theory – one is faced with an urn that contains 30 red balls and 60 balls that are either all yellow or all black, and one then draws a ball from the urn. This poses both "uncertainty" – whether the non-red balls are all yellow or all black – and "probability" – whether the ball is red or non-red, which is ⅓ vs. ⅔. Expressed preferences in choices faced with this situation reveal that people do not treat these types of imperfect knowledge the same. This difference in treatment is also termed "ambiguity aversion".