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.
Stupidity
In "Understanding Stupidity", James F. Welles defines stupidity this way: "The term may be used to designate a mentality which is considered to be informed, deliberate and maladaptive." Welles distinguishes stupidity from ignorance; one must know they are acting in their own worst interest. Secondly, it must be a choice, not a forced act or accident. Lastly, it requires the activity to be maladaptive, in that it is in the worst interest of the actor, and specifically done to prevent adaption to new data or existing circumstances."