Stupidity

Stupidity is the quality or condition of lacking intelligence, as opposed to being merely ignorant or uneducated. This quality can be attributed to both an individual or a person's actions, words or beliefs, or those of a group.

As an English word, it implies that the attributed party is not mentally retarded but rather is willfully ignorant and/or unintelligent, and displays poor use of judgement or insensitivity to nuances.

The adjective is also used as a general pejorative (e.g. I didn't borrow your stupid cap - go look for it yourself).

Manifested by the educated
Recently a great deal of attention has been paid to another class of stupidity: stupid actions by those who consider themselves educated and worldly. This is a fairly new topic for researchers and there are still few academic works on the subject.

The Encyclopedia of Stupidity by Matthijs van Boxsel is based on the author's contention that "stupidity is in fact the foundation of our civilization" and his idea that no one is intelligent enough to realise how stupid they are. This is not as stupid as it sounds if one includes in the definition of stupidity "unwitting self-destruction, the ability to act against one's best wishes".

In comedy
Generally, the stupidity featured in comedy is that of entertainers pretending to be stupid. The fool/buffoon has been a central character in most comedy. Today there is a wide array of television shows that showcase stupidity such as Jackass, as well as with many cartoon shows, such as Homer Simpson from The Simpsons, Phillip J. Fry from Futurama, or Peter Griffin from Family Guy.

Individual vs collective stupidity
In psychology, this is known as deindividuation in crowds, and can lead to behaviours usually not displayed outside the specific social situation. The behaviours occur because individuals will conform to perceived social norms in order to 'fit in' or project an impression of self as "normal".