The state of being voluntarily stupid, i.e. being so when you know better. Kurt Vonnegut offers the idea that many people don't express themselves due to fears that their opinions would offend others, resorting instead to parroting popular opinion or the opinions of those with whom they commiserate in the name of diplomacy. This is especially true with females, who have had to rely more strongly on diplomacy in the past when subjected to male domination.
Things produced by humans may also be stupid on purpose, such as TV shows, or politics.