Stupidity: Marked lack of intelligence or marked presence of unconscious ignorance.
Stupidity is, in various ways, one of the important 'unacknowledged legislators of mankind' and an important force of history. The main reason is that political leaders, priests and clergy abuse their authority to mislead the none-too-intelligent majority of those they lead.
If people were more intelligent, on average, or more learned (with real knowledge, that includes accurate knowledge about one's own ignorance), the vast majority of politics and religion either would not exist at all, or would have a different content and effect.
This does not only apply to ordinary men, but also to their leaders of all kinds: There also are the stupidity and ignorance of the intelligent, and indeed one of the weaknesses of democracy is that it allows the not-very-intelligent democratically elected to lead the not-at-all-intelligent majority. (A related weakness of monarchical systems of government are whole generations of hereditary kings without any special talents whatsoever.)
It is difficult to do much about this, apart from universal eugenetics, for which there is certainly not enough knowledge at present. Until then, the general rule is Schiller's "Gegen die Dummheit kämpfen selbst die Götter vergebens" - Against stupidity even the Gods battle in vain.