The Laws of Human Stupidity (Carlo Cipolla):
1: "Always and inevitably everyone underestimates the number of stupid individuals in circulation." Do not be fooled by traits assumed to be exclusive of stupidity such as education or career.
2: "The probability that a certain person be stupid is independent of any other characteristic of that person."
3: "A stupid person is a person who causes losses to another person or to a group of persons while himself deriving no gain and even possibly incurring losses." The corollary is that the exact nature of stupidity is difficult to predict and therefore guard against. (Refer to the QZ article covering these laws for an interesting graph comparing stupid people to all other types of people.)
4: "Non-stupid people always underestimate the damaging power of stupid individuals. In particular non-stupid people constantly forget that at all times and places and under any circumstances, to deal and/or associate with stupid people always turns out to be a costly mistake."
5: A stupid person is the most dangerous type of person - more dangerous than an intelligent but selfishly-motivated actor.