Carlo M. Cipolla's Laws of Stupidity

Zeroth LawThere is no upper bound on the amount of stupidity that can exist within any particular individual.
First LawWe always underestimate the number of stupid people. Not as obvious as it sounds, because: 1.     people we had thought to be rational and intelligent suddenly turn out to be unquestionably stupid; 2.     day after day we are hampered in whatever we do by stupid people who invariably turn up in the least appropriate places, and 3.     the underestimation is after the first law itself is already accounted for.
Second LawThe probability of a person being stupid is independent of any other characteristic of that person.
Third (and Golden) LawA stupid person is someone who causes damage to another person, or a group of people, without any advantage accruing to himself (or herself) — or even with some resultant self-damage.
Fourth LawNon-stupid people always underestimate the damaging power of stupid people.They constantly forget that at any moment, and in any circumstance, associating with stupid people invariably constitutes an expensive mistake.
Fifth LawA stupid person is the most dangerous person in existence.