The linked article below explains in detail what the author refers to as 'The Five universal rules of human stupidity".

Here is a short summary of those five laws:

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• Law 1: Always and inevitably, everyone underestimates the number of stupid individuals in circulation.

• Law 2: The probability that a certain person is stupid is independent of any other characteristic of that person. Example: Even a person who is a college professor with a Ph.D. can still be a stupid person.

• Law 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.

• Law 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 dealing and/or associating with stupid people always turns out to be a costly mistake.

• Law 5: A stupid person is the most dangerous type of person. And its corollary: A stupid person is more dangerous than a "Bandit" (see chart below).