JV sends along a link to The Basic Laws of Human Stupidity. In short:
1. "Always and inevitably everyone underestimates the number of stupid individuals in circulation."
Similarly, P. T. Barnum once said, "You’ll never go broke underestimating the intelligence of the American public."
2. "The probability that a certain person be stupid is independent of any other characteristic of that person."
The main problem I have with the second law is that it doesn't admit that self-selected groups may contain a disproportionate number of stupid people.
This idea was hard to accept and digest but too many experimental results proved its fundamental veracity. The Second Basic Law is an iron law, and it does not admit exceptions. The Women's Liberation Movement will support the Second Basic Law as it shows that stupid individuals are proportionately as numerous among men as among women. The underdeveloped of the Third World will probably take solace at the Second Basic Law as they can find in it the proof that after the developed are not so developed. Whether the Second Basic Law is liked or not, however, its implications are frightening: the Law implies that whether you move in distinguished circles or you take refuge among the head-hunters of Polynesia, whether you lock yourself into a monastery or decide to spend the rest of your life in the company of beautiful and lascivious women, you always have to face the same percentage of stupid people - which percentage (in accordance with the First Law) will always surpass your expectations.
There's a difference between the group of headhunters and the group of monks: headhunters don't normally make a decision to join their group, they're just born into it, whereas monks do. And stupid people are, obviously, inclined to join stupid groups. In fact, I'd argue that the Second Law is fundamentally wrong-headed, since membership in a destructive, stupid, self-selected group is one of the best ways to identify stupid people before they can hurt you.
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."
That's a pretty good definition. The author (Carlo M. Cipolla) also defines three other types of people: bandits, who help themselves while hurting others; helpless people, who hurt themselves and help others; and intelligent people who, who help others while helping themselves.
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."