From an article about the US justice system, but the relevance to misbehaving schoolchildren (or simply schoolchildren whose behaviour one doesn't like) is obvious:
Cesare Beccaria—the Italian criminologist from whom Jeremy Bentham borrowed not only the term “utility” but many of his ideas for criminal-justice reform—identified three characteristics that determine the deterrent efficacy of a threatened punishment: its swiftness, its certainty, and its severity. Of the three, severity is least important. If punishment is swift and certain, it need not be severe to be efficacious. If punishment is uncertain and delayed, it will not be efficacious even if it is severe. (It was only two and a half centuries after Beccaria that psychologists and behavioral economists discovered that some degree of excessive present-orientation, and excessive discounting of the risk of large losses, is normal.) The sort of bad gamble represented by most offenses tends to attract precisely those whose departures from rationality are most egregious.
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Not only is severity an inadequate substitute for swiftness and certainty, it actually interferes with them. The more severe a punishment is, the more due process (leading to delay) is required to impose it, and (if severity is measured in sentence length) the less often it can be imposed before the prisons fill up.
eterrent efficacy of a threatened punishment: its swiftness, its certainty, and its severity.
Interestingly, these correspond to delay, expectancy and value in the procrastination equation. It's interesting to see "negative" values used to form a kind of anti-motivation.
See also: Boring Advice Repository, Solved Problems Repository, Grad Student Advice Repository
I often find that my understanding of the world is strongly informed by a few key concepts. For example, I've repeatedly found the concept of opportunity cost to be a useful frame. My previous post on privileging the question is in some sense about the opportunity cost of paying attention to certain kinds of questions (namely that you don't get to use that attention on other kinds of questions). Efficient charity can also be thought of in terms of the opportunity cost of donating inefficiently to charity. I've also found the concept of incentive structure very useful for thinking about the behavior of groups of people in aggregate (see perverse incentive).
I'd like people to use this thread to post examples of concepts they've found particularly useful for understanding the world. I'm personally more interested in concepts that don't come from the Sequences, but comments describing a concept from the Sequences and explaining why you've found it useful may help people new to the Sequences. ("Useful" should be interpreted broadly: a concept specific to a particular field might be useful more generally as a metaphor.)