Try this experiment: Grab a pencil and, as quickly as possible, write down a string of 20 random numbers. You might think there’s no pattern to the numbers, but chances are you’re wrong. Psychologists have found that when making up long strings of numbers, people tend to overuse small digits, underestimate the number of times digits will repeat themselves, overuse pairs of adjacent digits like 2-3 or 7-8, and underuse pairs of distant digits like 1-7 or 9-2. In short, people aren’t very good at making up numbers that seem random.
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