In 1990, a seemingly innocuous puzzle published in Parade magazine sparked what might be called the Great Probability War. Thousands of readers, including doctorate holders in mathematics, bombarded the magazine with angry letters. Their target? Marilyn vos Savant, whose solution to the “Monty Hall Problem” they declared not just wrong, but offensively wrong. Even Paul Erdős, one of the 20th century’s most brilliant mathematicians, initially dismissed her answer. They were all mistaken.
The puzzle, based on the American game show “Let’s Make a Deal,” seems simple enough. A contestant faces three doors. Behind one is a car, and behind the others are goats. After the contestant picks a door, the host (who knows what lies behind each) opens another to reveal a goat. Should the contestant switch to the remaining door? The counterintuitive answer—that switching doubles one’s chances of winning—has been making heads spin for decades.
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