https://www.morningstar.com/authors/208/john-rekenthaler
The initial way to view the stock market is as a zero-sum game. With any stock trade, one side wins, because it buys a security that increases in price, or because it sells one that declines. The other side loses, by the same amount. In aggregate, then, the stock market's collective trades amount to nothing at all.
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In summary, describing the U.S. stock market as zero-sum before costs, then negative-sum after costs, is a fine approximation of the truth. Investors who know no further than that understand enough to succeed. It is, however, not the complete truth. The full story is somewhat more complex.