There are many important things about the stock market that can change, such as the behaviour of market participants and their level of collective knowledge. I believe an interesting example of this can be seen in the 2008/09 financial crisis. The period was an economic calamity and stock prices fell sharply. During the crisis, the S&P 500, a broad index for US stocks, fell by nearly 57% from peak to trough. But then-Federal Reserve chair Ben Bernanke prevented an even worse disaster from happening. Bernanke was a scholar on the Great Depression that happened in the 1930s. In a wonderful 2002 speech for the birthday gala of celebrated economist Milton Friedman, Bernanke laid out the mistakes the US government had made during the Great Depression. He ended the speech saying (emphasis is mine):

“I would like to say to Milton and Anna: Regarding the Great Depression. You’re right, we did it. We’re very sorry. But thanks to you, we won’t do it again.”

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