John Hussman on the "Money on the sidelines" argument:
"When Ricky sells his money market fund to buy stocks, money does not go "into" the stock market. All that happens is that previously issued securities change hands. See, Nicky has to buy the commercial paper that Ricky's money fund liquidates, and the cash that Nicky previously held now goes to Ricky, who uses it to buy shares of stock from Mickey. In the end, Mickey gets the cash that Nicky used to hold, Nicky gets the commercial paper that Ricky used to hold (via the money market fund), and Ricky gets the shares that Mickey used to hold. Money doesn't go into the stock market - it goes through it. After these trades, there is exactly as much money on the "sidelines" as before - exactly the same number of dollars, exactly the same number of shares, and exactly the same amount of commercial paper.
Now, if Ricky is more eager to buy stock than Mickey is to sell, the share price increases, which means that Ricky has to sell more of his money market fund to buy the shares than otherwise. Thus, compared to the alternative where Mickey is the eager party (in which case the stock price declines), Nicky ends up with a little more commmercial paper and a little less cash, Ricky ends up with a little bit less in money market funds, and Mickey ends up with a little more of the cash that Nicky used to hold. So even if the stock price changes, it does not follow that there is any more or less money on the sidelines. It's just that the final distribution of existing securities among various individuals may be a bit different, depending on who was the eager party. Regardless, every buy trade that actually executes in the market is matched with a sell."
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