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We have focused on the monetary tsunami set in motion by central banks, but there is another force contributing to the record-high valuations in the US stock market. That force is the shift towards passive and ETF-focused investing that began more than two decades ago and has come to dominate flows within the stock market.

So-called “passive” strategies use rules-based investing, often to track an index by holding all of its constituent components or a representative sample of those components. There is no discretion on the part of the asset manager. For example, money going into an S&P500 index fund will be allocated to all of the stocks in the S&P500 according to their weight in the index, meaning that the stocks with the highest market capitalizations will receive the lion’s share of the money flowing into such a fund.

Due to passive investing, the more expensive a stock becomes the more investment it will attract and the more expensive it will become. For example, in the S&P500 Index the current weighting of Apple is 500-times greater than that of Xerox, so when money flows into an S&P500 index fund the proportion that gets allocated to the purchase of Apple shares will be automatically 500-times greater than the proportion that gets allocated to the purchase of Xerox shares. Therefore, rather than a relatively high valuation stemming from past outperformance being an impediment to future relative strength, it will tend to create additional relative strength.

This wouldn’t be a major issue if passive investing constituted a small part of the market, but the strategy has grown to be by far the most important source of demand for stocks in the US. This means that the largest net buyer of US equities each month is price-insensitive (value blind).

Summing up the above, every month a large amount of money flows into funds that allocate with no consideration of value.

Furthermore, many “active” fund managers now trade ETFs rather than individual stocks and many of these ETFs are rules-based. For example, rather than go to the trouble of selecting/monitoring the stocks of individual oil companies, these days an active manager who is bullish on oil is likely to buy shares of the Energy Select Sector ETF (XLE). This ETF tracks a market-cap-weighted index of US energy companies in the S&P 500, so the more expensive an oil company becomes the greater will be its weighting in XLE and the larger the amount of money that will be allocated to it whenever the demand for the ETF pushes the ETF’s price above its net asset value.

The increasing popularity of ETFs among “active” managers tends to cause the stocks that have the largest weightings in ETFs to become relatively strong, regardless of whether the strength is warranted based on the performances of the underlying businesses. That is, the increasing use of ETFs by active managers exacerbates the effect on the market-wide valuation of the increasing popularity of passive investing.

A consequence is that the market no longer mean-reverts the way it used to. In theory, it could keep getting more expensive ad infinitum.

In practice, it won’t get more expensive ad infinitum because at some point something will happen (for example, a major inflation scare that causes the Fed to slam its foot on the monetary brake) that causes the direction of the passive flows to reverse. This is part of the explanation for why the March-2020 decline was exceptional. In March-2020, the decision to shut down large parts of the economy in reaction to a virus caused the massive price-insensitive buyer to become a net seller for a short period. The result for the S&P500 Index was the quickest-ever 35% decline from an all-time high.

In conclusion, be wary of confident claims to the effect that today’s record-high valuations imply that a major top is close in terms of time or price. The reality is that valuations could go much higher. Also be wary of bullish complacency, because at some point the flows will reverse. The risk that flows will reverse with little warning is why we are about 35% in cash despite our expectation that the equity bull market will continue for at least a few more months.