Summary
- Most professional investors see price/earnings (PE) ratios as inversely proportional to interest rates.
- Mathematically, this would be expressed as: PE = k / IR.
- Assuming that the interest rate on 10-year Treasuries could rise to 1%, and the k multiplier to rise to the pre-COVID level of 0.39, the expected S&P 500 PE ratio would then be 38.66.
- This would bring the S&P 500 up to 4,148. Still, not as expensive, in PE ratio terms, as in either the 2000s dot-com bubble or the Great Recession.
Interest rates and PE ratios
Most professional investors, including the late Benjamin Graham (), see PE ratios as inversely proportional to interest rates for Treasuries. Mathematically, this would be expressed as follows, where PE = the PE ratio of an equity security, IR = a relevant interest rate, and k = a multiplier.
PE = k / IR
There are a couple of key reasons for this relationship. One is that Treasuries become less attractive as an investment when interest rates go down. Since the Treasuries market is very large, a little over $21 trillion at the time of this writing, even a fraction of it moving to equities would push their PE ratios up significant.
Another key reason for the relationship above is that investing in Treasuries when interest rates are low becomes risky in terms of principal preservation. Treasury prices are inversely related to the interest they pay, or their yields. When yields are very low, the tendency is for them to go up.
Interest rates on 10-year Treasuries
The figure below shows the interest rates for the 10-year Treasury notes from January 2007 to July 2020. Those rates, which influence a number of other consumer-relevant rates (e.g., those for mortgages), go from 4.76% to 0.55% during the period.
While the interest rates have been going down over the years, they went up significantly during the recovery from the Great Recession. The same may happen during the recovery from the COVID recession. A move from 0.55% to 1% would be significant.
S&P 500 PE ratios
The figure below shows the PE ratios for the S&P 500 from January 2007 to July 2020. Note that the PE ratio for July 2020 is nowhere near the peak during the Great Recession.
The k multipliers
Finally, the figure below shows the k multipliers for the relationship between the S&P 500 PE ratios and interest rates on 10-year Treasuries from January 2007 to July 2020. An all-time low was reached for the multiplier in July 2020.
In a low interest rate environment, a high k multiplier would typically be associated with a high PE ratio. The multiplier increase, if it happens, should lag the interest rate reduction. Investors need time to be convinced that interest rates will remain subdued.
S&P 500 above 4,100 soon?
Assuming that the interest rate on 10-year Treasuries could rise to 1%, and the k multiplier to rise to the pre-COVID level of 0.39 (which is below its historical average), the expected S&P 500 PE ratio would then be 38.66. This would bring the S&P 500 up to 4,148. Still, not as expensive, in PE ratio terms, as in either the 2000s dot-com bubble or the Great Recession.
This PE ratio expansion for the S&P 500 is already happening at the time of this writing, with the S&P 500 hitting an all-time high. Our main point in this post is that, if interest rates on Treasuries remain low, we may see more of that - even with bankruptcies among small businesses, significant unemployment, and high volatility (including downward corrections).
