Interest Rates

US interest rates are coming up from a long stretch of all-time lows. Broadly speaking, this means that investments made in low risk products (e.g., bonds) are paying little in returns, and so are not highly demanded by investors. Traditionally risk-averse investors have little else to do with cash right now other than to put it into riskier equities, which drives the stock market higher. This model looks at the relative performance of the US stock market given current interest rates. As of September 30, 2024, the US stock market is Overvalued relative to a normal interest rate environment .

Below is our composite chart showing this model, which is a sum of US Treasury interest rates relative to historical average (red), and the S&P500 price relative to its own exponential trend line (blue). Read the detail below to understand the inputs and limitations of this model.

There are two core reasons that stock markets and interest rates tend to move inversely with one another.

• Lower Corporate Profits. As market interest rates rise, that means that firms who wish to borrow money in order to fund profitable projects will need to pay more in interest payments. This will necessarily lower profits. In some cases, it means the firms will not be able to do the project at all. Lower profits mean lower stock prices, since stock prices are fundamentally a measure of all future profits of a firm. The opposite is true as well - as rates fall firms are able to borrow more, which increases profits and expands economic output.
• Less Demand for Equity. As market interest rates rise investors are able to earn higher yields by investing in debt instruments (bonds, etc) rather than equities (stocks). This lowers the demand for stocks, which lowers their prices. Likewise, when interest rates are very low, investors seeking a return on their cash don't have many available choices, and tend to get pushed into riskier assets (lower quality bonds, stocks), in order to make a return. This drives those prices higher.

There is plenty more to know about the inverse correlation between stocks and bonds. Check out these resources for more info.

Suffice it to say here that what we are primarily concerned with is the question: Is the current market fairly valued when considering current interest rates?

Interest Rates

The chart below shows the Ten Year Treasury Bond rate over the last ~60 years, from 1962 to present day. Rates spiked in the '70s and '80s as a response to high inflation at the time, and since then have been on a pretty steady downward trajectory.

Trendlines

The average (arithmetic mean) of this rate over the entire timeframe is 5.85%. Rates haven't been in that neighborhood for about 20 years, but history does suggest that a healthy economy should revert back up to higher rates eventually. ("Eventually" here is quite a loaded term, and speculating on future Fed monetary policy decisions, or on fundamental changes to Fed policy and philosophy, are outside of the scope of this article, but only a Google search away).

Below is the same chart as above, only this time with the 5.85% average trend line drawn in. The chart to the right, in blue, shows the S&P500 (inflation-adjusted) over the same time frame, as well is its corresponding trendline. [This comes directly from our S&P500 Model detail, so look there for more detail on the data and trendline.]

Detrending the Data

Instead of looking at the raw values for each of these charts, we can instead think about each of them in terms of their performance relative to their respective trend lines. Are they above or below the respective trend? In the two charts below we map each chart in terms of the number of standard deviations above/below the value of each's respective trend line.

You can see in the left chart that the most current 10 Year Treasury interest rate is 0.69 standard deviations below normal. Likewise, you can see in the chart to the right that the S&P500 is currently 2.0 standard deviations above its trend line.

Comparison

We should generally expect that when interest rates are relatively high, stocks should be relatively low. What ought to be very interesting to us is when interest rates and stock prices are in the same phase (both high or both low, relative to their respective trend lines). Since we are already expressing data in both charts in terms of standard deviations above/below trend, we can just sum the two charts together to see where they are out of phase.

Summary

This last chart shows the complete model. The two relative performance indicators for interest rates (red) and stocks (blue) have been combined, showing a composite value in purple. When greater than zero, this indicates that stocks are high given where interest rates are. The peak here is during the 2000 internet bubble. During that time stock prices were exceptionally higher than the preceding trend, but bond prices were right around their historical average... meaning that even though investors had other good options to invest their cash, and despite the high interest rates firms needed to pay in order to borrow money, stocks were still priced very high. That's a clear bubble - which we all know in retrospect popped loudly and abruptly.

On those merits, we are not in a similar bubble today. As of September 30, 2024, the 10Y Treasury bond rate was 3.81%, which is 0.69 standard deviations below normal. Likewise, the S&P500 value of $5,762 is 0.9 standard deviations above its own respective trendline. Summed together, this gives a composite value of 1.33 standard deviations above normal, indicating that stocks are currently Overvalued.

The below table cites all data and sources used in constructing the charts, or otherwise referred to, on this page.

Item	Source
10 Year Treasury Bond Rate	Board of Governors of the Federal Reserve System (US), 10-Year Treasury Constant Maturity Rate [DGS10], retrieved from FRED, Federal Reserve Bank of St. Louis;
SP500 Price	Yahoo! Finance S&P500 Daily Close Values
CPI	U.S. Bureau of Labor Statistics, Consumer Price Index for All Urban Consumers: All Items [CPIAUCNS], retrieved from FRED, Federal Reserve Bank of St. Louis;