During a 2001 interview with Fortune magazine, famed investor Warren Buffett commented on his favorite stock market valuation indicator, calling it “probably the best single measure of where valuations stand at any given moment.” With his comments, the oracle of Omaha propelled the market-cap-to-GDP ratio to center stage, increasing its popularity almost overnight. Was he correct? Is this a reliable indicator of stock valuations? I do not believe it is correct, and here’s why.
To calculate the valuation of the U.S. stock market using this indicator, you simply divide the total market cap of all publicly traded U.S. stocks by GDP. The rationale behind it seems sound. After all, stock prices are based on profits, and corporate profitability is connected to economic growth. Therefore, if the economy expands, it is reasonable to assume that corporate profits will rise and push stock prices higher. However, if stock prices expand at a faster rate than the economy, stocks will become overvalued. Here is an overly simplified example to provide clarity.
Assume there are 1,000 publicly traded companies in the U.S. When you multiply the number of outstanding shares of each company by their price, the total market cap is $1.0 billion (i.e., the numerator). If GDP is also $1.0 billion (the denominator), the ratio would be 100% ($1.0 billion / $1.0 billion), indicating that stocks are fairly valued. Now let’s assume it is 12 months later and total stock market cap has risen to $1.5 billion. If GDP remained at $1.0 billion, the ratio would be 150% ($1.5 billion / $1.0 billion). Hence, stocks would be 50% overvalued. It’s that simple, but there are some major flaws in the formula.
To understand where the tool falls short, we will examine a graph of the indicator’s market valuation from December 31, 1970 to August 4, 2016. The red-dotted line indicates “fair value,” the point when stock market cap and GDP were equal.