What Is the Equity Premium Puzzle (EPP)?

The equity premium puzzle (EPP) refers to the excessively high historical outperformance of stocks over Treasury bills, which is difficult to explain. The equity risk premium, which is usually defined as equity returns minus the return of Treasury bills, is estimated to be between 5% and 8% in the United States. The premium is supposed to reflect the relative risk of stocks compared to "risk-free" government securities. However, the puzzle arises because this unexpectedly large percentage implies an unreasonably high level of risk aversion among investors.

Key Takeaways

  • The equity premium puzzle (EPP) refers to the excessively high historical outperformance of stocks over Treasury bills, which is difficult to explain.
  • Theoretically, the premium should actually be much lower than the historical average of between 5% and 8%.
  • The prospect theory by Daniel Kahneman and Amos Tversky, the role of personal debt, the importance of liquidity, the impact of government regulation, and tax considerations have been applied to the puzzle.
  • Previous lack of knowledge, the decline of the U.S. dollar relative to gold, the benefits of diversification, and population growth are all possible solutions to the equity premium puzzle.

Understanding the Equity Premium Puzzle (EPP)

The equity premium puzzle (EPP) was first formalized in a study by Rajnish Mehra and Edward C. Prescott in 1985. It remains a mystery to financial academics to this day. Notably, Professor Prescott won the Nobel Memorial Prize in Economics in 2004 for his work on business cycles and demonstrating that "society could gain from a prior commitment to economic policy," according to a statement by the prize organization.

Some academics believe the equity risk premium is too large to reflect a "proper" level of compensation that would result from investor risk aversion. Therefore, the premium should actually be much lower than the historical average of between 5% and 8%.

Some of the mystery surrounding the equity premium puzzle involves the variance of the premium over time. Estimates for the first half of the 20th century put the equity risk premium at near 5%. In the second half of that century, the equity premium went up to over 8%. The equity premium for the first half may be lower because the U.S. was still on the gold standard, limiting the impact of inflation on government securities. Many measures of stock market valuation, such as the P/E 10 ratio, also help to explain the different equity premiums. U.S. stock valuations were above average in 1900, relatively low in 1950, and at record highs in 2000.

Since the introduction of the EPP, many academics attempted to solve, or at least partly explain, the equity premium puzzle. The prospect theory by Daniel Kahneman and Amos Tversky, the role of personal debt, the value of liquidity, the impact of government regulation, and tax considerations have been applied to the puzzle.

Regardless of the explanation, the fact remains that investors have been rewarded handsomely for holding stocks instead of "risk-free" Treasury bills.

Special Considerations

Given its variance and status as an anomaly, there are substantial questions about the durability of the equity premium. Perhaps, the real reason for the seemingly excessive equity risk premium is that investors did not realize how much more stocks returned. A large portion of the market's returns come from dividends, which are obscured by media coverage of daily price movements. As people realized the long-term benefits of stock ownership, valuation levels generally trended up. The end result could be lower returns for stocks, which would solve the equity premium puzzle.

The "risk-free" nature and value of Treasury bills is another crucial consideration. Are Treasury bills genuinely risk-free? Of course not. Many governments have inflated their currencies and defaulted on their debts. Even the credibility of the U.S. government varies from one year to the next. Arguably, gold is the risk-free asset. Measured against gold, the equity premium since 1970 is far less impressive. From this point of view, the high equity premium is explained by the decline of the U.S. dollar against gold rather than objectively high returns for stocks.

The aggregation of stocks may also play a part in the equity risk premium puzzle. Individual stocks are far riskier than the stock market as a whole. In many cases, investors were compensated for the higher risk of holding particular stocks rather than overall market risk. The traditional idea was that an investor would directly buy shares in a few companies. Ideas about diversification, mutual funds, and index funds came later. By diversifying, investors can reduce risk without reducing returns, potentially explaining the excessive equity risk premium at the heart of the equity premium puzzle.

Finally, demographics may play a significant role in stock market returns and explaining the equity premium puzzle. Intuitively, businesses need more customers to grow. When the population is rising, the average business automatically gets new customers and grows. During the 20th century, populations in most countries were increasing, which supported business growth and higher stock market returns. Empirically, stock markets in Japan and many European countries performed poorly as their populations started to decline. Perhaps, rising populations created the equity premium puzzle.