What the CAPM Formula Tells Us
The CAPM (Capital Asset Pricing Model), developed by William Sharpe and others in the 1960s, explains the expected return of an individual asset in relation to its risk. The basic formula is: Expected Return = Risk-Free Rate + Beta × Market Risk Premium. The risk-free rate is the government bond yield (approximately 0.5-1% in Japan), and the market risk premium is the return of the overall stock market minus the risk-free rate (historically around 5-7% per year).
What this formula means is that when investors take on additional risk, they can expect a return commensurate with that risk. However, CAPM only rewards non-diversifiable risk (systematic risk). The bankruptcy risk of an individual company or industry-specific risks can be eliminated through diversification, so the market does not pay a premium for these risks. This concept provides the theoretical justification for why diversification is rational.
How to Read Beta Values and Use Them in Practice
Beta measures an individual asset's sensitivity to the overall market. A beta of 1.0 means the asset moves in line with the market, 1.5 means it fluctuates 1.5 times as much, and 0.5 means half as much. For example, Toyota Motor has a beta of roughly 0.9-1.1, moving close to the market average, while emerging technology companies often have betas of 1.5-2.0, gaining significantly in rising markets but falling sharply in downturns.
In practice, beta values are used for portfolio risk adjustment. books on equity analysis and risk metrics explain how investors nearing retirement can increase their allocation to low-beta stocks (utilities, consumer staples sectors), while younger investors can include high-beta stocks to pursue higher returns.
Limitations of CAPM and Advanced Models
CAPM is a theoretically elegant model, but it cannot fully explain real-world markets. The biggest issue is that beta alone does not adequately explain differences in stock returns. Fama and French proposed the three-factor model in 1993, showing that in addition to beta, firm size (small-cap effect) and value (value stock effect) also influence returns. This was further expanded in 2015 into a five-factor model that includes profitability and investment patterns.
What matters most for individual investors is not the precision of CAPM but the fundamental principle that risk and return are two sides of the same coin. guides to factor investing introduce practical applications of multi-factor models that evolved from CAPM.
Next Steps for Applying CAPM to Your Investment Decisions
To put CAPM knowledge into practice, start by determining the overall beta of your portfolio. You can look up individual stock betas on your brokerage's tools or Yahoo Finance, then calculate the weighted average based on each holding's proportion. If your portfolio beta exceeds 1.3, your portfolio carries higher risk than the market average; below 0.7, it is conservative. Compare this against your risk tolerance to judge whether the level is appropriate.
Next, try calculating the expected return of your holdings using CAPM with the risk-free rate and market risk premium. Using the current Japanese 10-year government bond yield (approximately 1%) as the risk-free rate and a historical equity risk premium of about 5-6%, a stock with a beta of 1.2 has an expected return of roughly 7-8.2%. Check whether you are being adequately compensated for the risk you are taking, and if any holdings offer insufficient returns relative to their risk, consider replacing them - that is the first step toward rational portfolio improvement using CAPM.