CAPM: Capital Asset Pricing Model for Traders

The Capital Asset Pricing Model (CAPM), developed by Sharpe, Lintner, and Mossin in the 1960s, extends Modern Portfolio Theory by specifying the relationship between risk and expected return for any asset held in a well-diversified portfolio.

The core equation

CAPM states that an asset's expected return equals the risk-free rate plus a risk premium proportional to its beta:

$$E(R_i) = R_f + \beta_i \big(E(R_m) - R_f\big)$$

where:

• $R_f$ is the risk-free rate
• $\beta_i$ is the asset's sensitivity to market returns
• $E(R_m)$ is the expected market return
• $E(R_m) - R_f$ is the market risk premium

Beta is defined as:

$$\beta_i = \frac{\mathrm{Cov}(R_i, R_m)}{\sigma_m^2}$$

Systematic versus unsystematic risk

CAPM draws a sharp line between two risk types. Systematic risk is market-wide risk that cannot be diversified away — recessions, rate shocks, geopolitical events. Unsystematic risk is asset-specific and shrinks toward zero in a diversified portfolio.

CAPM rewards only systematic risk with expected return, because unsystematic risk can be diversified away for free. This is why a stock with high idiosyncratic volatility but low beta earns no risk premium under CAPM.

Practical use for traders

• Required return benchmark: Before entering a trade, CAPM gives a hurdle rate. If a strategy's expected return barely exceeds $R_f + \beta(R_m - R_f)$, the risk premium is thin.
• Performance evaluation: Alpha ($\alpha$) is the excess return above CAPM's prediction. A positive alpha means the strategy added value beyond its market exposure.
• Hedging: Knowing a position's beta tells you how many index futures contracts offset its market exposure.

Limitations

CAPM assumes a single-period horizon, homogeneous expectations, a risk-free lending and borrowing rate, and that all investors hold the market portfolio. Real markets violate every one of these. Beta is unstable through time, the market portfolio is unobservable, and multi-factor models (Fama-French, APT) explain returns better empirically.

For traders, CAPM's value is conceptual: separate market exposure from skill, demand a premium for bearing systematic risk, and use beta as a rough hedge ratio. Treat its output as a baseline, not a verdict.