The model and its logic
CAPM rests on the idea that the only risk investors should be compensated for is systematic (market) risk — the risk that cannot be diversified away. Idiosyncratic (company-specific) risk can always be eliminated by holding a diversified portfolio, so the market pays no premium for bearing it. Beta (β) measures how much an asset moves relative to the market: a beta of 1.5 means the asset moves 1.5% for every 1% market move. The equity risk premium (ERP) is the excess return of the market over the risk-free rate. In CAPM, expected return = risk-free rate + β × ERP. A high-beta asset commands a higher expected return to compensate for greater systematic risk.
Empirical failures of CAPM
CAPM predicts a positive linear relationship between beta and expected return. Empirical tests since the 1970s have consistently found this relationship is much flatter than predicted, or even inverted. The low-volatility anomaly (sometimes called "low beta" anomaly) — one of the most robust findings in empirical finance — shows that low-beta stocks have historically generated returns similar to or higher than high-beta stocks on an absolute basis, and dramatically better risk-adjusted returns. CAPM cannot explain the value premium (cheap stocks outperform), the size effect (small-caps outperform), or the momentum effect — all documented since the 1990s.
Fama-French and beyond
In response to CAPM’s empirical failures, Fama and French (1993) introduced a three-factor model: market beta, a size factor (SMB: small minus big), and a value factor (HML: high minus low book-to-market). Carhart (1997) added a fourth factor — momentum (WML: winners minus losers). These multi-factor models explain historical cross-sectional return variation far better than CAPM. Later extensions added profitability (QMJ: quality minus junk) and investment (CMA: conservative minus aggressive). The Fama-French five-factor model is now the standard academic benchmark, though practitioners often use proprietary multi-factor models with more variables.
“CAPM is the wrong model, but it is a useful wrong model. It gave finance a language for thinking about risk and return systematically for the first time.”
What this means for you
CAPM’s most practical legacy is the insight that risk matters and must be compensated — you should not hold a riskier asset unless you expect a higher return. Its most important failure is the suggestion that beta alone captures risk: it does not. Factor investing (value, quality, momentum, low volatility) emerged precisely because CAPM missed dimensions of risk and return that practitioners found empirically. For cost-of-equity calculations in corporate finance, CAPM remains the standard despite its flaws — simply because it is tractable, widely understood, and "wrong in a standard way" that professionals have learned to adjust for.