Fama-French Three-Factor Model

The Fama-French Three-Factor Model was a landmark development in empirical finance that challenged the sufficiency of the single-factor CAPM. Fama and French documented that two characteristics — company size (measured by market capitalization) and value (measured by the ratio of book equity to market equity) — explained significant cross-sectional variation in stock returns that the market factor alone could not capture.

The model has three components. The market factor (Mkt-RF) is the excess return of the broad equity market over the risk-free rate, identical to the CAPM factor. The size factor (SMB, for Small Minus Big) is the return of a portfolio long small-capitalization stocks and short large-capitalization stocks. The value factor (HML, for High Minus Low) is the return of a portfolio long high book-to-market (value) stocks and short low book-to-market (growth) stocks.

Empirical evidence showed that portfolios with high loadings on SMB and HML earned higher average returns than the CAPM would predict. Fama and French interpreted this as evidence that size and value represent compensated risk factors — investors demand extra return for bearing the systematic risks associated with small-cap and value stocks.

An alternative interpretation, associated with behavioral finance, is that the value and size premiums arise from investor mispricing. Growth stocks may be overvalued due to excessive optimism about future earnings, and value stocks undervalued due to excessive pessimism. From this view, the premiums represent an opportunity rather than compensation for risk.

The Three-Factor Model became the standard benchmark for evaluating mutual fund and hedge fund performance, replacing CAPM-based alpha. A manager's excess return must be measured relative to all three factor exposures before the residual can be attributed to stock-picking skill.