Finance likes to think of itself as a science. In the 1960s the capital-asset pricing model (CAPM) offered a breakthrough: a simple equation linking an asset’s expected return to its measurable risk. It was clean, testable and mathematically satisfying. A generation of textbooks enshrined it as gospel. Markets, however, have been stubbornly irreverent. Sixty years of data have left CAPM looking like a monument to misplaced certainty.
Rob Arnott of Research Affiliates and Edward McQuarrie, an emeritus professor at Santa Clara University, believe the flaw is fundamental. CAPM assumes investors are equally wary of volatility whether it is good or bad. Reality is skewed. Losses are dreaded; windfalls are welcomed. The pair argue pricing assets demands a “fear theory” that captures this imbalance.
Fear, in their telling, comes in two forms. The first is fear of loss (FOL), which drives investors to shun downside surprises. This can be measured by semivariance: how far returns fall below their average. The second is fear of missing out (FOMO), the pull towards lottery-like bets with small odds of outsized payoffs. Both instincts run deep in human psychology and, unlike CAPM’s symmetric notion of variance, they operate independently.
Behavioural finance has long pointed to the quirks of human decision-making: from prospect theory’s loss aversion to Robert Shiller’s narrative economics. Arnott and McQuarrie’s novelty is in marrying these insights to a direct challenge of CAPM’s empirical record. They note the celebrated equity premium—the extra return from holding shares rather than safe assets—has been inconsistent. In some eras it disappears. McQuarrie’s historical work finds in America’s own market, entire lifetimes have seen bonds outperform equities. An investor who bought shares in 1804 waited almost a century to pull ahead, only to fall behind again in the Depression. In many foreign markets bonds have dominated for decades at a time.
Part of the problem is the data on which CAPM was built.Before short-term Treasury bills were issued, analysts treated long-term government bonds as the “risk-free” benchmark. That meant comparing two risky assets, flattering equities in the historical tables. Adjust the record and extended stretches where stocks fail to beat bonds come into view. Arnott’s probability arithmetic shows with an equity premium of a mere two percentage points, it could take centuries before one could be statistically confident that shares will win. Mortal investors have less patience.
Fear theory seeks to replace CAPM’s single neat ratio with a messier set of moving parts. FOL and FOMO have both diversifiable and non-diversifiable components, influenced by market narratives, policy shifts as well as geopolitical shocks. Their relative dominance will change over time, producing the market’s familiar mood swings between exuberance and despair. Factor premiums such as momentum—returns to chasing recent winners—may be explained as temporary triumphs of one fear over the other.
This is as much a philosophical shift as an economic one. Karl Popper would say a theory that fails repeated tests should be abandoned. Thomas Kuhn would counter entrenched paradigms survive until replaced by something better. CAPM has persisted not only through academic inertia but also because it is easy to teach and compute. Fear theory, being more complex, will have to prove itself both explanatory and practical.
Investors have been told for decades that bearing risk is the price of higher returns. History shows the bill sometimes arrives without the dessert. If fear is the true currency of markets, finance will have to learn to account for it, and to accept that emotions, not equations, may at last set the price. ■
