Are Stocks Really Less Volatile in the Long Run?

Abstract:     
Conventional wisdom views stocks as less volatile over long horizons than over short horizons due to mean reversion induced by return predictability. In contrast, we find stocks are substantially more volatile over long horizons from an investor's perspective. This perspective recognizes that parameters are uncertain, even with two centuries of data, and that observable predictors imperfectly deliver the conditional expected return. Mean reversion contributes strongly to reducing long-horizon variance, but it is more than offset by various uncertainties faced by the investor, so that annualized 30-year variance is nearly 1.5 times the 1-year variance. The same uncertainties also make target-date funds undesirable to a class of investors who would otherwise find them appealing.

Keywords: stock, volatility, target-date funds, Bayesian, predictive system, predictive variance

JEL Classifications: G12

Working Paper Series