Zvi Bodie, Ph.D, in a recent article in the Journal of Financial Planning highlights a few of the traps that lie in wait for those trying to understand the nature of risk in the capital markets:
Stocks as a hedge against inflation. While many have argued that investors with long time horizons should own stocks as a means of hedging against inflation, there is no evidence that stocks offer an effective hedge, even in the long run. In fact, empirical studies show that stock returns are largely uncorrelated with inflation. Not only that, but stocks have often performed very poorly during periods of high inflation, such as experienced in the 1970s. The idea that stocks should be included in a glide path as an effective hedge against inflation is not justified by the facts.
The fallacy of time diversification. The idea that the risk of holding risky assets somehow decreases with the length of the holding period has perhaps been around as long as investing itself. That this is a fallacy is well documented. A simple way to understand this is to consider the riskiness of an asset, or portfolio of assets, in terms of the cost to insure that it will earn at least the risk-free rate of return over time. Bodie (1995) shows that the cost of this insurance increases with the time horizon, and the empirical evidence supports this conclusion. Such insurance can be replicated by purchasing a put option, and the actual prices of put options traded in the capital markets do in fact increase with the length of their horizons.
Reliance on probability statistics as a measure of risk. Probability theory has strongly influenced modern economics, including the area of lifecycle finance. In fact, we can learn much about its application—and limitations—from its 17th century founders, Blaise Pascal and Pierre de Fermat. It was Pascal who so famously reasoned that knowing the probability of an event was not enough. The consequences of the event matter, too. Thus, risk has two dimensions. One involves the probabilities of certain events. The other involves the consequences of those events. In terms of the defined contribution plan objective, we can think of this decomposition of risk in terms of (1) the probability that a given funding level objective will be met and, if not, then (2) the magnitude by which it could fall short of its objective. Any risk measure that does not address both dimensions is flawed. Risk measurement is not the same thing as probability measurement.