a fundamental challenge in delegated investment management is that many quantitative rules are vulnerable to agents who act to boost measured returns by selling insurance against unlikely events–that is, by writing deep out-of-the-money puts. An example is that if you hire an agent to manage your equity portfolio, and compensate the agent based on performance relative to the S&P 500, the agent can beat the benchmark simply by holding the S&P 500 and stealthily writing puts against it, since this put-writing both raises the mean and lowers the measured variance of the portfolio.7 Of course, put-writing also introduces low-probability risks that may make you, as the end investor, worse off, but if your measurement system doesn’t capture these risks adequately–which is often difficult to do unless one knows what to look for–then the put-writing strategy will create the appearance of outperformance.
The whole speech is a must-read. One more excerpt:
Quantifying risk-taking in credit markets is difficult in real time, precisely because risks are often taken in opaque ways that escape conventional measurement practices. So we should be humble about our ability to see the whole picture, and should interpret those clues that we do see accordingly.