If you believe that value matters but valuations do not necessarily mean revert, you should move your portfolio of risky assets around pretty aggressively as valuations shift among the various risky assets. But you should keep a fairly constant allocation to risky assets over time except in the rare instances where valuations are so extreme that risky assets are actually priced to lose out to lower-risk assets.
That strategy will outperform a naïve strategy over time, but if valuations do mean revert, it is substantially sub-optimal. If valuations mean revert, you can improve the risk/reward trade-offs of your portfolio substantially by adjusting how much risk you take through time, taking more risk when the return to risk is high, and less when it is low.
- GMO