Today’s Market Environment
Future stock returns depend on dividend income, the growth of the underlying earnings, and changes in the valuation multiples placed on those earnings. If the current dividend yield is below its historical average, then future stock returns will also tend to be lower. When price-earnings multiples are high, markets tend to exhibit mean reversion and relatively lower future returns are to be expected.

Returns on bonds, meanwhile, depend on the initial bond yield and on subsequent yield changes. Low bond yields will tend to translate into lower returns because there is less income and there could be capital losses if interest rates rise.

Sustainable withdrawal rates are intricately related to the returns provided by the underlying investment portfolio. And with sequence-of-return risk, the returns experienced in early retirement will weigh disproportionately on the final outcome. The current market conditions are much more relevant to this approach, yet many financial planning software programs unfortunately still default their Monte Carlo simulations to higher historical average returns.

That’s another reason we must question the relevance of conclusions based on what worked in the past. The U.S. historical record is too short to determine how much can be safely withdrawn from a rather aggressive investment portfolio. The combination of low bond yields and high stock market valuations today suggests that the situation is different now. Today we are dealing with a situation in which Shiller’s cyclically adjusted price-to-earnings ratio (PE10) is well above historical averages, while bond yields are at historic lows.

Figure 2 demonstrates how today’s high-valuation/low-yield situation has been quite rare in U.S. history, indicating that we are in uncharted territory when trying to determine whether the 4% rule will remain a safe strategy. Historical simulations don’t analyze this possibility, but with Monte Carlo simulations we can adjust our capital-market expectations to better account for the types of returns that are more likely to be experienced in the future.

In January 2015, the 10-Year Treasury rate was 1.88%. This is about 2.8 percentage points less than the historical average of 4.7%. Today’s retirees will be more strained to spend principal to achieve a 4% sustainable withdrawal rate. Even if we assume the historical risk premium for stocks and other asset characteristics remain the same, but adjust the average return on stocks and bonds downward to reflect today’s lower bond yields, we will obtain higher failure rates for the 4% rule. But we must also consider that Shiller’s PE10 registered a value of 26.67 in January 2015. This is quite high, and a statistical regression of the subsequent 10-year average for the equity premium over bonds suggest that the equity premium will average 1.55% for the next 10 years when starting from this PE10 value.

Given that we require 30 years of data to calculate past sustainable withdrawal rates, 1941 is the only starting year with such low interest rates, and 1929 is the only year with such lofty stock market valuations for which we can know the subsequent outcomes.

There have been no past years with both of these rare events happening simultaneously. Again, this is uncharted territory.