Mar 25, 2009

Spending in retirement: The 4% rule

Bill Sharpe et. al. prepared an interesting analysis of The 4% Rule (At What Price?) that examines the traditional advice given to retirees: plan on spending 4% of your initial 60% stock/40% bond portfolio every year in retirement to acheive a >90% success rate (confidence interval) of having enough money for the rest of your life. The 4% rule is parroted by many retail investment advisers - and therefore, "it behooves us to be familiar with the rule’s approach, features, and flaws." They find that this strategy, while popular, is suboptimal in terms of overpayments and surpluses (i.e. you are paying more than you need to or spending less than you could). Common modifications to the plan, such as adjusting spending depending on market returns (also known as the glide-path strategy), don't do much to remedy the situation. Part of the problem lies in the fundamental mismatch between the cash flows - the investor seeks a constant stream of payments while the portfolio's returns are variable.

The authors make an interesting comparison between the risky investor that buys stocks and bonds versus one that guarantees his retirement spending by buying zero-coupon inflation-index government bonds (TIPS). For the very risk averse investors, this 100% TIPS portfolio provides the highest utility and dominates other investment strategies. This is an application of financial economics. Investors that are willing to take on more risk with a stock/bond portfolio can still achieve a higher utility by moving away from the 4% rule. But the riskless portfolio provides a minimum cost option that all other investment/spending plans should be judged against.

What does all this mean? Basically, that blind adherence to the 4% rule is inefficient. Unfortunately the paper stops short of developing a new framework that takes into account investors' preferences. However, it does conclude that once surpluses or overpayments are identified, they can be appropriately valued, which is the first step towards improving the spending and investing decisions.

This graph plots the static 4% rule at point A for an investor that has $100 to pay for a retirement plan. You can clearly see that an individual would prefer other points under the curve. Depending on their risk tolerance, they might move up and obtain a higher utility for the same cost, or move to the left and get the same benefits for less cost.

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