Retirement Decumulation

Core Concepts

Sequence-of-Returns Risk

Two retirees earning identical average returns can finish with very different wealth if the returns arrive in a different order while money is being withdrawn. Without withdrawals, order is irrelevant — multiplication commutes. With withdrawals, dollars sold after a decline are gone permanently and never participate in the recovery, so poor early returns do disproportionate damage. The danger zone is roughly the five to ten years on either side of the retirement date. Mitigants: flexible spending rules (guardrails), a cash/short-bond buffer, reduced equity exposure near retirement (or a rising equity glide path), and part-time income that lowers the withdrawal rate in early years.

Safe Withdrawal Rate (SWR) Framework

Bengen's 1994 study (extended by the 1998 Trinity study) found that an initial withdrawal of 4% of the portfolio, adjusted for inflation each year thereafter, survived every rolling 30-year US historical period with 50-75% stocks — the "4% rule." Its assumptions are also its criticisms: it relies on US historical returns (an unusually strong market), a fixed 30-year horizon, rigid inflation-adjusted spending with zero flexibility, and it ignores fees and taxes. Longer retirements, high starting valuations, or lower expected returns argue for 3-3.5% initial rates; dynamic rules (guardrails, amortization-based, RMD-style percent-of-balance) support higher initial rates because spending flexes with the portfolio.

Guardrails (Guyton-Klinger-Style Decision Rules)

A concrete dynamic rule set:

• Initial rate: withdraw, e.g., 5.0% of the starting portfolio in year one.
• Inflation rule: each year, increase the prior dollar withdrawal by inflation.
• Guardrails: compute the current withdrawal rate = this year's withdrawal / current portfolio. Set guardrails at plus or minus 20% of the initial rate (for 5.0%: upper 6.0%, lower 4.0%).
• Capital-preservation rule: if the current rate rises above the upper guardrail, cut the withdrawal 10%.
• Prosperity rule: if the current rate falls below the lower guardrail, raise the withdrawal 10%.

The full Guyton-Klinger rule set also skips the inflation increase after a negative-return year. The trade-off: a higher starting income than the 4% rule, paid for with variable spending — the retiree must actually take the cuts when triggered.