Why Sequence Matters
Two investors can experience identical average returns over a 30-year retirement but accumulate very different outcomes if the timing of those returns differs. This is sequence-of-returns risk: the order in which gains and losses occur matters enormously when money is being withdrawn from a portfolio, even if the arithmetic average return is the same.
During the accumulation phase — when an investor is saving and adding money — sequence of returns matters relatively little. A bear market early in the saving period is, in a narrow sense, a benefit: the investor is buying more units at lower prices, which compounds favourably when the market recovers. A bear market late in accumulation is more damaging but still manageable, because the investor can delay retirement if necessary.
The distribution phase is fundamentally different. A retiree who experiences a severe bear market in the first years of retirement is forced to sell depressed assets at low prices to fund living expenses. This permanently reduces the portfolio's size, leaving fewer assets to participate in the eventual recovery. The mathematics compound against the investor: a smaller base produces smaller absolute gains, which makes it harder to rebuild. A bear market late in retirement — when the portfolio is already smaller — produces far less damage in absolute terms.
Bengen's 4% Rule
In 1994, financial planner William Bengen published a landmark study in the Journal of Financial Planning, "Determining Withdrawal Rates Using Historical Data." His question was concrete: what is the maximum percentage of a portfolio that a retiree can withdraw annually, adjusted for inflation, without running out of money over a 30-year retirement?
Bengen tested this against every 30-year rolling period in US market history, including those starting with the worst market conditions on record — the Great Depression, the stagflation of the 1970s. His answer: a withdrawal rate of 4% of the initial portfolio value, adjusted annually for inflation, survived every historical 30-year period with a portfolio of 50–75% equities.
This became known as the 4% rule. A retiree with a £500,000 portfolio could withdraw £20,000 in the first year, then adjust upward for inflation each year, and would not have run out of money in any historical 30-year period. The rule was based on the worst-case historical sequences — specifically, it was calibrated to ensure survival even under the most damaging return sequences in the data.
Limitations of the 4% Rule
Bengen's rule has important limitations. It was calibrated to US historical data, which reflects one of the best-performing stock markets in history. Applied globally, with lower base return expectations, the safe withdrawal rate may be closer to 3–3.5% than 4%.
The rule assumes a 30-year retirement. A retiree who retires at 55 may need 40 years of portfolio survival. For longer retirement horizons, the safe withdrawal rate is lower.
Current bond yields and equity valuations in many markets differ substantially from their historical averages, and several researchers — including Wade Pfau — have argued that forward-looking safe withdrawal rates in many markets are below 4%.
Finally, the 4% rule produces a fixed real income. Real retirees can adapt: spending less in bad years, working part-time, drawing on housing equity, or adjusting lifestyle. A dynamic withdrawal strategy that responds to portfolio performance can sustain higher average spending than a rigid rule.
The Practical Implication for Allocation
The sequence-of-returns analysis explains why the allocation at retirement matters as much as the allocation during accumulation. An all-equity portfolio maximises expected wealth on average but creates devastating sequence-of-returns exposure if a crash occurs in the first few years of retirement. A moderate equity allocation (Bengen's 50–75%) reduces this vulnerability while preserving enough growth to sustain 30 years of inflation-adjusted withdrawals.