The order in which returns occur—not just the long-term average—determines whether a portfolio can sustain withdrawals and retirement spending. Poor returns early in retirement or late in accumulation can derail plans even if long-term average returns are excellent.
Simulate two 30-year portfolios: one with strong returns first then weak, one with weak returns first then strong. Observe the difference in ending wealth and sustainable withdrawal amounts.
As long as average returns are good, sequence doesn't matter; the stock market always recovers 'in the long run'; retirees don't need to adjust risk exposure; sequence risk only affects retirees.
You already understand expected returns and asset allocation — that a portfolio's long-run average return is what drives wealth accumulation over time. Sequence of returns risk exposes a critical assumption buried in that logic: the order of returns matters just as much as the average, but *only* when money is flowing in or out of the portfolio. During the accumulation phase when you are purely adding money, the sequence matters far less. The problem emerges sharply when you begin withdrawing.
Here is the intuition. Imagine two retirees, each starting with $1,000,000 and withdrawing $50,000 per year. Over 20 years, both experience the same set of annual returns — say an average of 6% — but in opposite order. Retiree A gets the bad years first, then good years. Retiree B gets the good years first, then bad. Arithmetic says they should end up with similar wealth, since the average is identical. But they do not. Retiree A runs out of money. Retiree B is fine. Why? Because Retiree A was forced to sell shares to fund withdrawals during the bad years when share prices were low — a phenomenon called selling into a downturn. Those sold shares are gone; they cannot participate in the subsequent recovery. Retiree B still owned shares when prices recovered and compounded from a larger base. The math is not commutative when cash is leaving the account.
The connection to arithmetic sequences helps clarify the mechanics: each withdrawal depletes the base that future returns compound on. A 30% market drop in year two of retirement followed by $50,000 in withdrawals means your portfolio starts year three substantially smaller — not just because of the loss, but because you withdrew from an already-reduced base. A 30% recovery in year three now applies to that smaller number. The sequencing of the -30% and the +30% around the withdrawal has a permanent asymmetric effect.
This has direct implications for how you should manage risk as you approach and enter retirement. The standard advice to "stay the course" and "the market always recovers" applies well to pure accumulators. For retirees or near-retirees, it can be dangerously incomplete. Strategies to mitigate sequence risk include: holding 1-3 years of expenses in cash or short-term bonds (a "buffer" that allows you to avoid selling equities during a downturn), reducing equity exposure in the years surrounding retirement (the "glide path"), and maintaining flexible withdrawal rates that can be temporarily reduced if markets fall early in retirement. None of these eliminate risk, but they reduce the probability that a bad early sequence permanently derails the plan.