Plans Fail When Assumptions Fail

January 6, 2026

Every retirement plan is built on a return assumption. When that assumption is wrong, the plan does not adjust — the outcome does. Most investors do not know what return their plan requires.

The Hidden Dependency in Every Retirement Plan

Every retirement plan contains a return assumption. It may be stated explicitly — "we are assuming 7% annual growth" — or it may be embedded invisibly in the projections. Either way, the plan's viability depends entirely on that assumption being correct over the relevant time horizon.

Most investors do not know what return their plan requires. They know what their advisor projected. They do not know the minimum rate of return below which the plan breaks — the point at which the income the plan is designed to generate can no longer be sustained without depleting the principal ahead of schedule.

Every plan depends on performance. When returns fall short, the plan eventually breaks — regardless of intent. Return assumptions are not guarantees. When returns fall short, the math does not adjust — the outcome does.

The Math Does Not Adjust

This is the critical point that most retirement planning conversations obscure: the math in a retirement plan is fixed. The income need is fixed. The time horizon is fixed. The only variable is the return the portfolio actually generates.

When returns fall short of the assumption, the plan does not automatically recalibrate. The income continues to be drawn. The principal continues to be depleted. The shortfall accumulates silently until it becomes visible — usually at the point when it is too late to correct without significant lifestyle adjustment.

A plan that assumed 7% and delivered 4.5% over a 20-year retirement does not produce a slightly worse outcome. It may produce a catastrophic one — not because the investor made a mistake, but because the assumption was never stress-tested against the possibility of being wrong.

Why Assumptions Fail

Return assumptions fail for three reasons, and all three are structural rather than exceptional.

First, sequence of returns risk. Even if the average return over a 20-year period matches the assumption, the order in which those returns occur matters enormously. A significant loss in the early years of retirement — when the portfolio is at its largest and withdrawals are beginning — can permanently impair the plan's ability to recover, even if subsequent years are strong.

Second, inflation. A plan that assumes 7% nominal return and 2% inflation has a real return assumption of 5%. If inflation runs at 4% — as it did from 2021 to 2023 — the real return is cut in half without any change in the nominal portfolio performance. The income need grows faster than the plan anticipated.

Third, the assumption itself may simply be too optimistic. A 7% return assumption for a 60/40 portfolio in a low-yield environment is not conservative. It is aspirational. Plans built on aspirational assumptions are structurally fragile.

The Required Return Standard

The alternative to assumption-based planning is requirement-based planning. Instead of asking "what return can we reasonably expect?" the question becomes "what return does this plan require to succeed?" — and then building a strategy specifically designed to pursue that return, rather than a generic allocation that may or may not deliver it.

This is the Required Return: the specific rate of return the portfolio must achieve to fund the investor's income without shortfall over the relevant time horizon. It is not a projection. It is a mathematical requirement derived from the income need, the portfolio size, and the time horizon.

When the Required Return is known, the portfolio can be constructed with a specific purpose. When it is unknown, the portfolio is constructed with a general hope.

The Question to Ask

Ask your advisor: What is the minimum rate of return my portfolio must achieve for this plan to remain viable? And: What happens to the plan if returns average 2% below that assumption for the first five years of retirement?

If those questions cannot be answered precisely, the plan has not been stress-tested. And a plan that has not been stress-tested is not a plan — it is a projection built on an assumption that has never been challenged.