FIRE glossary
Trinity Study
The 1998 academic paper that tested fixed annual withdrawal rates against historical US market returns and became the origin of the "4% rule."
What the study actually did
In 1998, three professors at Trinity University tested a simple question: if you retired in any given year between 1926 and 1995 and withdrew a fixed percentage of your starting portfolio each year — adjusting that amount upward for inflation every year after — how often did the money last the full retirement? They ran this test across rolling historical windows using real US stock and bond returns, at different withdrawal rates and different stock/bond mixes. The 4% figure that became famous is the rate that survived nearly all of the 30-year windows they tested at a roughly 50/50 portfolio split — not a theoretical maximum, and not a rate calculated from first principles. It's an empirical result from one specific dataset.
Why it became "the 4% rule"
The study's core finding was easy to compress into a single memorable number, and that number spread far beyond the original paper's careful caveats. The original authors tested several rates and portfolio mixes, not just 4%, and were explicit that results varied with the underlying assumptions. What travelled into popular use was the single most quoted output — 4% — stripped of the surrounding context about the specific market, timeframe, and horizon it was tested against. That's the gap between the study and the folklore built on top of it.
Where it stops applying cleanly
Three specific limits are worth naming. First, the data is entirely US markets — a globally diversified portfolio, or one concentrated in a different country, faced a different historical sequence of returns. Second, the study tested 30-year retirement windows; early retirement often means a 40–50+ year horizon, where more years of exposure to a bad early sequence of returns pushes the historically "safe" rate lower (see sequence-of-returns risk). Third, it's a backward-looking test against one realised history, not a forecast — a future 30-year period is under no obligation to resemble any of the ones already in the sample. None of these limits mean the study was wrong; they mean its conclusions were scoped narrower than the popular "4% rule" shorthand suggests.
Across borders
The Trinity Study itself carries no cross-border content at all — it modelled a US retiree drawing from a US-dollar, US-market portfolio and paying no attention to currency, residency or tax. Applying its conclusions to a life lived across borders means checking assumptions the original research never tested: a portfolio spread across multiple currencies doesn't move in lockstep with a US 50/50 stock-bond mix, and the withdrawal itself may be taxed very differently depending on where you're resident when you take it. Ember's tax gross-up (see safe-withdrawal-rate) and country pages handle that second part explicitly — the study's market-return assumptions don't need to.
Common questions
Is the Trinity Study still considered reliable?
It's a real, widely-cited piece of research, not a myth — but it's one historical dataset (US markets, 1926–1995) tested over 30-year windows, and treating its headline 4% figure as a universal law rather than one study's result is the common misreading. Later research using the same methodology on different horizons and portfolios has produced different numbers.
Does the Trinity Study apply to a 40+ year early retirement?
Not directly — it only tested 30-year retirement windows. A longer early-retirement horizon carries more exposure to a bad early sequence of returns, which is why researchers focused on early retirement tend to favour a more conservative rate than 4% for horizons well beyond 30 years.
Did the Trinity Study say 4% always works?
No — it found that a 4% initial withdrawal, with the amount then rising each year for inflation, survived nearly all of the historical 30-year periods it tested against a roughly 50/50 stock-and-bond portfolio. "Nearly all" is not "all," and the study never claimed a guarantee.
Related terms
See trinity study in your numbers
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