The Trinity Study: What It Actually Concluded (and What It Didn't)

The Trinity Study tested fixed percentage withdrawal rates from 3% to 12% against every rolling historical window from 1926 to 1995. For each combination of withdrawal rate × asset allocation (100% stocks, 75/25, 50/50, 25/75, 100% bonds) × horizon (15, 20, 25, 30 years), it counted the percentage of rolling periods where the portfolio lasted the full horizon without running out. The headline finding: at 4% withdrawal, a 50/50 or 75/25 stock/bond portfolio succeeded in 95–100% of 30-year historical periods. At 5%, success dropped to 83%. At 6%, 68%. The 2011 update extended the data through 2009 (covering the dot-com crash and 2008–09 financial crisis) and confirmed similar results.

The study was authored by Philip L. Cooley, Carl M. Hubbard, and Daniel T. Walz — all finance professors at Trinity University in San Antonio. It was published in the AAII Journal (American Association of Individual Investors) in February 1998. The paper built directly on Bengen (1994) but extended the analysis across five portfolio allocations and four time horizons, making it the more comprehensive reference point. The FIRE movement adopted the Trinity Study as canonical evidence that 25× annual spending was enough to retire. The 2011 update, 'Portfolio Success Rates: Where to Draw the Line', confirmed the original findings with extended data.

Three major limitations the Trinity Study does not address. First, the data is entirely US-centric — specifically US large-cap equity and intermediate Treasury bonds. Non-US investors or globally-diversified portfolios may have different risk profiles. Second, the longest horizon tested was 30 years; extending the same methodology to 40–50 year horizons (required for early retirees) produces much lower safe rates (~3.25–3.5%). Third, the study assumes a constant inflation-adjusted withdrawal amount — it does not model dynamic strategies like Guyton-Klinger Guardrails or floor-and-upside approaches, which have been shown to improve outcomes significantly at any horizon. Finally, the data covers a period of declining interest rates and rising equity valuations — the forward-looking environment of 2020s retirees may be structurally different, which is Wade Pfau's critique.

• “Original 1998 study, AAII Journal” — Cooley, Hubbard, Walz (1998)
• “2011 update with data through 2009” — Cooley, Hubbard, Walz (2011), 'Portfolio Success Rates'
• “Extensions to longer horizons” — Pfau, Finke, Blanchett (2013)

• Cooley, Hubbard, Walz (1998), 'Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable', AAII Journal (accessed 2026-04-17)
• Bengen (1994), 'Determining Withdrawal Rates Using Historical Data', Journal of Financial Planning (accessed 2026-04-17)
• Bogleheads Wiki — Safe Withdrawal Rates (accessed 2026-04-17)

Last verified: 2026-04-17