The Trinity Study: Foundation of FIRE Math

The Trinity Study is the research behind the 4% rule. Published in 1998 by three professors at Trinity University, it analyzed historical data to determine safe withdrawal rates for retirement portfolios. Understanding this study helps you evaluate whether the 4% rule applies to your situation.

What the Researchers Did

Professors Philip Cooley, Carl Hubbard, and Daniel Walz examined stock and bond returns from 1926 to 1995. They tested various portfolio allocations (100% stocks down to 100% bonds) and various withdrawal rates (3% to 12%) across rolling periods of 15 to 30 years.

For each combination, they asked: what percentage of historical periods would the portfolio have survived? "Survived" meant not running out of money before the end of the period.

Key Findings

4% withdrawal rate with 50-75% stocks: Success rate of roughly 95-98% over 30-year periods. This became the basis for the "4% rule."

Stocks improve success: Counter-intuitively, portfolios with more stocks (50-75%) had higher success rates than those with mostly bonds. Stocks provided growth that outpaced inflation over long periods.

Lower withdrawal rates are safer: At 3% withdrawal, success rates were nearly 100% regardless of allocation. Higher rates became increasingly risky.

Important Caveats

30 years may not be enough: The study focused on 30-year retirement periods. If you're planning for 40-50 years, the research doesn't directly apply.

Historical data, not predictions: Past performance included specific conditions (world wars, the Great Depression, various bull markets) that may not repeat. Future returns could differ systematically.

Inflation-adjusted withdrawals: The 4% rule assumes you increase withdrawals annually with inflation. In bad markets, this can deplete portfolios faster.

Success means survival, not comfort: A "successful" outcome meant having at least $1 left at the end. Some scenarios ended with much less than started, even if technically surviving.

Updates and Modern Context

The study has been updated multiple times, most recently including data through 2009. The findings have held up, though some researchers now suggest 3.5% or lower might be more appropriate given current valuations and lower bond yields.

The original study also assumed simple rebalanced portfolios of stocks and bonds. Modern retirement planners often incorporate more nuanced strategies: variable withdrawal rates, guardrails, and dynamic asset allocation.

Practical Application

The Trinity Study provides a reasonable starting point, not a guarantee. For shorter retirements (20-25 years), 4% is probably conservative. For very long retirements (40+ years) or those wanting extra safety, 3-3.5% may be wiser. Your personal situation, risk tolerance, and flexibility matter as much as the historical data.

This article is for educational purposes only and does not constitute investment advice.