The 1998 Trinity Study: Solid Foundations of the 4% Rule
The 4% rule originates from a landmark study published in 1998 by three professors from Trinity University: Philip L. Cooley, Carl M. Hubbard, and Daniel T. Walz. Their article, “Retirement Spending: Choosing a Sustainable Withdrawal Rate”, published in the Journal of Financial Planning, laid the groundwork for a quantitative method to determine a safe initial withdrawal rate on a stock/bond portfolio.
The methodology used is based on the analysis of 98 distinct historical 30-year withdrawal periods, covering the period from 1926 to 1995. The tested portfolio was a classic 50% stocks (S&P 500) / 50% bonds (long-term U.S. Treasury bonds) allocation. For each period, the authors simulated a fixed withdrawal adjusted annually for inflation and calculated the probability that the capital would not be depleted at the end of the 30 years.
The striking result was a 98% success rate for an initial withdrawal rate of 4%. In other words, in 96 of the 98 periods, an initial annual withdrawal equivalent to 4% of the capital, adjusted for inflation, did not exhaust the portfolio after 30 years. This benchmark was quickly adopted as a reference in financial planning and the FIRE (Financial Independence, Retire Early) movement.
Critical Analysis of the Trinity Methodology
Despite apparent robustness, several elements limit the universal scope of this study:
- Historical data limited to the United States: The study relies exclusively on U.S. markets from 1926 to 1995, a period marked by strong economic growth and high stock market returns. The results are therefore not directly transferable to other geographic regions or emerging markets.
- Retirement duration fixed at 30 years: The chosen duration corresponds to a ret