What Is Regression to the Mean - An Inconvenient Truth from Statistics
Regression to the Mean is a statistical phenomenon where extreme measurements tend to move closer to the average on subsequent measurements. Discovered by Francis Galton in the 19th century, this principle operates powerfully in the investment world. A fund that dramatically outperforms the market average in one year tends to see its performance drift back toward (or below) the average in subsequent years. According to the SPIVA report by S&P Dow Jones Indices, of active funds that ranked in the top 25% over a five-year period, only about 20% maintained a top-25% ranking over the following five years.
Multiple factors explain this phenomenon. Part of strong performance is attributable to luck rather than skill, and luck doesn't persist. Additionally, money flows into top-performing funds, inflating assets under management and making nimble investing more difficult. Furthermore, a particular investment style (growth, value, etc.) may have temporarily aligned with market conditions, and the advantage disappears as the environment shifts.
The Trap Investors Fall Into When Selecting Funds Based on Past Performance
Mutual fund advertisements prominently display figures like "3-year return of XX%," but selecting funds based on these numbers is irrational behavior that ignores regression to the mean. Morningstar research has shown that investing in 5-star (highest-rated) funds produces future returns that are not statistically different from 3-star funds. Past strong performance not only fails to guarantee future success - it is virtually useless as a predictive indicator. Books on the pitfalls of fund selection explain with abundant data why performance-based fund selection fails.
Rational Fund Selection with Regression to the Mean in Mind
Investors who understand regression to the mean should focus not on past returns but on structural factors that influence future returns. The most reliable predictor is "cost." Funds with lower expense ratios statistically tend to deliver higher future returns - a relationship that is clear and consistent. This is because cost is the only factor that reliably drags down returns. The next most important factors are "consistency of investment policy" and "fund company governance."
Regression to the mean also helps explain why index funds outperform most active funds over the long term. Books on index investing and fund selection detail how active fund outperformance regresses to the mean, while index funds consistently deliver market-average returns minus costs. The lower the cost, the smaller the gap, and the result is that index funds end up beating the majority of active funds.
Next Actions for Applying Regression to the Mean to Your Investments
Start by comparing the past 5-year returns of any active funds you hold against an index fund in the same category. Include the expense ratio difference in your calculation and verify whether the active fund is truly delivering returns that justify its higher cost. If it underperforms the index after costs, consider switching to a low-cost index fund.
As a next step, shift your fund selection criteria from "past returns" to "low expense ratio," "consistency of investment policy," and "trustworthiness of the fund company." Use our compound interest calculator to project the asset difference over 30 years between two funds with a 0.5% difference in expense ratios, and experience firsthand the impact that costs have on long-term returns.