📖John Bogle

Reversion to the Mean

🌱 Beginner★★★★☆

Past fund performance rarely predicts future success.

💬

Fund returns tend to revert to the mean. Yesterday's winners become tomorrow's losers, and vice versa.

— Common Sense on Mutual Funds，1999

🏠 Everyday Analogy

Risk control is like a seatbelt. It does not make the ride faster, but it keeps you alive when conditions suddenly turn against you.

📖 Core Interpretation

Chasing past performance is futile. Hot funds cool down, and cold funds heat up.
💎 Key Insight：Bogle demonstrated that chasing last year's hot funds is a losing strategy. Performance mean reversion means that top-performing funds tend to regress toward average, while laggards sometimes recover. This happens because outperformance often results from style tilts or concentrated bets that work temporarily. As assets flow into winners, they become harder to manage and advantages disappear. Academic research shows virtually no persistence in actively managed fund performance. The lesson: ignore past returns when selecting funds. Focus instead on low costs, which are the only reliable predictor of future net returns.

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❓ Why It Matters

Performance is largely random in the short term. Mean reversion is inevitable.

🎯 How to Practice

Don't chase last year's winners. Stick with your asset allocation.

🎙️ Master's Voice

Past performance is not a predictor of future performance.
Bogle showed that last year's winning funds rarely repeated. Chasing hot funds led to poor results.

⚔️ Practical Guide

✅ Decision Checklist

  • Am I chasing past performance?
  • Am I expecting repetition?
  • Is my approach forward-looking?

📋 Action Steps

  1. Ignore past performance
  2. Don't chase hot funds
  3. Focus on costs and diversification

🚨 Warning Signs

  • Performance chasing
  • Expecting repetition
  • Backward-looking

⚠️ Common Pitfalls

✕Ignoring persistent skill in rare cases
✕Over-rotating to losers

📚 Case Studies

1
Vanguard 500 vs. Janus Twenty in the Tech Bubble (2000)
In the late 1990s, Janus Twenty, a concentrated growth fund heavy in tech, dramatically outperformed the broad-market Vanguard 500 Index Fund. Investors poured billions into Janus based on its stellar recent returns, while the plain-vanilla index fund looked dull and lagging.
✨ Outcome：After the 2000–2002 tech crash, Janus Twenty’s performance collapsed and badly trailed the S&P 500 over the full cycle. The once-mediocre index fund pulled ahead, illustrating Bogle’s point that hot funds often cool and revert toward market averages.
2
Emerging Markets Funds After 2003–2007 Boom (2008)
From 2003–2007, emerging markets equity funds (e.g., Templeton and Fidelity EM funds) vastly outpaced U.S. stock index funds. Their 30%+ annualized gains attracted heavy inflows, while U.S. broad-market index funds appeared comparatively sluggish and unexciting.
✨ Outcome：In 2008, emerging markets plunged more than U.S. stocks, and their subsequent decade-long returns lagged the S&P 500. Late-arriving investors who chased the prior outperformance fared poorly, reinforcing that sector and regional leaders often revert toward the long-run global equity mean.

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