Reversion to the Mean: Why Hot Funds Always Cool Off (Chapter 10)

Book: Common Sense on Mutual Funds: Fully Updated 10th Anniversary Edition by John C. Bogle ISBN: 978-0-470-59748-4

Bogle opens Part III of the book with what might be the most important concept for any investor to understand. It’s called reversion to the mean, or RTM if you want to sound smart at parties. And he subtitles this chapter “Sir Isaac Newton’s Revenge on Wall Street,” which honestly tells you everything you need to know about where this is going.

Here’s the basic idea. What goes up must come down. The hot fund everyone’s talking about? Give it a few years. It’ll come back to earth. The fund that’s been embarrassing itself for a decade? It might surprise you.

This is gravity for the financial markets. And just like actual gravity, you can ignore it all you want. But it doesn’t care.

The Data Is Pretty Clear

Bogle didn’t just throw around opinions here. He went back and looked at actual fund performance data spanning decades. He took mutual funds from the 1970s and tracked what happened to them in the 1980s. And the pattern was obvious.

Funds that ranked in the top quartile during the 1970s? They fell hard in the 1980s. Their stellar performance didn’t stick. Meanwhile, funds that had been sitting in the bottom quartile during the 1970s? Many of them recovered and moved toward the middle of the pack.

Performance reverted to the average. Over and over again.

Now here’s where it gets interesting. And kind of frustrating. The reversion isn’t perfectly symmetrical. Funds that were doing well tend to come back to average. But funds that were doing badly don’t always fully recover. And the reason is costs.

Costs Are the Anchor

This is the part that really stuck with me. A fund with high expenses has a permanent drag on its performance. So when a high-cost fund happens to be doing badly, it doesn’t just revert to the mean. It reverts to a mean that’s already been pulled down by its own fee structure.

Think about it this way. If two funds both have average stock-picking ability, but one charges 1.5% and the other charges 0.2%, the expensive fund is going to consistently underperform. Its “mean” is lower than the market’s mean. So reversion to the mean for that fund just means returning to its own mediocre baseline.

High costs are the one thing that persists. Good performance doesn’t persist. Bad performance sometimes doesn’t persist. But costs? Always there. Every single year. And that’s why it matters so much.

It Happens to Sectors Too

This isn’t just about individual funds. Entire market sectors follow the same pattern. Tech stocks boomed in the late 1990s. Everyone and their cousin was pouring money into anything with “.com” in the name. And then? You know what happened.

The dot-com bust was reversion to the mean playing out in real time. Tech had gotten way ahead of itself, and it snapped back. Hard.

Same thing with the financial crisis in 2008. The sectors and strategies that had been crushing it for years were the ones that got destroyed. Meanwhile, the boring stuff that nobody was excited about held up relatively better.

Bogle was writing about this pattern before the dot-com crash happened. And in his 10th anniversary update, he got to say “I told you so” about both the dot-com bust and the 2008 crisis. Both were textbook examples of RTM playing out on a massive scale.

The Stock Market Itself Reverts

Here’s something that most people don’t think about. It’s not just funds and sectors. The overall stock market reverts to its long-term average returns over time.

If the market has been returning 15% a year for a while, that’s above the historical average. Eventually, returns will pull back. Maybe through a crash, maybe through a long period of mediocre returns. But over decades, the market tends to settle around its long-term average.

This works the other way too. After a terrible decade for stocks, the next decade tends to be better than average. Not because of some cosmic fairness rule, but because valuations get cheap after bad periods, and that sets up higher future returns.

Bogle used this idea to make some pretty accurate forecasts about future market returns. He wasn’t predicting short-term moves. He was just looking at where valuations sat relative to history and saying “eventually this reverts.”

The Biggest Mistake Investors Make

So what’s the practical takeaway? It’s simple: never chase past performance.

This is the most common mistake in investing. And the entire financial industry is set up to encourage it. Funds advertise their best recent returns. Media features the hottest funds. Platforms sort by performance. Everything is designed to make you buy the thing that already went up.

But if RTM is real, and the data strongly suggests it is, then buying the best performing fund is often the worst thing you can do. You’re buying at the top. You’re about to ride the reversion back down to average. Or worse, below average if the fund has high costs.

The funds that were crushing it over the last five years? Statistically, they’re likely to underperform over the next five years. Not because they suddenly got bad at their jobs. But because some combination of luck, market conditions, and style tailwinds that drove their outperformance is going to shift.

The Crown Jewels

Bogle talks about what he calls the “crown jewels” of sound investing. And they’re not complicated. Diversify broadly. Keep costs low. Stay invested for the long term. Don’t chase performance. Don’t try to time the market.

These aren’t exciting principles. Nobody’s going to make a TikTok about “keep your expense ratios low.” But they work. And they work precisely because they’re built on understanding reversion to the mean instead of fighting it.

If you accept that most outperformance is temporary, then you stop wasting energy trying to find the next hot fund. You stop paying high fees for active management that’s probably going to revert to average anyway. You build a simple, low-cost portfolio and let time do the heavy lifting.

How This Applies Today

Nothing about this has changed. If anything, the information age has made performance chasing worse. You can see real-time returns on your phone. Social media amplifies whatever’s working right now. Crypto, meme stocks, AI funds, whatever the current obsession is.

But the math hasn’t changed. RTM is still real. The funds and sectors that are leading the pack right now will eventually come back to the pack. And the investors who bought in because of recent performance will be disappointed.

So the next time someone tells you about an amazing fund that’s returned 30% a year for the last three years, just remember Newton. What goes up must come down. And in the financial markets, it almost always does.

Previous: Selecting Superior Funds (Chapter 9) Next: Investment Relativism (Chapter 11)