LONDON, April 10 —
- Long-Term Trend: U.S. small-cap stocks have outperformed the broader market by about 2 percentage points per year over the past century.
- Value Premium: Stocks trading cheaply relative to book value have historically delivered higher returns since 1926.
- Market Cycles: Performance between large-cap and small-cap companies tends to move in cycles rather than permanent trends.
- Key Concept: Mean reversion suggests that unusually high or low profitability often returns toward historical averages.
- Modern Challenge: Dominance of large technology firms has weakened the typical mean-reversion pattern in recent years.
- Future Trigger: Increased competition and investment spending in areas like AI infrastructure could restore competitive balance.
Understanding Mean Reversion
Mean reversion is one of the oldest ideas in finance. The theory suggests that extreme outcomes—whether high profits or deep losses—tend to move back toward historical averages over time.
For investors, this often means yesterday’s winners eventually slow down while struggling companies recover.
Historical Evidence
Long-term market data shows that smaller companies and undervalued stocks have often outperformed broader market averages.
Investor Jeremy Grantham, co-founder of asset manager GMO, found that small-cap stock performance has historically followed cycles. Periods of underperformance are often followed by bursts of strong returns.
According to Grantham, these cycles reflect basic economic forces: when profits soar, competitors enter the market; when profits collapse, companies cut costs and restructure.
The Mechanics Behind the Strategy
Grantham argued that the extra returns from small-cap stocks often came from portfolio rebalancing.
As successful firms grow and leave the small-cap category, new struggling companies enter the index. Investors who continually sell winners and buy underperformers benefit if fortunes revert toward the average.
The same logic has historically supported value investing, where stocks priced cheaply relative to their fundamentals eventually recover as profitability stabilizes.
Why It Has Been Harder Recently
In the past decade, mean reversion has appeared weaker in the United States.
Large technology companies have maintained unusually high profit margins while dominating markets for extended periods. Share buybacks and market concentration have also supported valuations.
Some investors worry that these dynamics reflect structural changes in the modern economy.
Signs of a Possible Shift
Even so, mean reversion may not have disappeared.
Rising investment in artificial intelligence infrastructure and increased competition between major technology firms could pressure profit margins.
Political scrutiny of large corporations may also force changes that restore competitive dynamics.
Timing Remains the Hard Part
For investors, the key challenge is timing.
Mean reversion tends to work over long periods but can take years—or even decades—to play out.
As Grantham has argued, the principle may be reliable, but waiting for markets to return to trend can test even the most patient investors.
