The Size Effect and Small-Cap Premium
The size effect refers to the historical tendency of small-capitalization stocks to generate higher returns than large-capitalization stocks. First documented by Rolf Banz in 1981, this anomaly became a cornerstone of the Fama-French factor model.
Understanding the Size Premium
Small-cap stocks are typically defined as companies in the bottom deciles of market capitalization. The Fama-French methodology uses NYSE median market cap as the breakpoint, classifying all stocks below this threshold as "small."
Magnitude: Historically, small stocks have outperformed large stocks by approximately 2-3% annually, though this premium has been much weaker (and often negative) since the 1980s.
Why Might Small Stocks Outperform?
Several theories attempt to explain the size premium:
- Liquidity Risk: Small stocks are generally less liquid, with higher trading costs and greater price impact. Investors demand compensation for this liquidity risk.
- Information Asymmetry: Small companies have less analyst coverage and public information, creating opportunities for informed investors and risks for others.
- Operating Leverage: Small companies may have higher operating leverage, making their earnings more sensitive to economic conditions.
- Institutional Constraints: Many large institutional investors cannot efficiently invest in small stocks due to capacity constraints, potentially leaving them underpriced.
The Disappearing Size Premium
One of the most debated topics in asset pricing is whether the size premium still exists. Several factors have been cited for its apparent decline:
Publication Effect: After the size effect was widely publicized in the 1980s, investor attention and capital flows may have arbitraged away much of the premium.
Data Mining Concerns: Some researchers argue the original findings may have been partly due to survivorship bias and data snooping.
Changing Market Structure: The growth of index funds, ETFs, and algorithmic trading may have improved price efficiency across all market caps.
Equal Weight vs. Market Weight
One way to gain exposure to the size factor is through equal-weighted indices, which give the same weight to each stock regardless of market cap. This inherently overweights small stocks relative to market-cap-weighted indices.
Recent research shows that equal-weight strategies have significantly underperformed cap-weight strategies over the past decade, particularly among large caps. This reflects both the weak size premium and the exceptional performance of mega-cap technology stocks.
Banz, R. W. (1981). "The relationship between return and market value of common stocks." Journal of Financial Economics, 9(1), 3-18.