Backtesting Equal Weight vs. Market Cap Weight: The Small-Cap Tilt Effect
# Backtesting Equal Weight vs. Market Cap Weight: The Small-Cap Tilt Effect
The debate between equal-weight and market-cap-weight indexing has important implications for portfolio construction. Equal-weighting inherently tilts toward smaller companies and requires regular rebalancing. We backtested both approaches to determine the magnitude and consistency of any difference.
The Core Difference
In a cap-weighted S&P 500 index, the top 10 stocks comprise approximately 30% of the portfolio. In an equal-weight version, each of the 500 stocks holds 0.2%, giving the same weight to a $3 trillion company as a $15 billion company. This creates a systematic small-cap and value tilt.
Long-Term Results (1990-2025)
Equal-weight S&P 500 returned 11.8% annualized versus 10.3% for cap-weight. The 1.5% annual advantage compounded to a massive difference over 35 years: $100,000 grew to $4.2 million versus $2.8 million. However, equal-weight achieved this with modestly higher volatility (16.8% vs 15.2%).
Risk-Adjusted Comparison
The Sharpe ratio for equal-weight was 0.59 versus 0.55 for cap-weight. While equal-weight had higher total returns, the risk-adjusted advantage was modest. Maximum drawdowns were similar: 55% for cap-weight in 2008-2009 versus 59% for equal-weight (reflecting higher small-cap beta during crises).
Decade-by-Decade Analysis
The 1990s: cap-weight won by 2.1% annually (large-cap tech dominance). The 2000s: equal-weight won by 4.3% annually (dot-com bust punished large caps). The 2010s: cap-weight won by 1.8% annually (FAANG dominance). The 2020s through 2025: mixed results, with cap-weight leading through 2024 before equal-weight caught up in 2025.
The Concentration Risk Argument
In 2024, the top 7 stocks (Magnificent Seven) comprised over 30% of the cap-weighted S&P 500. This concentration means cap-weighted investors had enormous single-stock risk. Equal-weight avoids this concentration, providing genuine diversification benefits that do not show up in simple return statistics.
Rebalancing Alpha
Equal-weight strategies require quarterly rebalancing to maintain equal weights. This rebalancing mechanically sells recent winners and buys recent losers, implementing a contrarian strategy. Our analysis shows this rebalancing contributed approximately 0.5% of the 1.5% annual outperformance, with the remaining 1.0% coming from the small-cap tilt itself.
Sector Exposure Differences
Cap-weight currently overweights technology (30%+) and underweights materials, utilities, and real estate. Equal-weight provides approximately 9% to each sector. During tech selloffs (2000-2002, 2022), equal-weight benefits from lower tech exposure. During tech rallies, it suffers. This sector neutrality is a feature or bug depending on your views.
Transaction Costs of Equal-Weight Rebalancing
Quarterly rebalancing of an equal-weight portfolio creates approximately 20-30% annual turnover. With modern commission-free trading, direct costs are minimal. However, market impact for institutional-sized portfolios is meaningful. We estimate the turnover cost reduces the equal-weight advantage by 0.2-0.4% annually for large accounts.
The Factor Decomposition
Decomposing equal-weight returns into factor exposures reveals: approximately 0.6% from size (small-cap tilt), 0.4% from value (rebalancing sells growth winners), and 0.5% from the rebalancing premium itself. Understanding these sources helps predict when equal-weight will under or outperform.
Practical Recommendations
Equal-weight indexing offers a simple way to capture size and value premiums without explicit factor investing. It works best as a core portfolio holding in tax-advantaged accounts (due to turnover). For taxable accounts, the tax drag of rebalancing partially offsets the return advantage. The strongest argument for equal-weight is diversification: avoiding the concentration risk of cap-weighted indices that become bets on a handful of mega-cap stocks.