Index rebalances — when indexes periodically add or remove stocks based on market cap or other criteria — create shifts that can affect momentum strategies. Most major indexes, like the S&P 500*, are market-cap weighted. This means that stocks that have recently performed well and grown in market capitalization get added or increase their weighting, while underperformers get removed or reduced. This creates a “buy high, sell low” dynamic. Since momentum stocks are often the high-performing “winners,” they are added to the index, causing a surge in demand from passive index funds that must now buy them. Conversely, underperforming stocks are sold off, often with a significant price dip.
The Reversal Effect
The reversal effect can create contrarian investment opportunities as it signifies a correction in short-term price movements. Stocks that are added to an index often experience a price boost as index-tracking funds buy them, but this demand can sometimes push the price beyond its fundamental value, leading to a subsequent reversal known as a reversal effect. Similarly, stocks removed from an index may be oversold and may offer an attractive entry point for value investors who believe the company is now undervalued.
Momentum-based indexes tend to have a higher turnover rate than value or broad market indexes because the list of stocks changes more frequently. This requires frequent rebalancing to maintain the desired factor exposure. This higher turnover can lead to increased trading costs, which can eat into returns.
The momentum factor captures the power of trends — both positive and negative — rooted in investor behavior and market psychology. When paired thoughtfully with value investing, it can create a more balanced and adaptive investment strategy, capable of navigating a variety of market conditions and shifts caused by index rebalances.
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