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Factor investing is the practice of targeting specific, persistent drivers of returns across asset classes. Rather than selecting individual stocks based on company-specific analysis, factor investors construct portfolios that are systematically tilted toward characteristics that have historically been associated with higher returns. The academic foundation for factor investing was laid by Eugene Fama and Kenneth French with their three-factor model in 1993, which showed that value and size factors explained a significant portion of stock returns beyond market beta.
The most widely recognized equity factors are value, momentum, quality, size, and low volatility. Value captures the tendency for cheap stocks (as measured by price-to-book, price-to-earnings, or other valuation metrics) to outperform expensive ones over time. Momentum captures the tendency for recent winners to continue outperforming and recent losers to continue underperforming over periods of 3 to 12 months. Quality captures the premium earned by financially sound companies with stable earnings, low leverage, and high profitability.
For a characteristic to qualify as a legitimate factor, it should meet several criteria. First, it must be persistent, meaning it works across long time periods and does not depend on a specific economic regime. Second, it must be pervasive, working across different geographies and asset classes. Third, it must be robust to reasonable variations in its definition. Fourth, it should have a rational economic explanation, either rooted in risk-based compensation or a behavioral bias. Fifth, it must be investable after accounting for transaction costs.
Factor investing can be implemented in several ways. The simplest approach is smart beta ETFs, which mechanically tilt toward one or more factors at low cost. More sophisticated investors build custom factor portfolios using quantitative screens and optimization techniques. The most advanced approach involves multi-factor models that dynamically adjust factor weights based on the current market environment, economic cycle, or valuation spreads between factor portfolios.
A critical concept in factor investing is factor cyclicality. No factor works all the time. Value underperformed dramatically during the 2017-2020 period, leading many to question its validity. Momentum periodically experiences sharp reversals, as it did in March 2009. Understanding that factor premia are harvested over full market cycles, not in every quarter, is essential for maintaining discipline and avoiding the common mistake of abandoning a factor strategy precisely when it is about to recover.