S&P 500 stock 12-month momentum and 1-month reversal are highly statistically significant in t-statistics, with sector and factor momentum together accounting for most momentum but reversal mostly stock-specific (Jan 1995–Dec 2020; gross returns; descriptive; not a trading test).
What drives 12-month (with skip-month) momentum and 1-month reversal effects among U.S. common stock returns? In their July 2021 paper entitled "Mapping out Momentum", Yimou Li and David Turkington decompose momentum and reversal effects into distinct industry/sector, factor (size, value, profitability, investment) and stock-specific contributions. In addition to full-sample results, they look at:
High and low volatility states, as defined by a threshold of 25 for average daily CBOE Volatility Index (VIX) during the month of stock return measurement.
Contributions of past winners versus past losers.
Two subsamples with breakpoint December 2009.
They focus on S&P 500 stocks to avoid concerns that any anomalies are due to market frictions or are not exploitable on a large scale. They assume a 3-day implementation lag in computing next-month returns. They examine statistical significance (t-statistic) rather than magnitude of anomaly returns. Using S&P 500 stock, sector/industry and factor data and daily VIX levels during January 1995 through December 2020, they find that:
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