Mean-reversion investing

Mean-reversion investing is a contrarian strategy based on the premise that stock prices that move far from historical norms — whether extremely high or extremely low valuations — will eventually revert back toward the average. Investors buy the most depressed and sell the most appreciated.

The mean-reversion thesis

The premise is statistical: stock prices oscillate around a central tendency (the mean). Temporary overshoots — a stock down 60% in a month, or up 100% in three months — tend to correct. The strategy is to identify these extremes and bet on the reversal.

This is not the same as value investing, which bets on intrinsic value. A mean-reversion investor simply believes that whatever the mean is, prices currently far above or below it will eventually return. The mean could move (upward or downward), but the expectation is that current prices are unsustainable.

Types of mean reversion

Short-term price reversion targets stocks that have moved sharply on the daily or weekly basis. A stock down 15% in a day on panic selling may bounce 5–10% over the next week as shorts cover and buyers step in. This is more akin to swing trading than long-term investing.

Valuation reversion targets stocks trading at historically extreme valuations. A stock at 8x earnings when its 20-year average is 15x is positioned for either earnings growth or multiple expansion — both of which could drive the stock higher toward the mean.

Sector reversion bets that sectors far from their historical valuations will revert. Energy stocks after a crash, or tech stocks after a euphoria peak.

Identifying extremes

Common tools include:

• Bollinger Bands. Price bands around a moving average. Extreme moves outside the bands are flagged as potential reversions.
• Price deviation from long-term averages. A stock trading 50% below its 200-day moving average or 100% above it.
• Valuation percentiles. A stock at its lowest P/E in 20 years, or highest in 20 years.
• Breadth and sentiment extremes. When put/call ratios, short interest, or bullish sentiment hit extremes.

The reversal trap

Mean reversion fails when the “mean” has genuinely shifted. A company in secular decline may have a new, lower mean. A high-growth company may have a new, higher mean. What looked like an extreme valuation becomes the new normal.

Additionally, trends can persist for years before reverting, causing mean-reversion bets to tie up capital and suffer drawdowns before winning. This requires deep conviction and patience.

See also