What is the Betting Against Beta (BAB) factor, and how does leverage aversion create a persistent return premium for low-beta stocks?

Question

AcadiFi · Accepted Answer

The Betting Against Beta (BAB) factor formalizes the low-beta anomaly into a tradeable long-short portfolio. It goes long leveraged low-beta stocks and short de-leveraged high-beta stocks, creating a market-neutral strategy that profits from the empirical flatness of the security market line. **The Flat Security Market Line** CAPM predicts a linear relationship between beta and expected return. Empirically, the line is much flatter — low-beta stocks earn more than predicted and high-beta stocks earn less. BAB exploits this pricing error. **Worked Example — Stonebridge BAB Strategy:** Stonebridge constructs a BAB portfolio from the Russell 3000: *Step 1:* Sort all stocks by estimated beta (trailing 12-month, shrunk toward 1.0) *Step 2:* Form two portfolios: - Low-beta portfolio: Bottom tercile (avg beta = 0.62) — leveraged to beta 1.0 - High-beta portfolio: Top tercile (avg beta = 1.48) — de-leveraged to beta 1.0 *Step 3:* Go long the leveraged low-beta portfolio, short the de-leveraged high-beta portfolio | Component | Raw Beta | Target Beta | Leverage | Capital | |---|---|---|---|---| | Low-beta (long) | 0.62 | 1.00 | 1.61x | $250M × 1.61 = $403M | | High-beta (short) | 1.48 | 1.00 | 0.68x | $250M × 0.68 = $169M | | **Net market beta** | — | **0.00** | — | — | \`\`\`mermaid flowchart TD A["CAPM Predicts:
Higher β = Higher Return"] --> B["Empirical Reality:
SML Is Flat"] B --> C["Low-β Stocks
Earn Too Much"] B --> D["High-β Stocks
Earn Too Little"] C --> E["Leverage Low-β
to β = 1.0"] D --> F["De-leverage High-β
to β = 1.0"] E --> G["Long Position"] F --> H["Short Position"] G --> I["BAB Portfolio
Market-Neutral"] H --> I \`\`\` **Why the Premium Exists — Leverage Aversion Theory:** 1. **Borrowing constraints** — many investors (pension funds, mutual funds, retail) face limits on leverage. Unable to lever up low-beta assets to achieve desired returns, they instead buy high-beta assets as leverage substitutes. 2. **Demand-driven pricing** — this excess demand for high-beta stocks inflates their prices, compressing expected returns. Low-beta stocks are under-demanded and underpriced. 3. **Margin requirements** — real-world margin and collateral requirements make levered low-beta strategies more expensive to implement than the theory suggests, preserving the anomaly. 4. **Funding liquidity risk** — BAB returns deteriorate when funding markets tighten (e.g., 2008-2009) because the strategy requires leverage, and leverage costs spike during crises. **Historical Performance:** - BAB has earned approximately 7-9% annually across 20+ equity markets since 1926 - The factor exists in bonds, credit, and commodities — not just equities - The premium is time-varying: stronger after periods of tight funding, weaker after loose funding For CFA Level II, understand that BAB isolates the CAPM mispricing that leverage constraints create in every asset class. Review our CFA Level II Equity course for factor construction methodology.

What is the Betting Against Beta (BAB) factor, and how does leverage aversion create a persistent return premium for low-beta stocks?

Master Level II with our CFA Course

Related Questions

Practice Questions