Beta measures how much a stock moves relative to the market. But beta isn't fixed — it drifts over time as business conditions, market regimes, and investor behavior change. Beta drift can silently change your portfolio's risk profile.
Beta (β) measures the sensitivity of a stock's returns to the returns of a benchmark (typically the S&P 500 or SPY). A beta of 1.0 means the stock moves in line with the market — if SPY rises 1%, the stock rises approximately 1%. A beta of 1.5 means the stock amplifies market moves by 50% — if SPY rises 1%, the stock tends to rise 1.5%.
Beta is the slope of the regression line between the stock's returns and the market's returns over a lookback period:
β = Covariance(stock returns, market returns) / Variance(market returns)
Different lookback periods give different betas. A 21-day beta captures recent behavior; a 120-day beta captures longer-term average behavior. The gap between these is where beta drift becomes visible.
Beta drift is the change in a stock's beta over time. A stock with a 120-day beta of 1.2 but a 21-day beta of 1.8 has drifted significantly higher — it's now moving much more aggressively with the market than its long-term average suggests.
Beta drift matters because most portfolio risk models use a fixed beta assumption. If a stock's beta has drifted, your actual portfolio risk is different from what your model says it is. A portfolio you think has moderate market exposure may actually have high exposure — or vice versa.
Example: NVDA had a 120-day beta of ~1.4 in early 2023. By mid-2023, driven by AI enthusiasm, its 21-day beta had drifted to 1.9+. Traders using the 1.4 figure underestimated how much NVDA would move on market-wide risk events.
For active traders and portfolio managers, monitoring beta drift across a watchlist helps with: