Direct answer: Portfolio rebalancing is the process of restoring a portfolio's asset weights to the original target allocation after market movements cause drift. A 60/40 portfolio that grows to 75/25 during a bull market without rebalancing now carries more risk than intended. Rebalancing reverses that drift. It is primarily a risk management tool: it keeps the portfolio's volatility and drawdown potential aligned with what the investor deliberately chose. The often-cited return enhancement from rebalancing is real but modest and secondary to its risk control purpose.
Portfolio Rebalancing: What It Is and Why Investors Care
What is portfolio rebalancing and how does drift happen?
Portfolio rebalancing is the act of returning asset class weights to their target allocation after market price movements have caused them to drift. It involves selling a portion of the assets that have grown above their target weight and buying more of the assets that have fallen below their target.
Drift is a mathematical inevitability in any multi-asset portfolio where assets grow at different rates. Consider a $600,000 equity / $400,000 bond portfolio (60/40) where equities return 15% per year and bonds return 3% per year:
| Year | Equity Value | Bond Value | Total | Actual Equity % |
|---|---|---|---|---|
| Start | $600,000 | $400,000 | $1,000,000 | 60% |
| Year 1 | $690,000 | $412,000 | $1,102,000 | 62.6% |
| Year 3 | $912,150 | $436,727 | $1,348,877 | 67.6% |
| Year 5 | $1,205,677 | $463,710 | $1,669,387 | 72.2% |
After five years of strong equity performance, a portfolio that started at 60/40 has drifted to approximately 72/28 without any deliberate decision by the investor. This drifted portfolio will now produce a much larger drawdown during the next equity bear market than the investor originally accepted when choosing a 60/40 allocation.
Rebalancing restores the 60/40 target by selling approximately $100,000 of equities and buying $100,000 of bonds. The portfolio returns to the risk profile the investor deliberately selected.
Does rebalancing improve returns or just control risk?
The "rebalancing bonus" is a widely cited concept suggesting that systematic rebalancing improves long-run returns by systematically selling assets that have recently outperformed (selling high) and buying assets that have recently underperformed (buying low). The concept is theoretically sound when asset class returns are mean-reverting and correlations between asset classes are low.
In practice, the evidence is nuanced. During sustained trends, rebalancing reduces returns by trimming winners. From 2010 to 2021, a U.S. equity-heavy portfolio that was never rebalanced significantly outperformed one that was regularly rebalanced back to a global equity target, because U.S. equities outperformed other asset classes for an extended period. Rebalancing into the underperformer (international equities, bonds) consistently cost return during those years.
The honest summary: rebalancing is a risk management discipline first. It keeps portfolio risk aligned with the investor's stated tolerance. Any return benefit is secondary, uncertain, and regime-dependent. An investor who rebalances to preserve risk control should not expect it to add return on average; the goal is to prevent the portfolio from unknowingly taking on more risk than intended during extended bull markets.
What is the difference between calendar rebalancing and threshold rebalancing?
Calendar rebalancing reviews and potentially rebalances the portfolio on a fixed schedule, regardless of how much drift has occurred. Common cadences are annual, semi-annual, or quarterly. The advantage is simplicity and predictability: the investor knows exactly when a rebalancing review will occur. The disadvantage is that the portfolio might drift significantly between scheduled reviews during volatile periods, or might be reviewed frequently during stable periods when little action is needed.
Threshold rebalancing triggers a rebalancing action when any asset class drifts beyond a specified tolerance band from its target allocation. Common thresholds are an absolute drift of 5 percentage points from target, or a relative drift of 25% of the target weight. For a 60/40 portfolio:
- Absolute 5-point band: rebalance if equities drift above 65% or below 55%, bonds above 45% or below 35%.
- Relative 25% band: rebalance if equities drift above 75% (60% x 1.25) or below 45% (60% x 0.75).
Research comparing the two approaches finds similar long-term outcomes, with threshold rebalancing typically generating slightly fewer transactions (and therefore lower costs) because it avoids unnecessary rebalancing during stable periods. A practical combination is an annual check that only triggers a rebalance if a threshold has been breached, capturing the structure of both approaches: a defined review cadence with action only when drift is meaningful.
Frequently Asked Questions
What is portfolio rebalancing and how does drift happen?
Portfolio rebalancing is the process of returning a portfolio's asset class weights to their original target allocation after market movements have caused them to drift. Drift happens automatically when different assets grow at different rates. A 60/40 stocks/bonds portfolio where stocks return 15% annually and bonds return 3% will drift to approximately 67/33 after three years and 72/28 after five years without rebalancing. Rebalancing reverses this drift by selling a portion of the outperforming asset and buying the underperformer to restore the original weights and, crucially, the original risk profile.
Does rebalancing improve returns or just control risk?
Rebalancing is primarily a risk management tool, not a return enhancement strategy. The frequently cited "rebalancing bonus" is real when asset class returns are mean-reverting and correlations are low, but in trending markets where the outperforming asset continues to lead, rebalancing reduces returns by trimming the winner. From 2010 to 2021, regular rebalancing back to a global equity target cost return because U.S. equities outperformed other asset classes persistently. The primary purpose of rebalancing is to maintain the risk profile the investor deliberately chose, keeping potential drawdowns aligned with stated tolerance. Any return benefit is secondary and regime-dependent.
What is the difference between calendar rebalancing and threshold rebalancing?
Calendar rebalancing triggers a portfolio review on a fixed schedule (annually, quarterly) regardless of actual drift. It is simple and predictable. Threshold rebalancing triggers action when any asset class drifts beyond a specified band from target, commonly 5 percentage points or 25% in relative terms. A 60/40 portfolio with a 5-point band rebalances if stocks drift above 65% or below 55%. Research finds similar long-run outcomes from both approaches; threshold rebalancing typically generates fewer transactions in stable markets. A practical combination is an annual check that only triggers a rebalance when a threshold is breached, capturing the predictability of calendar rebalancing and the efficiency of threshold rebalancing.