Why Rebalance: The Evidence
Why Rebalance: The Evidence
The case for rebalancing rests on two claims: it controls risk, and it enhances returns. The first is robust; the second is modest but measurable. Together, they justify the modest costs and friction of the practice.
Key takeaways
- Rebalancing reduces portfolio volatility by constraining overweight positions before they become dangerously large.
- Historical data shows rebalancing adds a small but consistent return boost—typically 0.1% to 0.5% per year depending on asset classes, frequency, and costs.
- The primary benefit is risk control; return enhancement is secondary and not guaranteed over any single period.
- A portfolio that drifts without rebalancing becomes gradually riskier than you planned.
- The case for rebalancing is strongest in accounts where costs are zero or near-zero (IRAs, ISAs, 401(k)s).
Risk control: the first pillar
A 60/40 portfolio is designed to weather a typical market downturn without forcing you to abandon the plan or face a breakdown in your retirement. That design assumes you maintain the 60/40 weights. If you do not rebalance and a long bull market pushes you to 75/25, you now have a riskier portfolio than you chose. A subsequent market crash will hurt more.
In the 2007–2008 financial crisis, the S&P 500 fell roughly 57% peak-to-trough. A 60/40 portfolio (assuming historical volatilities and correlations) would have declined around 28%. A 75/25 portfolio, left to drift during the preceding bull run, would have fallen closer to 35%. That is not a small difference when you are already stressed. Rebalancing forces you to maintain the risk profile you planned.
Vanguard's research, updated regularly, shows that rebalancing reduces realised portfolio volatility compared to a buy-and-hold approach—often by 0.2% to 0.5% per year, depending on asset class volatility and the correlation between them. In a 2019 study, Vanguard found that a rebalanced 60/40 portfolio had a lower maximum drawdown and lower volatility than a drifting allocation. The cost of that protection was negligible transaction costs in a monthly rebalancing regime.
This is not guesswork. It is mechanical: if you keep Asset A at 60% instead of letting it grow to 75%, your portfolio swings smaller when Asset A falls. Volatility reduction is the guarantee of rebalancing, assuming non-zero correlations between assets.
Return enhancement: the second pillar
The claim that rebalancing improves returns (not just reduces risk) is more nuanced. When you rebalance, you sell recent winners (high valuations, likely to mean-revert) and buy recent losers (low valuations, likely to recover). Over time, this "contrarian" behaviour captures what researchers call the rebalancing bonus or rebalancing return.
The rebalancing bonus is typically small: 0.1% to 0.5% per year in US equity–bond portfolios. It is not small enough to ignore, but it is not large enough to justify expensive rebalancing. The bonus emerges from market mean reversion—the tendency of extreme valuations to correct—which is documented but not guaranteed in any given year. In a decade of strong momentum (winners keep winning), rebalancing can drag performance. In volatile sideways markets, it shines.
Ibbotson Associates studied monthly and quarterly rebalancing of a 60/40 portfolio from 1926 to 2013. Monthly rebalancing (assuming zero costs) added 0.21% per year to returns. Quarterly rebalancing added 0.18% per year. Annual rebalancing added 0.14% per year. In all cases, the benefit was present but modest. When transaction costs were included, the benefit shrank, and annual or threshold-based rebalancing looked more attractive.
Research by Arnott, Beck, Kalesnik, and West (2016) examined a broader range of asset classes and rebalancing approaches. They found that rebalancing benefits were strongest when assets had high volatility (more tendency to drift) and low correlation (less tendency to move in lockstep). A portfolio of US equities and international equities showed larger rebalancing bonuses than a 60/40 equity–bond portfolio, because equities themselves are more volatile and less correlated with each other.
When rebalancing adds the most value
The empirical evidence suggests rebalancing is most valuable:
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For volatile portfolios with low correlations. A three-fund portfolio (US equities, international equities, bonds) benefits more from rebalancing than a simple US equity–bond portfolio.
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With low costs. The rebalancing bonus erodes quickly if you pay 0.5% in commissions per trade or trigger large capital gains taxes. In a Roth IRA or ISA, the bonus is undiminished. In a taxable account, it may be consumed by taxes.
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In volatile, sideways markets. The benefit of "buy low, sell high" is largest when assets oscillate rather than sustain momentum. The 2010–2015 period, with multiple sharp corrections and recoveries, would have been a good rebalancing environment. The 2016–2021 period, with relentless momentum in mega-cap tech, would have penalised rebalancing.
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Over long time horizons. The rebalancing bonus is small. It requires decades to compound into a meaningful advantage. A 30-year investor will see it more clearly than a 5-year investor.
The evidence against frequent rebalancing in taxable accounts
Arnott et al. also examined the tax costs of rebalancing. In a taxable account, rebalancing triggers capital gains tax, which can exceed the rebalancing bonus. They found that threshold-based rebalancing (sell only when allocations drift beyond 5% to 10%) outperformed calendar-based rebalancing (sell on a fixed schedule) in taxable accounts. The threshold approach captures the benefits of rebalancing while minimizing tax realisations.
This finding has large practical implications. If you are an investor with substantial positions in taxable brokerage accounts, annual or quarterly rebalancing may be counterproductive. Threshold-based rebalancing, combined with directing new contributions to underweight assets, is likely optimal.
The strong case: risk control
While the return enhancement from rebalancing is modest and uncertain, the risk control benefit is robust. Academic evidence and simple logic agree: rebalancing keeps your portfolio close to your target allocation, which keeps volatility predictable and drawdowns manageable. For a retiree depending on the portfolio, that stability is worth the modest cost of execution. For a young accumulator, it is worth the discipline of maintenance.
Evidence in context
Next
Now that we understand the case for rebalancing, the next question is the mechanism: should you rebalance on a calendar (e.g., every January) or on a threshold (e.g., when allocations drift 5%)? Each approach has merits and trade-offs, which we explore next.