Direct answer: Rebalancing is primarily a risk-control process: it restores a target allocation after market movements, prevents the portfolio from becoming unintentionally concentrated, and forces disciplined selling of relative winners to buy relative laggards. The evidence on whether rebalancing improves returns is mixed; it sometimes adds return through a 'rebalancing bonus' in mean-reverting markets and sometimes reduces return in trending markets. Its primary evidence-backed benefit is risk control, not return enhancement.
What the Evidence Says About Rebalancing
Key Takeaways
- A 60/40 portfolio left unrebalanced from 2012 to 2021 would have drifted to approximately 80/20, significantly increasing equity risk without the investor making an explicit decision to do so.
- The 'rebalancing bonus' (rebalancing adding return over buy-and-hold) is realized only when asset classes mean-revert; in trending markets (U.S. equities outperforming bonds continuously, 2012 to 2021), rebalancing reduced return by selling winners.
- Annual or semi-annual rebalancing has similar outcomes to threshold-based rebalancing (rebalance when allocation drifts 5+ percentage points) with lower transaction costs than monthly rebalancing.
- Tax-aware rebalancing (using new contributions, dividends, and bond coupons to buy underweight assets before selling overweight assets) can reduce the tax drag of rebalancing in taxable accounts.
- The primary evidence-backed case for rebalancing is that it prevents unintended concentration, not that it reliably adds return.
The Risk Control Case for Rebalancing
The clearest evidence for rebalancing is as a risk control mechanism. A portfolio started at 60% equity/40% bonds in 2012 with no rebalancing would have drifted to approximately 80% equity/20% bonds by 2021 due to equity outperformance. The investor's actual risk exposure in 2021 was dramatically higher than their target allocation, even without any intentional decision to increase risk. Rebalancing restores the intended allocation, ensuring that the investor's risk exposure reflects their deliberate choice rather than the market's random drift. This is not a return argument; it is an argument about risk governance.
The 'Rebalancing Bonus' and When It Applies
Some researchers, including William Bernstein, have argued for a 'rebalancing bonus': a return premium from systematically selling high and buying low within the portfolio. The mechanism requires that asset classes mean-revert (relative performance is cyclical rather than persistent). In environments with mean reversion (e.g., stocks vs. bonds where poor equity performance is followed by recovery, and good bond performance is followed by compression), rebalancing can add 0.2% to 0.5% annually. In trending markets (e.g., U.S. equities continuously outperforming international equities from 2010 to 2021), rebalancing subtracts return by reducing exposure to the winner. The bonus is conditional, not guaranteed.
Rebalancing Frequency Evidence
Studies of rebalancing frequency (monthly, quarterly, annual, threshold-based) consistently find that the frequency chosen has less impact on outcomes than the decision to rebalance at all. Annual and threshold-based (5% drift trigger) rebalancing produces similar risk-adjusted results to more frequent rebalancing, with lower transaction costs. Monthly rebalancing generates unnecessary trading in trending markets and may be harmful after taxes. The practical recommendation from most research: annual or threshold-based rebalancing captures the risk-control benefit with minimal cost.
Rebalancing in Taxable Versus Tax-Deferred Accounts
The tax implications of rebalancing differ significantly by account type. In tax-deferred accounts (IRAs, 401ks), rebalancing is generally free of immediate tax consequences; rebalance as frequently as the risk-control benefit warrants. In taxable accounts, selling appreciated assets to rebalance triggers capital gains taxes, which can exceed the rebalancing benefit. Tax-aware techniques: direct new contributions to underweight asset classes, reinvest dividends and coupons into underweight assets, harvest losses in overweight positions to offset gains, and use tax-loss harvested proceeds to rebalance without triggering net taxes.
Frequently Asked Questions
Should I rebalance in a down market?
The decision to rebalance should be driven by allocation drift, not by market direction. If a market decline has caused your equity allocation to fall below target, rebalancing means buying more equities (buying the dip, systematically). If a market rally has caused equities to exceed target, rebalancing means selling some equities. In both cases, you are restoring the intended risk exposure. Deciding to skip rebalancing during a down market because it 'feels wrong to buy more equities' defeats the purpose of having a target allocation.
What is the threshold at which I should rebalance?
Research suggests that rebalancing when any asset class drifts 5 percentage points or more from target (e.g., target 60% equities, rebalance when it reaches 65% or 55%) produces outcomes similar to time-based annual rebalancing with lower transaction frequency. More specific thresholds depend on the volatility of the asset classes and the investor's tolerance for interim allocation drift.
Is there a version of rebalancing that works for factor or tilted portfolios?
Yes, but with important nuance. A portfolio with explicit factor tilts (e.g., a small-cap value tilt) should rebalance to its target factor exposure, not just its geographic or asset-class allocation. This requires understanding which funds provide which factor exposures and can be more complex to implement. The same risk-control logic applies: prevent unintentional factor drift that was not part of the original investment thesis.