Portfolio & Risk
Rebalancing and Risk Budgeting: Maintaining Portfolio Targets
Rebalancing restores a portfolio to its intended asset allocation after market drift. Risk budgeting assigns explicit risk limits to each portfolio component, so the portfolio's risk profile reflects deliberate decisions rather than drift.
Direct Answer
Rebalancing is the process of selling asset classes that have grown above their target weight and buying those that have fallen below, restoring the portfolio to its intended allocation. Without rebalancing, equity bull markets cause a conservative 60/40 portfolio to silently become an 80/20 or 90/10 portfolio. Risk budgeting extends this discipline to the risk dimension: instead of targeting capital weights, a risk budget targets each component's contribution to total portfolio risk. Risk parity is the limiting case where all components contribute equally.
Key Takeaways
- Market drift turns a conservative 60/40 portfolio into a riskier one over time. Rebalancing counteracts this by selling winners and buying laggards.
- Calendar rebalancing is simple and predictable. Threshold rebalancing responds to actual drift and often results in lower transaction frequency.
- Tax-aware rebalancing directs new contributions to underweight assets, sells in tax-advantaged accounts first, and applies wider bands in taxable accounts.
- Risk budgets assign explicit risk limits per component, keeping the portfolio's actual risk aligned with its intended risk.
- Volatility targeting adjusts total portfolio exposure dynamically to maintain consistent risk over time.
Why portfolios need rebalancing
A portfolio that is not rebalanced will drift toward its highest-returning asset class. From 2009 through 2021, a portfolio starting at 60% global equities and 40% bonds would have drifted to roughly 80% equities without rebalancing, because equities outperformed bonds by a wide margin over that period. The investor set a 60% equity target because that level of equity exposure matched their risk tolerance. By allowing drift, they ended up holding a portfolio that matched neither their intended risk level nor their stated allocation.
Rebalancing is mean-reversion in practice: it systematically sells what has outperformed and buys what has underperformed, which enforces a form of disciplined value-seeking. It also forces the investor to stay committed to their allocation during market dislocations, buying equities when they fall and trimming them when they rise.
Calendar vs threshold rebalancing
Calendar rebalancing occurs at fixed time intervals regardless of how much the portfolio has drifted. Annual or semi-annual rebalancing is common for long-term investment portfolios. The advantage is simplicity and predictability; the disadvantage is that the portfolio may be rebalanced when drift is negligible (wasting transaction costs) or may be significantly off-target between rebalancing dates.
Threshold rebalancing triggers when an asset class drifts more than a set amount from its target. Common thresholds are absolute (rebalance when equities exceed 65% in a 60% target portfolio) or relative (rebalance when any asset class deviates by more than 25% of its target weight). Threshold rebalancing responds to actual portfolio state rather than arbitrary calendar dates.
Research on rebalancing frequency generally finds that annual rebalancing captures most of the benefit of more frequent rebalancing without generating excessive transaction costs. Threshold-based approaches are favored in environments with high transaction costs or high intraday volatility.
Tax-aware rebalancing
Selling overweight positions in a taxable account triggers capital gains taxes, which reduces the net benefit of rebalancing. Tax-aware rebalancing uses several techniques to achieve the same allocation goal at lower tax cost.
New contributions: direct new deposits toward underweight asset classes rather than selling overweight ones. This rebalances without triggering any taxable event.
Account location: sell overweight positions in tax-advantaged accounts (IRA, 401k) where gains are tax-deferred or tax-free. Hold the same allocation across the entire portfolio but do the selling where taxes are smallest.
Tax-loss harvesting: sell assets that are below their cost basis to realize losses, then reinvest in similar (but not substantially identical) assets. The realized losses offset gains elsewhere in the portfolio.
Wider bands in taxable accounts: tolerate more drift before rebalancing in taxable accounts, accepting a slightly off-target allocation in exchange for deferred taxes.
Risk budgeting and risk parity
A risk budget assigns each portfolio component an explicit limit on how much portfolio risk it can contribute. Risk is typically measured as a contribution to total portfolio volatility or as a Value at Risk share. For example: equities 60% of risk, bonds 30%, alternatives 10%. A portfolio where equities have drifted to contribute 75% of risk is risk-overweight in equities even if its capital weight is still 60%.
Risk parity is the extreme version: every asset class contributes equally to total portfolio risk. Because equities are much more volatile than bonds, a risk-parity portfolio holds far more bonds by dollar weight than a capital-weight portfolio. Ray Dalio's All Weather Portfolio is one well-known implementation.
Risk budgets are more sophisticated than capital budgets because they account for the varying volatility and correlation of each component. A 10% allocation to commodities may contribute more portfolio risk than a 30% allocation to investment-grade bonds, depending on their volatilities and correlations with other holdings.
Volatility targeting
Volatility targeting adjusts the portfolio's overall exposure dynamically to maintain a constant annualized volatility level. When realized volatility rises above the target, the portfolio reduces exposure (holds more cash or shorts a futures contract on the index). When volatility falls below the target, it increases exposure.
The practical effect is that the portfolio reduces its market exposure precisely during crises, when volatility spikes, and increases it during calm periods. This tends to reduce the depth of drawdowns compared with a fixed-weight portfolio. The tradeoff is that the portfolio may miss early-stage rallies when volatility remains elevated after a sell-off.
Where to go next
- Rebalancing deep dives: detailed guides on specific rebalancing methods, band design, and tax-aware strategies.
- Asset Allocation: setting the target allocation that rebalancing restores.
- Diversification and Correlation: how correlation affects the rebalancing benefit.
- Portfolio Management: the full portfolio construction and management framework.
FAQ
Why do portfolios need to be rebalanced?
Portfolios drift from their target allocations because different asset classes grow at different rates. A portfolio set at 60% stocks and 40% bonds will drift toward 70% or 80% stocks in a prolonged equity bull market. Rebalancing sells the overweight asset and buys the underweight one to restore the target. Without rebalancing, the portfolio's risk profile becomes increasingly different from what the investor intended.
What is the difference between calendar rebalancing and threshold rebalancing?
Calendar rebalancing triggers at fixed time intervals (annually, quarterly) regardless of how much the portfolio has drifted. Threshold rebalancing triggers when an asset class drifts more than a set amount from its target, such as 5 percentage points or 25% of the target weight. Calendar rebalancing is simpler; threshold rebalancing responds to actual drift and typically results in fewer transactions in stable markets and more in volatile ones.
What is tax-aware rebalancing?
Tax-aware rebalancing minimizes the tax cost of restoring target allocations. The main techniques are: using new contributions to buy underweight assets rather than selling overweight ones, selling overweight positions in tax-deferred accounts (IRA, 401k) where gains are not taxed immediately, harvesting losses in taxable accounts to offset gains, and applying wider rebalancing bands in taxable accounts to reduce transaction frequency.
What is a risk budget?
A risk budget assigns explicit risk limits to each portfolio component, typically measured as a contribution to total portfolio volatility or Value at Risk. For example, a portfolio might budget 60% of total risk to equities, 30% to bonds, and 10% to alternatives. Risk parity is an extreme form of risk budgeting where each asset class is given equal risk contribution. Risk budgets shift attention from capital allocation to risk allocation.
What is volatility targeting?
Volatility targeting is a strategy that adjusts overall portfolio leverage or exposure to maintain a constant level of portfolio volatility. When market volatility rises, the portfolio reduces exposure; when volatility falls, it increases exposure. The goal is to produce a portfolio with more stable risk over time than a fixed-weight portfolio. Volatility targeting can reduce the severity of drawdowns during high-volatility crises.
References
This material is for educational and informational purposes only. It does not constitute personalized investment, legal, tax, or financial advice and does not recommend any specific security or financial product. Investing involves risk, including possible loss of principal.