Portfolio Rebalancing: Why and How to Do It

Why Portfolios Drift

When you set a target asset allocation — say, 70% stocks and 30% bonds — you are expressing a deliberate risk preference. But markets do not hold still. If stocks outperform bonds for several years, your portfolio might drift to 85% stocks and 15% bonds. You now have more risk than you intended, concentrated in assets that have already risen significantly.

Rebalancing is the process of selling assets that have grown above their target weight and buying assets that have fallen below, restoring the portfolio to its original allocation.

The Mechanical Benefit

Rebalancing enforces a systematic buy-low/sell-high discipline. When you rebalance, you are selling assets that have risen (relatively expensive) and buying assets that have fallen (relatively cheap). Over time, this can improve risk-adjusted returns compared to a portfolio that is never rebalanced.

Research on the magnitude of this benefit is mixed — in a strongly trending market, rebalancing can reduce returns by trimming the winning asset. The primary benefit of rebalancing is risk management, not return enhancement.

Rebalancing Methods

There are two common approaches. Calendar rebalancing restores the target allocation on a fixed schedule — quarterly, semi-annually, or annually. It is simple and predictable. Threshold rebalancing triggers a rebalance when any asset class drifts more than a set percentage (e.g., 5 percentage points) from its target. It is more responsive to market movements but requires more monitoring.

Many financial planners recommend annual rebalancing as a reasonable balance between responsiveness and transaction costs.

Tax Considerations

In taxable accounts, rebalancing by selling appreciated assets triggers capital gains taxes. Strategies to minimize this include: rebalancing primarily within tax-advantaged accounts (401(k), IRA), directing new contributions to underweight asset classes rather than selling, and using dividends and interest to rebalance without selling.

Behavioral Value

Beyond the mechanics, rebalancing has significant behavioral value. It provides a structured process for making portfolio decisions, reducing the influence of emotion and recency bias. Investors who rebalance regularly are less likely to panic-sell during downturns (because they are already buying the falling asset) and less likely to chase performance during rallies (because they are selling the rising asset).

Educational Context

This article is for educational purposes only and does not constitute investment advice. Rebalancing involves transaction costs and potential tax consequences.