Portfolio Rebalancing — Restoring Target Allocations

Every portfolio has an intended allocation — a deliberate spread across assets, sectors, or risk categories that reflects an investment strategy. Over time, market movements cause holdings to drift from these targets: winners grow larger, losers shrink, and the portfolio's actual risk profile diverges from the intended one. Rebalancing corrects this drift.

Why Drift Happens

Suppose a portfolio is constructed with 25% in each of four sectors. If one sector doubles and the others stay flat, that sector now represents roughly 40% of the portfolio — far above the intended 25%. The portfolio is now more concentrated in that sector than planned, regardless of whether the investor made any active decision to increase that exposure.

Drift is not inherently bad. If the outperforming sector continues to outperform, drift increases returns. But it also increases the portfolio's sensitivity to that sector reversing — a risk that was not part of the original design.

Rebalancing Methods

• Calendar rebalancing: Rebalance on a fixed schedule — quarterly, semi-annually, annually. Simple to implement, consistent discipline, but may rebalance when markets are calm and drift is minimal.
• Threshold rebalancing: Rebalance when any position drifts beyond a set percentage from its target (e.g., ±5%). More responsive to actual market conditions, but harder to systematise.
• Hybrid rebalancing: Review on a fixed schedule; only rebalance if positions are beyond threshold. Balances discipline with responsiveness.

The Rebalancing Return

Rebalancing has a counter-intuitive property: in volatile, mean-reverting markets, it can improve returns beyond the target allocation held constant. By systematically selling what has risen (partially) and buying what has fallen, it enforces a buy-low/sell-high discipline that is psychologically difficult for most investors to maintain without a systematic rule.

This benefit disappears in strongly trending markets. If one asset class simply outperforms for 10 years straight with no mean reversion, frequent rebalancing reduces returns by repeatedly trimming the winner.

Rebalancing Costs

Rebalancing is not free:

• Transaction costs: Brokerage commissions and bid-ask spreads accumulate over many rebalancing events.
• Taxes: In taxable accounts, selling appreciated positions triggers capital gains taxes. Tax-efficient rebalancing (using new contributions, or rebalancing in tax-advantaged accounts first) can mitigate this.
• Opportunity cost: Trimming a position that continues to outperform after the rebalance has a real opportunity cost.