What Is Portfolio Drift and Why Does It Matter?
Suppose you start with a portfolio of 60% equity and 40% debt — a target allocation suited to your risk profile. After a strong equity bull run, your portfolio might look like 75% equity and 25% debt without you having done anything. Your risk profile has not changed. But your portfolio's risk has increased significantly — you just did not notice it happen.
This is portfolio drift. It is silent, automatic, and universal. Every portfolio that is not rebalanced drifts over time. And drift means the portfolio you actually hold is no longer the portfolio you agreed to hold.
A 2020 illustration: A portfolio that started 2020 at 60/40 (equity/debt) — without rebalancing through the COVID crash and subsequent recovery — ended 2021 at roughly 72/28. The investor had quietly absorbed 20% more equity risk than their risk profile prescribed, without any conscious decision.
Three Approaches to Rebalancing
1. Calendar-Based Rebalancing
Rebalance on a fixed schedule — annually, semi-annually, or quarterly — regardless of how much the portfolio has drifted. Simple to implement and remember. The downside: if drift is small, you incur transaction costs and potential taxes for little benefit. If drift is large, you may wait too long.
2. Threshold-Based Rebalancing
Rebalance when any asset class drifts beyond a defined threshold from its target — typically ±5% (relative) or ±5 percentage points (absolute). This is the approach we use at Aryzen and the one most supported by research.
Example: Target allocation is 60% equity. You set a ±5 percentage point band. Rebalancing is triggered when equity exceeds 65% or falls below 55%. This captures significant drift without over-trading on minor fluctuations.
3. Hybrid Approach
Check the portfolio on a calendar basis (e.g., quarterly), but only rebalance if drift exceeds a threshold. This combines structure with efficiency — you are not rebalancing unnecessarily, but you are also not ignoring the portfolio between scheduled reviews.
The Behavioural Case for Rebalancing
Rebalancing is mechanically simple but psychologically difficult. When equity markets are booming and your equity allocation has grown from 60% to 75%, rebalancing means selling some of your best-performing assets and buying more of your worst-performing ones. Every instinct says the opposite.
This is precisely why rebalancing works. By systematically selling high and buying low — not based on market predictions, but on preset rules — you exploit the mean-reverting nature of asset classes over time. You are buying debt when equity is expensive and buying equity when debt is relatively attractive. Not because you predicted anything, but because your target allocation demanded it.
Rebalancing is the mechanical implementation of "buy low, sell high" — without requiring you to predict markets or time anything. The discipline of the rule is the strategy.
Tax Implications of Rebalancing
In India, rebalancing can trigger capital gains tax. Before executing any rebalance, consider:
- Equity ETFs held >1 year: Long-term capital gains tax at 12.5% on gains above ₹1.25 lakh. Plan rebalancing to utilise the annual ₹1.25 lakh LTCG exemption efficiently.
- Equity ETFs held <1 year: Short-term capital gains at 20%. Where possible, delay rebalancing until the holding crosses the 1-year threshold.
- Debt funds: Gains taxed as income at your slab rate, regardless of holding period. There is no LTCG benefit for debt funds.
- New contributions first: Before selling anything, consider whether new SIP contributions can be directed to underweight asset classes to reduce the rebalancing required.
Tax-efficient rebalancing sequence: (1) Redirect new contributions to underweight classes first. (2) Sell overweight holdings that have crossed the 1-year mark for LTCG treatment. (3) Use the ₹1.25L LTCG exemption each financial year — do not waste it.
Practical Rebalancing: A Step-by-Step
- Record your target allocation — by asset class, with specific ETFs and target percentages.
- Review actual allocation quarterly — using your broker's portfolio statement or our Portfolio Analyser tool.
- Calculate drift — compare current to target. Flag any class that has drifted beyond your threshold.
- Check holding periods — before selling, verify which holdings have crossed 1 year for tax efficiency.
- Execute the minimum required trades — rebalance to the edge of the threshold band, not necessarily exactly to the target. This reduces transaction costs and future trade frequency.
- Document the rationale — record what you sold, what you bought, and why. This is good discipline and useful if you are questioned later.