Portfolio Rebalancing Frequency

What is Rebalancing?

Rebalancing is the process of adjusting your asset allocation back to your original target weights after they have drifted over time. For example, if you start with a 60% stock, 40% bond portfolio, and stocks perform very well, your allocation might shift to 70% stocks and 30% bonds. Rebalancing involves selling some of the appreciated stocks and buying bonds to restore the 60/40 allocation.

The key point is that asset weights change on their own. If left untouched, the best-performing assets will dominate your portfolio, leading you to take on more risk than initially designed. Rebalancing is less about maximizing returns and more about maintaining your risk level as intended. This article is for educational purposes only and does not recommend any specific trades.

Why Does Frequency Affect Asset Weights?

Rebalancing frequency refers to the time interval at which you review and adjust your asset weights. The longer the review interval, the further your assets may drift from their target before being brought back. For instance, if your target is 50% stocks and you don't review for a year, your stock allocation might exceed 60%. If you review monthly, it will stay closer to 50% with smaller fluctuations.

A shorter interval keeps your weights closer to the target but increases the number of trades. Trades incur transaction fees, the bid-ask spread (the difference between the selling and buying price), and capital gains taxes if held in a taxable account. Therefore, choosing a frequency is a trade-off between 'how closely you want to stick to your target' and 'how much you are willing to tolerate in transaction costs'.

Comparing Monthly, Quarterly, and Annual Rebalancing

Monthly rebalancing keeps weights most tightly aligned but involves frequent trades. In highly volatile periods, selling appreciated assets and buying depreciated ones can create a 'contrarian' effect, potentially yielding small diversification benefits. However, monthly fees and taxes can easily erode these gains. Quarterly rebalancing, with four reviews per year, offers a good balance between cost and precision.

Annual rebalancing, adjusted only once a year, results in the lowest frequency of transaction costs and taxes. However, asset weights can drift significantly over the year, leading to periods where you hold higher risk than intended for an extended time. For a hypothetical example, if you started with $1,000,000 allocated 60% to stocks and 40% to bonds, and a bull market caused stocks to grow to 70% of the portfolio (making it 70/30), an annual approach would leave that extra $100,000 worth of excess risk untouched until the next review.

There is weak evidence that any single frequency is inherently superior. Various long-term studies have shown small differences in risk-adjusted performance between monthly, quarterly, and annual rebalancing. When costs are factored in, overly frequent adjustments often become disadvantageous. Therefore, in practice, quarterly or annual rebalancing is a common choice.

Time-Based vs. Threshold-Based Rebalancing

Rebalancing rules generally fall into two categories. One is time-based, where you review according to a calendar, such as at the end of each quarter. The other is threshold-based, where you only adjust when an asset's weight deviates from its target by a certain margin. Thresholds are typically set as an absolute value, such as 'adjust if it deviates by more than 5 percentage points from the target'.

These two methods can be combined. A common compromise is a rule like 'review quarterly, but only adjust assets whose deviation exceeds the threshold.' This prevents trades in quarters where weights haven't moved much, reducing unnecessary costs while still preventing significant risk drift. The trick is to separate the frequency of review from the frequency of actual trades.

Pitfalls Created by Costs, Taxes, and Account Types

The biggest enemy of rebalancing is cost. In Korea, fees are charged for selling domestically listed ETFs and stocks. For US stocks, capital gains tax is imposed on amounts exceeding the basic deduction, so frequent selling in taxable accounts increases the tax burden. Adjusting within tax-deferred accounts like pension savings or IRPs (Individual Retirement Pensions, a Korea-specific retirement account type) eliminates the tax issue at the time of sale. Thus, the effective cost of rebalancing can vary depending on the account type.

Another pitfall is ignoring new cash inflows. If you contribute money monthly on a regular basis, simply allocating new funds to underweighted assets can achieve significant rebalancing without selling. This is called 'cash flow rebalancing' and is advantageous for regular investors as it avoids sales taxes and fees.

Finally, setting too narrow a deviation threshold or too short a frequency can lead to overreacting to short-term market fluctuations. If weights briefly drift off target only to quickly return, trading each time will only accumulate costs. It is generally better to keep rules simple and stick to them dispassionately.

Summary: Checkpoints for Choosing a Rebalancing Frequency

First, write down your target allocation and acceptable deviation. Example: 60% stocks, 40% bonds, with a 5 percentage point deviation. Set a review frequency that you can consistently follow, such as quarterly or annually, and limit actual trades to instances where the deviation threshold is exceeded. For taxable accounts, prioritize allocating new contributions to underweighted assets to reduce selling.

There is no single correct frequency. For tax-deferred accounts, frequent reviews may incur less burden. If transaction costs are high in a taxable account, a wider interval is more reasonable. Regardless of the method, consistency in setting rules beforehand and adhering to them will have a greater impact on your results than the choice of frequency itself.