Rebalancing

A portfolio's asset allocation is a decision, but markets ignore it. Let a 60% stock / 40% bond portfolio ride through a strong stock run and it becomes 70/30, carrying more risk than its owner ever chose. Rebalancing is the maintenance that undoes the drift: sell enough of the overweight asset and buy enough of the underweight one to restore the targets. It is unglamorous by design, and its main product is a portfolio whose risk level still matches the plan when the next downturn arrives.

Two standard triggers exist, and both work. Calendar rebalancing checks on a fixed schedule, commonly annually, and restores targets regardless of drift size. Threshold rebalancing acts only when an allocation strays by a chosen amount, for example five percentage points from target, which responds faster in turbulent markets and leaves the portfolio alone in calm ones. Hybrids that check on a schedule but act only past a threshold are popular because they bound both effort and drift. Frequency matters less than having a rule; rebalancing monthly versus annually changes little, while rebalancing never changes a lot.

Execution is where taxes enter. Selling winners in a taxable brokerage account realizes capital gains, so tax-aware investors rebalance in a deliberate order: first direct new contributions and any dividends toward whatever is underweight, then trade inside IRAs and 401(k)s where sales trigger no tax, and only then, if drift remains, sell in taxable accounts--ideally longer-held shares taxed at long-term rates. Many households can stay near target for years without a single taxable sale.

The behavioral point deserves respect. A rebalancing rule made you a systematic buyer of stocks in every crash and a systematic trimmer near every peak, exactly when instinct screams to do the opposite. The rule does not know the future; it simply enforces the plan against the person, which is often the plan's hardest job.

A hypothetical example. June holds $100,000 with a 60/40 target: $60,000 in a stock index fund, $40,000 in a bond fund. Over a strong year, stocks gain 30% and bonds gain 2%. Her stocks are now $78,000 and her bonds $40,800, a $118,800 portfolio sitting at roughly 66% stocks. If she uses a five-percentage-point threshold, the drift triggers a rebalance.

Restoring 60/40 means holding about $71,280 in stocks, so June sells roughly $6,720 of the stock fund and buys bonds with the proceeds. If the account is her IRA, the trade has no tax consequence. If it is a taxable account, she might instead redirect her monthly contributions into bonds and let the gap close over several months, avoiding any sale. Either way she has locked in a slice of the stock run and returned the portfolio to the risk level she actually chose.

Pros

• Keeps the portfolio's risk aligned with the plan instead of with recent market performance.
• Institutionalizes selling high and buying low, without requiring any forecast.
• A written rule short-circuits the urge to improvise during manias and panics.
• Often achievable with no tax cost, using contributions, dividends, and tax-advantaged accounts.

Cons

• In long bull markets, rebalancing trims the best performer and lags a never-rebalanced portfolio; risk control has a price.
• Careless selling in taxable accounts converts drift into a tax bill.
• Overdoing the frequency adds effort and potential costs with little added benefit.