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Portfolio Drift and Rebalancing: Restoring a Risk Allocation

For educational purposes only; not investment advice.

Direct answer

Section titled “Direct answer”

Portfolio drift is the difference between an investment portfolio’s actual weights and its chosen target weights. Prices, dividends, interest, withdrawals, contributions, and trades all cause drift. If equities rise from a 60% target to 72%, the portfolio now has more equity exposure than the plan specified even though the investor made no new decision.

Rebalancing means trading or directing cash flows to restore the target allocation or an allowed range around it. Its primary purpose is risk control and plan discipline, not guaranteed higher returns. Rebalancing can lag a sustained winner and can reduce returns after taxes and costs.

Measure drift and define a rule

Section titled “Measure drift and define a rule”

For each asset class:

Actual weight = current market value ÷ total portfolio market value

Absolute drift = actual weight - target weight

Relative drift = (actual weight - target weight) ÷ target weight

Common policies include calendar review, absolute tolerance bands, relative bands, or a combination. A 40% target with a ±5 percentage-point band permits 35%-45%. A 25% relative band permits 30%-50% because 40% × 25% = 10 percentage points. Those rules are not equivalent.

A policy should state the asset taxonomy, targets, bands, review frequency, treatment of cash flows, destination inside the band, tax-account ordering, and exceptional conditions. Review frequency and trading frequency are different: a portfolio can be checked monthly but traded only after a breach.

Rebalance broad risk exposures, not simply the security with the largest price decline. Several technology stocks may share one concentrated factor exposure, while a security can fall because its investment thesis deteriorated. Buying it is not automatically disciplined rebalancing.

Full-target and band rebalancing

Section titled “Full-target and band rebalancing”

A $100,000 portfolio targets 60% U.S. stocks, 30% Treasury securities, and 10% cash. Current values are $68,000, $25,000, and $7,000.

Asset Target value Current value Trade to target
U.S. stocks $60,000 $68,000 Sell $8,000
Treasuries $30,000 $25,000 Buy $5,000
Cash $10,000 $7,000 Retain $3,000

If the stock allocation instead has an allowed 55%-65% band, selling only $3,000 of stock brings it to the upper boundary, assuming the total remains $100,000. Directing a new $10,000 contribution entirely to Treasuries and cash changes the denominator to $110,000; weights and required trades must be recalculated after the flow rather than added mechanically to the old table.

For multiple assets, calculate all desired values from the same post-flow portfolio total. Purchases, sales, taxes, fees, and unsettled cash must balance; rounding each sleeve independently can create an unintended residual.

Implementation checklist

Section titled “Implementation checklist”
  • Revisit the target when time horizon, spending needs, income stability, liabilities, risk capacity, or constraints change. A new objective requires a new allocation, not restoration of the old one.
  • Use contributions, dividends, interest, and withdrawals to reduce drift before creating taxable sales when appropriate.
  • Check account type, tax lots, holding periods, realized gains and losses, and applicable tax rules. Tax consequences are jurisdiction- and investor-specific.
  • Include bid-ask spread, commissions, market impact, fund redemption restrictions, and minimum trade sizes.
  • Coordinate across all accounts that serve the same goal; rebalancing each account in isolation can leave the household allocation wrong.
  • Check look-through exposures in funds and overlapping securities rather than relying only on fund labels.
  • Keep an emergency-liquidity reserve separate if it is not part of the investment risk allocation.
  • Use written rules for threshold breaches, trade sequence, rounding, and incomplete fills, then retain execution records.

Thresholds that are too narrow can cause frequent costly trades; thresholds that are too wide permit substantial risk drift. No universal interval or band is optimal. The appropriate rule depends on volatility, correlations, taxes, costs, contribution size, and the consequences of exceeding the risk budget.

Common misconceptions

Section titled “Common misconceptions”
  • “Rebalancing always raises returns.” It primarily restores risk; return effects depend on the market path and costs.
  • “Every review requires a trade.” A policy can review frequently and trade only outside bands.
  • “Always return to the exact target.” Trading to the nearest boundary or using cash flows can reduce turnover.
  • “Buying the worst performer is rebalancing.” Trades should follow defined allocation weights, not a performance ranking.
  • “Five stocks mean diversification.” Correlated holdings can share the same sector, factor, currency, or economic risk.
  • “Tax-deferred and taxable accounts should be treated separately.” For one objective, total exposure matters, although account rules affect where trades occur.
Section titled “Related topics”

Authoritative sources

Section titled “Authoritative sources”