Portfolios drift. Winners grow to oversized weights; laggards shrink. Rebalancing is the disciplined act of selling relative strength and buying relative weakness to return to target allocation—mechanically uncomfortable, mathematically sensible over long horizons.
Why drift happens
A 60/40 stock/bond target can become 70/30 after an equity rally without any new purchases. Concentration risk rises silently. Rebalancing forces partial profit-taking and refills underweights.
Calendar vs threshold methods
Calendar: Review quarterly or semi-annually regardless of drift. Simple, may allow extended imbalances between dates.
Threshold: Rebalance when any sleeve drifts more than 5% (or 20% relative to target weight—define your rule in writing). More responsive, potentially higher turnover and taxes in taxable accounts.
Hybrid approaches are common: check quarterly, trade only if thresholds breached.
Tax and cost considerations
In taxable accounts, selling winners triggers gains. Use new contributions to rebalance when possible, harvest losses tactically where appropriate, and prefer tax-advantaged wrappers for frequent rebalancing. Trading costs and bid-ask spreads matter on small accounts.
Behavioral edge
Rebalancing is anti-momentum in the short term—you trim what feels best. That is the point. Rules written in calm markets protect you during euphoria and panic.
Implementation steps
- Document target weights and acceptable bands.
- Choose calendar, threshold, or hybrid trigger.
- Automate where your broker supports it; otherwise calendar reminders.
- Review after large macro shocks even if bands not hit.
Key takeaways
- Drift increases concentration risk; rebalancing restores plan.
- Pick calendar, threshold, or hybrid—and account for taxes.
- Rules beat emotion; contribute new cash to rebalance when possible.