Portfolio rebalancing keeps your risk level where you want it. Here's when to do it, how to do it, and why most investors overthink it.
Portfolio rebalancing is the process of returning your investments to their target asset allocation after market movements have shifted the percentages away from your original plan.
For example: if you start with a 70% stocks / 30% bonds portfolio, a bull market in stocks might push it to 80% / 20%. Rebalancing sells some stocks and buys bonds to restore the 70/30 target.
Markets move, and over time your carefully chosen asset allocation can drift significantly from your intended risk level. After a multi-year stock bull market, an investor who intended a balanced 60/40 portfolio might find themselves with 80% in stocks — taking far more risk than they planned.
Rebalancing also enforces a disciplined "buy low, sell high" behavior: you systematically sell what has grown expensive and buy what has become cheap relative to your targets.
Two common approaches:
The most tax-efficient way to rebalance is through your contributions — simply direct new money toward the underweight asset class rather than selling. If you're in a tax-advantaged account (Roth IRA, 401k), you can buy and sell freely without tax consequences.
In taxable accounts, selling investments that have appreciated triggers capital gains taxes. Minimize this by rebalancing with new contributions first, using tax-loss harvesting, or rebalancing inside tax-advantaged accounts.
Research shows rebalancing adds modest value — perhaps 0.2-0.5% annually — primarily through risk control rather than return enhancement. For simple portfolios, annual rebalancing or rebalancing with contributions is sufficient. Don't obsess over perfection.