A portfolio's target allocation — the intended mix of stocks, bonds, and other asset classes — rarely stays exactly in place on its own. Markets move at different rates, and left unchecked, that drift can quietly change how much risk you're actually carrying. This guide explains how automatic rebalancing addresses that, building on the robo-advisor evaluation framework.
Why Portfolios Drift
Suppose a target allocation is 70% stocks and 30% bonds. If stocks perform well over a period, that ratio can shift toward 80/20 without a single new investment decision being made — the drift happens purely because one asset class grew faster than another. Left unaddressed over years, this drift can meaningfully increase the portfolio's actual risk level beyond what was originally intended.
Two Common Rebalancing Approaches
- Threshold-based rebalancing triggers a correction whenever an asset class drifts beyond a defined percentage from its target — for example, a five-percentage-point deviation.
- Calendar-based rebalancing runs on a fixed schedule, such as quarterly or annually, regardless of how far drift has occurred by that date.
Some platforms combine both: monitoring continuously but only executing trades when both a scheduled check and a drift threshold are met.
| Method | Trigger | Tradeoff |
|---|---|---|
| Threshold-based | Allocation drifts past a set percentage | Reacts to actual drift, but timing is less predictable |
| Calendar-based | Fixed schedule (e.g., quarterly) | Predictable timing, but may act even with minimal drift, or miss drift between dates |
How Rebalancing Tries to Minimize Taxes
In a taxable account, rebalancing by selling appreciated holdings can trigger capital gains taxes. Many robo-advisors try to reduce this by first directing new contributions and reinvested dividends toward underweighted asset classes, rebalancing "with new money" before resorting to selling existing positions. This interacts with tax-loss harvesting, since both processes involve trading decisions within the same taxable account.
Why Rebalancing Matters Beyond "Buy Low, Sell High"
Rebalancing is sometimes framed as a way to systematically sell high and buy low, but its more fundamental purpose is risk control — keeping your actual exposure to market swings aligned with the level you originally chose, rather than letting it drift upward (or downward) unintentionally as markets move.
What to Ask Any Platform
- How frequently does the platform monitor for drift?
- What threshold, if any, triggers an actual rebalancing trade?
- How does it try to minimize taxable events in a taxable account specifically?
- Does the answer differ between taxable and tax-advantaged account types?
Common Mistakes to Avoid
- Assuming all platforms rebalance with the same frequency or method.
- Ignoring how rebalancing interacts with taxes in a taxable account.
- Treating rebalancing purely as a return-boosting tactic rather than a risk-control tool.
- Not asking how the platform prioritizes new contributions versus selling existing holdings.
Conclusion
Automatic rebalancing exists to keep your portfolio's actual risk level aligned with your intended target as markets move unevenly over time. The specific method and frequency vary by platform, so it's worth asking directly rather than assuming — especially if you're weighing this feature as part of a broader robo-advisor evaluation.