Rebalancing returns a portfolio to its intended allocation after markets have moved it. The mechanical consequence is selling what performed and buying what did not, which is the opposite of what most investors do unaided.
Drift changes the risk profile without any decision
A portfolio set to a chosen mix of assets does not stay there. Whichever component rises fastest becomes a larger share of the total.
After a long run in one asset class, the portfolio carries more exposure to it than the investor ever chose to take.
The drift is silent, and it is largest exactly when the risky asset has performed best, which is when tolerance for it feels highest.
The mechanism enforces a counter-cyclical trade
Restoring target weights requires selling part of the appreciated holding and using the proceeds to top up the lagging one.
No forecast is involved. The trade is generated by the arithmetic of the weights rather than by any view about what happens next.
That is precisely why it is difficult to execute, because it requires reducing the position that has been rewarding and adding to the one that has not.
Risk control is the primary benefit
The reliable outcome of rebalancing is that the portfolio's volatility stays near what was intended rather than rising with the strongest asset.
Any return improvement depends on whether asset classes revert, which happens in some periods and not in others.
Treating rebalancing as a risk discipline rather than a return strategy sets the right expectation, since the risk effect is dependable and the return effect is not.
Frequency involves a genuine trade-off
Rebalancing often keeps allocations tight but generates more transactions, more costs and, in taxable accounts, more realised gains.
Threshold rebalancing acts only when a weight drifts beyond a set band, which reduces activity while still capping how far the portfolio can wander.
Calendar rebalancing on a fixed schedule is simpler to administer and produces similar results, which is why many plans use it by default.
Cash flows can do the work without selling
Directing new contributions towards underweight assets moves the portfolio back towards target without selling anything.
In a taxable account this avoids realising gains, which can make the entire adjustment cost-free apart from the trading spread.
The same applies in reverse during withdrawals, where drawing from the overweight asset rebalances and funds spending in a single transaction.