Why Do People Say a Portfolio Needs Rebalancing From Time to Time?
Someone sets up a portfolio with a deliberate split between stocks and bonds, feels satisfied with the plan, and then checks back a couple of years later to find the split looks nothing like what they originally chose. Nobody touched it — so what happened?
The quick answer
A portfolio’s original mix of investments drifts over time because different assets grow, or shrink, at different rates, even when no money is added or withdrawn. Rebalancing means periodically adjusting the holdings back toward the original target mix, which usually involves trimming what has grown into a larger share and adding to what has shrunk into a smaller one.
Why drift happens even with no activity
Picture a portfolio that starts as a simple split between a stock fund and a bond fund. If the stock portion grows faster than the bond portion over a stretch of time, its share of the total portfolio increases, and the bond share shrinks proportionally, purely from performance differences. Nobody bought or sold anything — the mix just moved on its own because the two pieces didn’t grow at the same pace. Over years, that drift can turn a moderate original mix into something meaningfully more aggressive, or more conservative, than intended.
What rebalancing is actually doing
Rebalancing is the process of restoring a portfolio’s mix back toward its original target percentages, typically by selling a portion of whatever has grown beyond its target share and using the proceeds to buy more of whatever has fallen below its target share. It’s a mechanical process rather than a prediction about what will perform well next — the goal is maintaining a consistent risk level, not timing the market. This is a different question entirely from whether a viral investing trend is worth chasing, since rebalancing is about maintaining a plan already in place rather than reacting to a new one.
Common ways people approach the timing
There are a few general approaches to deciding when rebalancing happens. Some people rebalance on a fixed schedule, such as once a year, regardless of how far the mix has drifted. Others use a threshold approach, rebalancing only once a particular holding has drifted a set number of percentage points away from its target, whichever comes first. Neither approach is universally superior — they simply trade off simplicity against precision, and the right cadence depends on the account type, the fees involved, and how actively someone wants to manage it.
Why fees and taxes factor into the decision
Rebalancing inside a tax-advantaged account like a retirement plan usually doesn’t trigger a taxable event, but doing the same trades inside a regular taxable brokerage account can generate a taxable gain if something sold has appreciated. This is also where expense ratios and the fees tied to each fund become relevant, since frequent trading in and out of funds can add cost on top of any tax impact, especially compared with something as broad and low-turnover as an index fund approach.
Worth remembering
Rebalancing exists because markets don’t move all asset classes in lockstep, and an unmanaged portfolio naturally becomes something different from what was originally intended. Understanding drift, and picking a rebalancing rhythm that fits an account’s tax treatment and cost structure, matters more than treating any single rebalancing method as the only correct one.