Rebalancing asks you to sell some of what's been winning and buy more of what's been losing. That's exactly why it works — and exactly why almost nobody actually does it.
Rebalancing ranks among the least glamorous disciplines in investing and among the most neglected, and those two facts are connected. It consists of periodically restoring a portfolio to its intended proportions — trimming whatever has grown too large a share and topping up whatever has shrunk. Simple to describe. For entirely psychological reasons, remarkably hard to actually do.
The need for it arises because portfolios drift. Over time, whatever has performed well grows into a larger share of the total, while whatever has lagged shrinks. Left alone, a portfolio that started with deliberate proportions slowly turns into something its owner never chose, weighted more and more toward whatever has recently gone up. That drift isn't neutral. It steadily raises the portfolio's exposure to whatever has already run the furthest — which happens to be the exposure most vulnerable to a reversal.
Rebalancing corrects the drift, and the correction is exactly what makes it so hard to do. It means selling some of what's been performing well, which feels like abandoning a winner, and buying more of what's been performing poorly, which feels like throwing good money after bad. Every instinct pushes the other way. The holding that's risen feels like the one worth keeping; the one that's lagged feels like the one worth dumping. Rebalancing insists on the opposite of both instincts, and it offers no promise about when, exactly, the anticipated reversal will show up.
It's worth being honest about what rebalancing does and doesn't accomplish, because it sometimes gets oversold. It doesn't reliably boost returns — in a stretch where a single holding rises for years running, an investor who rebalanced away from it will end up with less than one who didn't. What rebalancing reliably does is control risk. It stops a portfolio from drifting, through sheer inaction, into something far more concentrated and far more exposed than its owner ever intended, and that risk control, not any boost to return, is the actual point of doing it.
This is how a lot of investors discover, too late, that they were never really diversified in the first place. A holding that's performed exceptionally for several years can come to dominate a portfolio without its owner noticing, until it falls — at which point they learn their portfolio's fate had quietly become tied to one position they never deliberately chose to bet on. Skipping rebalancing is, in effect, a decision to let recent performance dictate your exposures, which is rarely something anyone would choose on purpose if they saw it clearly.
The way to actually make rebalancing happen, given how strongly instinct resists it, is to take it out of the realm of judgment and make it mechanical. Rebalancing on a fixed schedule, or whenever a holding has drifted past a defined band, turns a decision that would otherwise require fighting your own instincts into a routine that doesn't ask that of you. You don't have to feel like selling the winner. You just follow the rule, and the rule doesn't care how you feel about it.
There's a cost to weigh against all this. Rebalancing generates transactions, and transactions generate costs and, depending on the account, tax consequences. Rebalance too often and you can burn through more than the discipline actually contributes. That argues for a sensible frequency — rebalancing once drift has become material rather than trivial captures most of the benefit while keeping the cost in check.
We think of rebalancing as a clean example of a discipline whose difficulty is entirely psychological and whose value is entirely real. An investor who can bring themselves to trim what's working and add to what isn't has proven they can follow a plan even when following it feels wrong, and that capacity is what every sound approach ultimately rests on. Most people skip rebalancing not because they don't understand it, but because it feels bad — and feeling bad is not a reason to neglect it.