The Mechanical Logic
Rebalancing has a structural property that is easy to state but psychologically difficult to act on: it automatically forces buying of underperforming assets and selling of outperforming ones. This is the classic "buy low, sell high" principle — achieved not through predictive skill but through the simple discipline of maintaining a fixed target allocation.
When equities fall sharply, a rebalancing investor sells bonds (which have held their value or risen) and buys equities (which are now cheaper). When equities surge, the opposite occurs: bonds are purchased with the proceeds of equity sales. The investor is not making a prediction about future prices. They are simply correcting the portfolio back to its intended weights. The buy-low, sell-high behaviour is the automatic consequence of that correction.
This is the key insight: rebalancing does not require market timing ability. It requires only the discipline to execute trades that feel deeply uncomfortable in the moment — buying equities during a crash, selling them during a euphoric rally.
During a Market Crash
The behavioural challenge of rebalancing is most severe during market crashes. In early 2009, as equity markets fell 50% from their 2007 peak, the correct rebalancing action for any investor with an equity allocation was to buy more equities at depressed prices. Most investors did the opposite — they reduced equity exposure or fled to cash, locking in losses.
An investor following a mechanical rebalancing schedule would have bought equities in 2009 and sold some of those positions as markets recovered through 2010–2013. The result would have been meaningfully better long-run performance than the average investor achieved through emotion-driven trading.
The same dynamic played out in March 2020 during the COVID-19 market crash. Equities fell approximately 35% in five weeks. A disciplined rebalancer bought. By the end of 2020, equity markets had fully recovered and then some. The investors who sold in panic and re-entered later captured only part of the recovery.
What the Academic Evidence Shows
Academic studies of rebalancing consistently find a nuanced result. Compared to a buy-and-hold portfolio that is never rebalanced, a rebalanced portfolio tends to produce slightly lower long-run absolute returns. This is because rebalancing trims the winners and supports the losers — in a long bull market for equities, trimming equities sacrifices some upside.
However, rebalancing significantly reduces volatility and maximum drawdown — the worst peak-to-trough decline the investor experiences. For most investors, this trade-off is favourable: modestly lower returns in exchange for a smoother ride and avoidance of catastrophic portfolio outcomes.
More importantly, the academic comparison is to a hypothetical investor who holds the drifted portfolio and never panics. Real investors do panic. The true comparison should be between a rebalanced portfolio and the actual outcomes of unmanaged portfolios, where drift leads to increased risk that eventually triggers panic selling at the worst possible moment.
Rebalancing as a Commitment Device
The deepest value of a rebalancing schedule is not statistical — it is psychological. Committing in advance to rebalance mechanically removes the decision from the moment of market stress. The investor does not need to ask "should I buy more equities now?" when the market has fallen 30% and every news headline is catastrophic. The schedule already gave the answer: yes. The commitment device transforms an emotionally agonising decision into a procedural one.
This is the same principle behind systematic investment plans: removing discretion from high-stress moments prevents the emotional errors that destroy long-run returns.