Rebalancing vs. market timing, why one works and one doesn't

Two investors look at their portfolios after a strong equity run. One says: "Stocks are up 30% this year, I should take some profits and rebalance." The other says: "Stocks are hot right now, I should let this ride and buy more."

Both are reacting to the same market conditions. One is following a sound strategy. The other is market timing. The distinction matters enormously over a long investing period.

What rebalancing is

Rebalancing is returning your portfolio to a target allocation when drift has pushed it away from that target.

Suppose your target is 70% stocks / 30% bonds. After a strong year for equities, you're at 78% stocks / 22% bonds. You sell some stocks and buy bonds to get back to 70/30.

Note what you're doing: selling what has recently gone up, buying what has recently gone down. This feels counterintuitive. It is also mechanically sound.

What market timing is

Market timing is adjusting your portfolio based on predictions about future market direction. "Stocks are overvalued, I'll move to cash now and buy back when they correct." Or: "The Fed is raising rates, bonds will fall, I should reduce duration."

The prediction might sound sophisticated. The track record of market timing, at both the professional and retail level, is poor.

A study from DALBAR (an independent research firm) consistently finds that average investor returns lag the market by 3-5 percentage points annually. The primary culprit is behavior: investors chase performance, sell during downturns, buy after rallies. This is market timing, even when investors don't call it that.

Why rebalancing outperforms market timing over time

Rebalancing doesn't require you to predict the future. It requires you to have a target allocation and execute a rule: when you drift beyond a threshold (say, ±5 percentage points), rebalance back.

This discipline has two effects:

1. It forces you to sell high and buy low systematically, not because you predicted anything correctly, but because you're restoring proportions.
2. It enforces a ceiling on risk. Without rebalancing, a strong equity run pushes your allocation above your risk tolerance. A subsequent correction hits harder because you have more exposure than you intended.

The Vanguard Balanced Index Fund, which maintains a 60/40 allocation through automatic rebalancing, has consistently outperformed the average balanced fund investor over long periods, not because of manager skill, but because of forced discipline.

A concrete example

January 2020: 70% stocks, 30% bonds. S&P 500 up 31% in 2019. Many investors had drifted to 78% stocks without rebalancing.

March 2020: COVID-19 crash. S&P 500 drops 34%.

The investor at 70/30 (rebalanced) held less equity and lost less in absolute terms. If they also rebalanced during the crash (buying stocks when they were down), they participated more fully in the recovery.

The investor who let equities run to 78% held more exposure into the drawdown and may have sold out of fear at the bottom.

How often to rebalance

The research supports threshold-based rebalancing over calendar-based rebalancing. Instead of rebalancing every January, rebalance whenever an asset class has drifted more than 5% from target.

Calendar-based rebalancing is fine if it's what keeps you consistent, the main value is executing the rule, not optimizing frequency.

Tax considerations matter. In taxable accounts, rebalancing triggers capital gains. Use new contributions to bring allocation back in line when possible. In tax-advantaged accounts (IRAs, 401(k)), rebalance freely.

The harder part

Rebalancing requires selling what's working and buying what isn't. During a strong bull market, that feels wrong. After a crash, buying more equities feels terrifying.

That's the psychological test. Investors who pass it, not by predicting the market, but by following a rule, consistently do better than investors who let emotion drive allocation.

How Prismfolio supports this

Prismfolio shows your current allocation against your target allocation, with drift highlighted when you're beyond your set threshold. When you see the drift percentage, the question "should I rebalance?" has a concrete answer, not a gut feeling.

The data makes the discipline easier to maintain.