Allocation Drift and Rebalancing

Without rebalancing, a 60/40 stock-bond allocation gradually drifts toward stocks as equities outperform. This increases risk and locks you into buying high, selling low. A discipline system fixes this.

Key takeaways

• A 60/40 portfolio that returns 8% equities and 3% bonds annually drifts to 70/30 within 5 years
• Bull runs make drift worse: from 2009–2019, a 60/40 became 85/15 without rebalancing
• Not rebalancing means you are overexposed to stocks at market peaks and underexposed at troughs
• Rebalancing forces you to buy stocks when they are down and sell when they are up—the opposite of emotional instinct
• A simple annual rebalancing rule is optimal for most investors

How drift happens: the math

A concrete example. You start with a 60/40 stock-bond portfolio:

• Stocks: $600,000 (60%)
• Bonds: $400,000 (40%)
• Total: $1,000,000

Over year one, stocks return 10% and bonds return 2%:

• Stocks: $600,000 × 1.10 = $660,000
• Bonds: $400,000 × 1.02 = $408,000
• Total: $1,068,000
• New allocation: 61.8% stocks, 38.2% bonds

The drift is 1.8% in one year. You are now slightly overexposed to stocks.

Over the next four years, assuming the same returns:

• Year 2: 63.5% stocks, 36.5% bonds
• Year 3: 65.1% stocks, 34.9% bonds
• Year 4: 66.8% stocks, 33.2% bonds
• Year 5: 68.4% stocks, 31.6% bonds

After five years of 8% stock returns and 2% bond returns, your 60/40 portfolio has drifted to roughly 68/32. You are 8% overexposed to stocks.

In a mild market, this does not seem dramatic. But in a bull market when stocks compound at 15% and bonds at 2%, the drift is severe:

• Year 1: 63% stocks, 37% bonds
• Year 2: 66% stocks, 34% bonds
• Year 3: 70% stocks, 30% bonds
• Year 4: 74% stocks, 26% bonds
• Year 5: 78% stocks, 22% bonds

After five years of the 2009–2013 bull run (similar to this pattern), a 60/40 portfolio drifted to roughly 78/22. You were now 18% overexposed to stocks and completely vulnerable to a market crash.

The consequences of drift

Drift has two negative consequences:

1. Increased risk: If stocks crash 30%, you lose more because you are 78% stocks instead of 60%. A 30% stock crash on 78% stocks costs 23.4% of portfolio value. On 60% stocks it costs 18%. The 5.4% difference is significant.

2. Forced buying high, selling low: Drift locks you into the worst possible market timing. You are overexposed to stocks after a bull run (when they are expensive and likely to crash). You are underexposed after a bear run (when they are cheap and likely to rally). This is the opposite of what you want.

From 2009–2019, a 60/40 portfolio that drifted to 80/20 was extremely overexposed to stocks at their most expensive valuations. When the COVID crash came in March 2020, this over-allocated investor suffered maximum losses. An investor who had rebalanced in 2019 (restoring 60/40) would have had both more cash to buy the dip and lower overall losses.

The cost of not rebalancing

From 2009–2019, a 60/40 portfolio that rebalanced annually returned roughly 7.8% annualized. A 60/40 portfolio that never rebalanced (drifted to 80/20 by 2019) returned roughly 8.5% annualized. So far, drift looks good—higher returns.

But then 2020 came. The COVID crash hit. The rebalanced 60/40 portfolio (still near 60/40) fell 15%. The drifted portfolio (now 80/20) fell 20%. More importantly, when the market recovered, the rebalanced portfolio had more cash and bonds to buy the dip. The drifted portfolio was fully invested in stocks and missed the rebalancing opportunity.

Over the full 2009–2024 period, a rebalanced 60/40 portfolio beat a drifted portfolio by roughly 0.3–0.5% annually. This compounds significantly. An investor who rebalanced had roughly 10% more wealth than one who did not.

When drift is most dangerous

Drift is most dangerous during bull runs. From 2009–2019, 2013–2017, and 2023–2024, stock returns far exceeded bond returns. During these periods, drift is rapid and dramatic. An investor who simply held and let drift occur became dangerously overexposed to equities, just as valuations reached peaks.

Conversely, during bear markets like 2000–2002 or 2008, drift works in your favor. Stocks underperform, so your allocation naturally becomes more conservative (smaller stock percentage). But when the recovery comes, you are underexposed and miss upside.

The pattern is clear: drift helps in bear markets and hurts in bull markets. Over a full cycle, the costs in bull markets exceed the benefits in bear markets.

Rebalancing methods

There are several ways to rebalance:

1. Calendar-based (annual): Rebalance once per year on a fixed date, regardless of market performance. This is the simplest and requires discipline, not market timing.

2. Threshold-based: Rebalance when allocation drifts more than X% from target. For example, if stocks drift above 65% (when your target is 60%), you rebalance. This requires more monitoring but is more tax-efficient.

3. Opportunistic: Rebalance when you have new contributions or withdrawals to make. If you contribute $1,000 per month, direct new contributions to whichever asset is underweighted. This is the most efficient for regular savers.

4. Automated: Use a target-date fund or a robo-advisor like Vanguard Personal Advisor Services or Betterment, which rebalances automatically.

For most individuals, calendar-based annual rebalancing is optimal. It is simple, requires no market timing skill, and has proven to improve returns over decades.

Mechanical rebalancing discipline

Here is a rebalancing framework:

1. Choose your target allocation: 60% stocks, 40% bonds (or 70/20/10 for 3-fund, or whatever matches your risk tolerance).
2. Set a calendar reminder: June 1st each year, or your birthday, or January 1st. Pick one date.
3. On that date, calculate drift: What percentage is each component now? Does it match your target?
4. Rebalance mechanically: If stocks are now 68% (instead of 60%), sell $80,000 of stocks and buy $80,000 of bonds. Do not think about whether stocks will go up or down. Just rebalance.
5. Repeat annually: Ignore all market news between rebalancing dates. Just rebalance mechanically once per year.

This discipline removes emotion. You are forced to:

• Sell stocks when they are high (after rallies), which feels bad but is correct.
• Buy stocks when they are low (after crashes), which feels scary but is correct.
• Avoid overthinking market conditions or valuations.

Tax efficiency of rebalancing

In tax-deferred accounts (401(k)s, IRAs, RRSP), rebalancing has no tax consequences. You can rebalance freely without worrying about capital gains.

In taxable accounts, rebalancing triggers capital gains taxes. A $80,000 sale of stocks that have appreciated $20,000 triggers $20,000 of capital gains, potentially $3,000–$7,400 in taxes (depending on your bracket). This is a real cost.

To minimize tax drag:

• Rebalance using new contributions: If you contribute $5,000 per month, direct contributions to underweighted assets instead of rebalancing.
• Harvest losses: When stocks have fallen, sell them to realize losses that offset gains from other sales.
• Rebalance in tax-deferred accounts: Keep the bulk of your portfolio in 401(k)s and IRAs where rebalancing is tax-free. Use taxable accounts only for emergency reserves or excess savings.
• Use target-date funds: They handle rebalancing internally within the fund, avoiding capital gains distributions.

Drift tolerance: how much is too much?

Some investors set a drift tolerance of 5%. They rebalance only when allocation drifts more than 5% from target. A 60/40 portfolio rebalances when stocks reach 65% or fall to 55%.

This approach reduces rebalancing frequency (and thus tax drag in taxable accounts) while still maintaining reasonable risk control. It is a middle ground between calendar-based and pure no-rebalancing.

For most investors, a 5% drift tolerance with annual checkups is good. You rebalance if needed, but you also avoid constant trading.

Rebalancing in a downturn

The hardest rebalancing happens in crashes. In March 2020, the S&P 500 fell 34%. A 60/40 portfolio fell to perhaps 45/55 (stock losses made bonds the larger component). To rebalance, you would sell bonds (the now-larger piece) and buy stocks (at depressed prices).

This feels insane. Stocks are crashing and you are forced to buy them. But this is exactly when rebalancing is most valuable. Those who rebalanced in March 2020 locked in buying stocks at $2,300 instead of holding at $3,400. By end of 2020, they had gains.

Mechanical discipline is hardest to maintain in crashes. But it is also most valuable. This is why calendar-based annual rebalancing is superior to discretionary rebalancing—it removes the emotion.

Rebalancing frequency and cost trade-offs

Frequency	Tax Drag	Behavioral Benefit	Complexity
Annual	Low	Moderate	Low
Semi-annual	Moderate	Higher	Moderate
Quarterly	Moderate-High	High	Moderate
Monthly	High	Very High	High
Target-date fund	None (internal)	Very High	None

For most investors, annual rebalancing is optimal. It balances the behavioral benefit of forced discipline against the tax and complexity costs.

Rebalancing decision tree

Related concepts

• ./01-what-is-asset-allocation.md
• ../../passive-investing/chapter-13-re-balancing-rules/01-why-rebalance.md

Next

Next we'll examine the final piece of allocation: portfolio-level allocation vs account-level placement—how to position assets across taxable and tax-deferred accounts.