Bonds and Rebalancing Fuel
Bonds and Rebalancing Fuel
Bonds serve as the cash machine of a diversified portfolio. When stocks fall sharply, selling bonds to buy stocks at depressed prices is not hedging—it is the rebalancing mechanism that makes equity diversification work.
Key takeaways
- Bonds provide the psychological and practical cushion to buy stocks when prices are lowest
- Systematic rebalancing forces you to sell bonds (and bonds go up when stocks crash) and deploy that to equities
- The more volatile your equity allocation, the more important bonds become as your rebalancing fuel tank
- Rebalancing only works if you have dry powder; bonds are that dry powder
- A portfolio that cannot rebalance during severe bear markets locks in losses and underperforms
Why rebalancing needs fuel
A 60/40 portfolio is not a static allocation. It drifts. When stocks soar, equities might creep to 70% of the portfolio; when stocks crater, equities might collapse to 40%. Rebalancing means selling the winners and buying the losers—but you can only do that if you have something to sell.
This is where bonds shine. When stocks fall 20%, 30%, or 40%, bonds typically hold steady or even gain value. A 10-year Treasury that yielded 3% becomes worth more when yields rise during a panic. Investors sell bonds at higher prices, harvesting the gains, and redeploy that cash into equities at lower prices. This is the entire premise of a balanced portfolio: you earn a return from the rebalancing trade itself.
Contrast this with a portfolio that is 100% stocks. During the 2008 crisis, when stocks fell 57%, a pure equity investor had no fuel to buy the dip. They either panicked and sold at the bottom, or they watched helplessly as their portfolio evaporated. There was no mechanism to force disciplined buying.
In 2020, when stocks fell 34% in 23 days, a balanced portfolio owner could sell bonds (which actually gained 8% during that period) and buy equities at a 34% discount. The rebalancing trade captured that opportunity. A 60/40 portfolio that rebalanced quarterly in 2020 would have outperformed a buy-and-hold 60/40 by roughly 1–2% annually by the time the recovery took hold.
The rebalancing discipline in bear markets
Rebalancing in a bull market is easy. Stocks go up, you trim them and buy bonds—you are locking in gains. Everyone is willing to sell winning positions when prices are high. The hard part comes during severe drawdowns.
Consider 2022. Stocks (S&P 500, VTI) fell 18%. Bonds fell too—a 10-year Treasury had negative total return of roughly 16%. Both asset classes lost money. In this regime, rebalancing did not work well because there was no diversification benefit. The crisis was inflation-driven, hitting both stocks and bonds simultaneously.
But look at 2008. Stocks fell 57%; the Barclays Aggregate Bond Index rose 5.2%. In March 2020, stocks fell 34%; Bloomberg Aggregate Bonds rose 1.9%. In 1987 (Black Monday), stocks fell 22% in one day; bonds gained. In 2000–2002, the tech crash hammered equities while bonds surged. In nearly every historical crisis except 2022 and parts of the 1970s, stocks and bonds moved in opposite directions, creating the classic diversification benefit.
A disciplined rebalancer in 2008 would have had the moral courage and the dry powder to buy stocks at 8–10x earnings ratios. A rebalancer in 2020 bought the S&P 500 at 15x forward earnings, just before a five-year bull run. The rebalancing trade earned you a massive premium on recovery.
How much rebalancing fuel do you need?
This depends on your volatility tolerance and your stock allocation. A portfolio with 80% stocks and 20% bonds needs much more discipline because the drag from rebalancing losses (on the losing 80% during rare negative years) is steeper. A portfolio with 50% stocks and 50% bonds rebalances more smoothly—each $100k down in stocks gets offset by gains in the $100k in bonds.
The practical rule is simple: the more aggressive your stock allocation, the more important bond allocations become as a rebalancing buffer. A 100% equity portfolio cannot rebalance. A 90/10 portfolio needs to rebalance constantly and will suffer sequence-of-returns risk if you lack the discipline or the fuel to execute. A 60/40 portfolio has enough fuel that the rebalancing mechanism works reliably.
During the period 1926–2023, the S&P 500 has experienced 17 bear markets (drops of 20% or more). In 14 of those 17 cases, bonds provided positive returns or at worst modest negative returns. Bonds supplied the dry powder to rebalance into the crash. In 2 of those cases (1973–1974, 1981–1982), bonds fell alongside stocks—but their duration exposure meant that a quick recovery in bond prices would have rewarded rebalancers. In only 1 case (2022) did bonds fail to rebalance meaningfully because both asset classes were driven by the same inflation shock.
The rebalancing calendar vs event-driven rebalancing
Many investors use a calendar approach: rebalance quarterly, semi-annually, or annually. This removes emotion and creates a mechanical discipline. If you own a 60/40 fund that drifts 10% off its target weights (say, 65/35 or 55/45), you set a hard rule that at the end of each quarter, you restore it to 60/40.
Others use threshold-based rebalancing: only rebalance when a position drifts more than 5% or 10% from its target. This is more efficient in fees and taxes, but it risks inaction during the exact moment when rebalancing matters most—a severe crash.
The best approach for most investors is a hybrid: rebalance on the calendar, but allow yourself to trigger an extra rebalance if a shock pushes allocations more than 10% off target. During the March 2020 crash, waiting until quarter-end would have meant locking in losses. But an investor who rebalanced in late March (selling bonds, buying stocks at 80-cent-dollar prices) captured a massive rebalancing premium within weeks.
How bonds enable a higher stock allocation
Without reliable rebalancing fuel, many investors are forced into lower equity allocations than they might otherwise tolerate. A conservative investor might hold 30% equities, not because that is their optimal risk-return trade-off, but because they lack confidence in their ability to rebalance when it matters.
Bonds change that calculus. If an investor knows that a 60/40 allocation includes a proven rebalancing mechanism, they may tolerate higher equity exposure than they otherwise would. The bonds are not there to match benchmark or to reduce volatility in normal times—they are there to mechanically force you to buy low and sell high.
This is why target-date funds, Vanguard's LifeStrategy funds, and most balanced funds structure themselves with significant bond allocations. It is not accident. The bonds provide the dry powder that makes the equity allocation sustainable over a multi-decade career.
The tax and fee implications of rebalancing fuel
Rebalancing in tax-deferred accounts (401k, IRA, Roth IRA, RRSP, SIPP) is tax-free, so the full benefit of rebalancing accrues to you. In taxable accounts, rebalancing triggers capital gains tax, which reduces the net benefit. A rebalancing trade in taxable that forces a 20% gain on bonds at a 20% federal rate costs you 4 percentage points of the rebalancing premium.
This is why many investors hold aggressive allocations (60/40, 50/50) in tax-deferred accounts and more conservative allocations (40/60, 30/70) in taxable. The tax friction of rebalancing in taxable means you need more dry powder to justify the costs.
Bond funds with lower turnover (index funds like BND, VBTLX, or AGG) are ideal rebalancing fuel because the fund itself does not force you to harvest gains prematurely. A high-turnover bond fund or active manager wastes rebalancing fuel on unnecessary tax drag.
Process
Next
You have now learned how bonds fuel rebalancing, the mechanism that makes diversified portfolios work in practice. But bonds alone don't insulate you from all risks—so the next article explores the question all rebalancing investors eventually face: when bonds aren't enough, should you hold cash instead?