Aggregate Bond Funds (BND, AGG)
Aggregate Bond Funds (BND, AGG)
Aggregate bond funds hold the broadest mix of U.S. bond types—Treasuries, corporates, mortgage-backed securities, and agencies—tracking the entire investable bond market in a single holding.
Key takeaways
- Aggregate bond funds own Treasuries, investment-grade corporates, mortgage-backed securities, and agencies in a single diversified portfolio.
- BND (Vanguard Total Bond Market ETF) and AGG (iShares Core U.S. Aggregate Bond ETF) are the two dominant passive options, each with over $100 billion in assets.
- These funds track the Bloomberg U.S. Aggregate Bond Index, the de facto standard for "the bond market" in the United States.
- Expense ratios are negligible (0.03% for BND, 0.03% for AGG), making them suitable for buy-and-hold investors.
- An aggregate fund is an all-in-one holding for investors who lack strong convictions about bond type allocation and simply want market exposure.
Composition and index methodology
The Bloomberg U.S. Aggregate Bond Index (the "Agg") includes nearly all fixed-rate, investment-grade U.S. debt—roughly 13,000 issues totaling over $30 trillion in value. The fund managers cannot possibly hold all 13,000 issues; instead, they hold a representative sample of 5,000 to 10,000 positions chosen to match the index's risk and return profile.
As of early 2025, a typical aggregate fund's allocation looks approximately like this:
- 40-42% U.S. Treasury bonds and TIPS
- 25-27% mortgage-backed securities (agency MBS)
- 20-22% investment-grade corporate bonds
- 8-10% U.S. agency debt (not mortgage-backed)
- 2-3% other (floating-rate bonds, munis, etc.)
Within Treasuries, the fund holds bonds across the entire yield curve: short bills (1-3 years), intermediate notes (3-10 years), and long bonds (20-30 years). This mix of maturities is crucial. It means an aggregate fund's duration—its sensitivity to interest rate changes—is moderate. As of 2025, the Agg's duration hovers around 5-6 years, meaning a 1% rise in interest rates would reduce bond prices approximately 5-6%.
Within mortgage-backed securities, the fund holds agency MBS (government-guaranteed) rather than non-agency MBS (subject to credit risk). Agency MBS offer higher yields than Treasuries but slightly longer effective duration due to the complexity of prepayment risk.
Diversification benefits
The power of the aggregate fund lies in its diversification. A single holding gives you exposure to thousands of individual bonds across multiple issuers, sectors, and geographies. An investor holding just the Agg is largely immune to the default risk of any single issuer. Even if a major corporation (say, a Fortune 500 company) defaulted, the impact on the aggregate fund's NAV would be a fraction of a percent.
This diversification extends to duration as well. Bond duration varies based on maturity and coupon. A 2-year Treasury has a duration of roughly 2 years; a 30-year Treasury has a duration around 20 years. By holding bonds across all maturities, an aggregate fund delivers a smooth, diversified duration profile. If short rates rise faster than long rates (a common scenario in the early stages of a rate-hiking cycle), the fund holds some long-duration bonds that will appreciate relatively, offsetting losses on shorter bonds.
Diversification across bond types is also powerful. Treasuries and corporates have different risk-return profiles. In a strong economy, corporates typically outperform Treasuries (wider spreads compress). In a weak economy, Treasuries outperform (widening spreads penalize corporates). By holding both, an aggregate fund smooths returns across economic regimes.
MBS introduce their own dynamics. When refinancing activity surges (because rates have fallen), MBS holders experience unexpected early prepayment, capping upside. But when rates are stable or rising, MBS behave like regular bonds. The aggregate fund's large MBS allocation (about 26%) is a stabilizer that most individual bond investors could not easily replicate on their own.
The case for aggregate as a core holding
For an investor assembling a simple portfolio, the aggregate bond fund is the obvious starting point. Why allocate across Treasury funds, corporate funds, and MBS separately when the aggregate fund does it all, with an expense ratio of 0.03% and the liquidity of one of the world's largest exchange-traded funds?
This is the same logic that justified the three-fund portfolio in equities (total stock market, developed ex-US, emerging markets). Aggregate bond funds are the fixed-income equivalent of VTI (Vanguard Total Stock Market ETF). They are the first piece of a bond allocation for any investor without strong, specific convictions about bond type allocation.
An aggregate fund also offers lazy-rebalancing benefits. Over time, if corporate spreads widen (corporates become cheaper relative to Treasuries), the fund's corporate allocation will drift down, and its Treasury allocation will drift up. Conversely, if spreads tighten, corporates will outperform and their allocation will rise. This rebalancing happens automatically, without the investor doing anything. In a sense, the fund rebalances toward attractive valuations without the investor needing foresight.
Performance and tracking
Both BND and AGG track essentially the same index and deliver nearly identical returns. BND (the Vanguard version) has been in existence longer and holds assets of over $200 billion. AGG (the iShares version) is smaller but similarly massive. Their expense ratios are both 0.03%, and their tracking error—the difference between fund return and index return—is typically under 0.01% annually.
Over the past 10 years (2014-2024), both funds returned approximately 2.3% annualized, including distributions. This is slightly below the index return due to the 0.03% expense ratio and small frictions from cash drag and transaction costs. For practical purposes, they are interchangeable. An investor owning either can be confident they own the entire U.S. bond market at minimal cost.
Liquidity and trading
Both BND and AGG are extraordinarily liquid. Each trades tens of millions of shares daily. Bid-ask spreads are typically just $0.01 per share, or about 0.015% on a $70 share price—negligible for a buy-and-hold investor. You can buy or sell $1 million of BND without materially moving the price.
This liquidity also means that aggregate bond funds are suitable for dollar-cost averaging. If you are accumulating a bond position slowly over time (say, $5,000 per month for a year), both BND and AGG are equally convenient, and the bid-ask spread you pay will be trivial.
Limitations and when to look beyond aggregate
Despite its merits, the aggregate bond fund is not suitable for all investors. If you have a strong conviction that long-term rates are heading higher, you might want to limit duration by holding more short-term bonds. An aggregate fund's 5-6 year duration may feel too long.
Conversely, if you believe rates are headed lower and want to maximize duration exposure (to capture capital gains), an aggregate fund may feel too short. You might prefer a fund of 20+ year Treasuries.
If you want higher yield and accept higher risk, the aggregate fund's 20-22% allocation to corporate bonds may feel insufficient. You might want to overweight corporate bond funds.
If you are in a high tax bracket and hold bonds in a taxable account, the aggregate fund's substantial MBS allocation may create inefficiencies. MBS distributions are less tax-efficient than Treasury distributions, and some of the fund's returns come from price appreciation that could be realized or deferred at your discretion if you owned bonds directly.
If you want inflation protection, the aggregate fund's 40% Treasury allocation includes only a handful of TIPS relative to nominal Treasuries. You might want to complement it with a dedicated TIPS fund.
For the vast majority of investors, though—especially those in a 401k or IRA where tax efficiency is irrelevant—an aggregate bond fund is the right and only holding they need for their bond allocation. It is diversified, liquid, cheap, and transparent.
Construction and rebalancing
The fund manager must make practical choices about how to hold the index. The Bloomberg Agg includes bonds as small as $1 million in notional value—far too small to trade economically. The manager constructs a representative sample of 5,000-8,000 issues, weighted by market value, that closely tracks the index. This sampling introduces minimal tracking error.
Monthly, the manager rebalances by adjusting position sizes to match the index's current weights. If corporate bonds have rallied and now represent 23% of the portfolio instead of the target 22%, the manager trims corporates and adds Treasuries. This rebalancing is done efficiently, with most activity focused on the bonds with the largest position changes.
As bonds approach maturity (especially near the end of their term), they are sold and proceeds reinvested in newly issued bonds or longer-duration holdings. This "rolling down the curve" is automatic and requires no shareholder decision.
Distributions and yield
An aggregate bond fund's yield is the weighted average yield of all holdings, minus the 0.03% expense ratio. As of early 2025, with Treasury yields around 4.0-4.5%, corporate yields around 5.0-5.5%, and mortgage yields around 4.5-5.0%, an aggregate fund yields roughly 4.2-4.5% annually. This yield is distributed monthly or quarterly.
In a taxable account, distributions are taxable in the year received, whether you reinvest or spend them. In a tax-deferred account, distributions compound untaxed until withdrawal.
Next
The aggregate bond fund is the foundation. But many investors find it useful to complement the aggregate with more specialized holdings to fine-tune duration, credit risk, or inflation exposure. Treasury funds are the natural next step—allowing you to tilt toward the safest part of the bond market while maintaining diversification within the Treasury curve.