Advertising
Most Americans holding cash in a brokerage account assume their money is working as hard as it can. In reality, millions of savers park idle funds in default sweep positions earning a fraction of a percent, while a money market fund inside the same account could be generating significantly more, sometimes three to five times the return, with comparable risk and no additional complexity.
This gap rarely comes from ignorance. It arises from a mismatch between surface-level familiarity and the deliberate decision-making required to capture the full value these funds offer.
What follows is a structured look at how money market funds work beyond the marketing language, covering fund types, key performance metrics, overlooked risks, and the choices that separate an efficient cash position from an underperforming one.
What a Money Market Fund Is and How It Works
A money market fund is a type of mutual fund that invests exclusively in short-term, high-quality debt instruments like U.S. Treasury bills, corporate commercial paper, certificates of deposit, and repurchase agreements. The fund’s goal is not growth but capital preservation, liquidity, and a modest income stream tied to prevailing short-term interest rates.
The mechanics of this stability come from strict regulatory requirements. In the United States, these funds are governed by SEC Rule 2a-7, which limits holdings to instruments with very short maturities (no more than 13 months per security) and requires the portfolio’s weighted average maturity to stay at or below 60 days. This compressed time horizon helps keep the fund’s value stable, even when broader markets fluctuate.
Most retail money market funds maintain a net asset value (NAV) of exactly $1.00 per share. Investors buy shares at $1.00 and expect to redeem them at $1.00, with income paid out as dividends. This stable price is the defining feature that separates them from bond funds, where the share price fluctuates with interest rate movements.
The Three Core Fund Types and What They Hold
Not every money market fund is the same. The differences between types have real consequences for yield, risk, and tax efficiency, so this is where deliberate selection begins.
- Government money market funds invest at least 99.5% of their assets in cash, U.S. Treasury securities, and fully collateralized repurchase agreements. Because they hold government-backed debt, they have the most conservative risk profile and often qualify for a state income tax exemption.
- Prime money market funds hold a broader mix, including corporate commercial paper and bank certificates of deposit, alongside government securities. They typically yield more than government funds but absorb more pressure during market stress.
- Municipal money market funds invest in short-term debt issued by state and local governments. Their income is generally exempt from federal income tax and sometimes state income tax, making them effective for high-bracket investors with taxable accounts.
The Metrics That Determine Real Returns
A money market fund’s headline yield is the starting point for analysis, not the end. Several other factors shape the actual return an investor receives.
7-Day SEC Yield and Expense Ratio
The 7-day SEC yield is the standardized metric for comparing money market funds. It annualizes the fund’s net income over the most recent seven days after deducting all expenses. Because it updates frequently, it is the most current indicator of what a fund is earning for investors right now.
While the published yield is already net of fees, the expense ratio still matters. Fee waivers, which are common in low-rate environments, can be withdrawn without notice, significantly shifting the net yield. The table below illustrates how expense ratios affect net yield for comparable government funds at a hypothetical 5.40% gross yield.
| Fund Example | Expense Ratio | Gross Yield (Hypothetical) | Net Yield After Fees | Annual Difference on $100,000 |
|---|---|---|---|---|
| Low-cost government fund | 0.09% | 5.40% | 5.31% | — |
| Higher-cost government fund | 0.42% | 5.40% | 4.98% | ~$330 less per year |
A 33-basis-point difference in the expense ratio on a $100,000 cash position amounts to roughly $330 per year. This is real money left behind by choosing a default option over a more deliberate one.
Tax Treatment and After-Tax Yield
For investors in high-tax states (such as California or New York), the state tax exemption on many government money market funds adds significant value. Government funds holding direct U.S. Treasury obligations pass through income that is generally exempt from state and local taxes.
Meanwhile, municipal money market funds produce income exempt from federal income tax, which can translate to much higher after-tax yields for those in top tax brackets.
Consider an investor in the 37% federal tax bracket. A municipal fund yielding 3.5% in tax-exempt income may deliver a tax-equivalent yield over 5.5%, outpacing many taxable alternatives with a similar risk profile. The math changes based on individual circumstances, but the principle is consistent: the fund with the highest published yield is not always the one with the highest after-tax return.
The Sweep Account Trap Most Investors Miss
One of the most consequential risks of holding cash at a brokerage has little to do with the funds themselves, relating to where uninvested cash lands by default.
Many brokerage platforms automatically route idle cash into a bank sweep program (a product affiliated with the firm’s banking subsidiary) rather than a money market fund. These sweep programs often pay interest rates well below what a comparable money market fund would deliver. The difference can be substantial, representing a meaningful gap that compounds silently over time.
To address this, investors should verify which cash feature their account uses. According to Schwab’s money market fund offerings, there are distinct differences between the bank sweep default and available money market funds. Investors typically must make a manual selection to access the higher-yielding option, a dynamic that exists at other major brokerages as well.
Understanding the Real Risks
Money market funds are widely considered safe, and that characterization is largely accurate. Still, several specific risks deserve a clear-eyed evaluation.
Breaking the Buck and Credit Risk
When a money market fund’s NAV drops below $1.00 per share, it has “broken the buck.” This event is extremely rare, having only happened twice in the retail sector, in 1994 and during the 2008 financial crisis. The 2023 SEC reforms, which took effect in 2024, were designed to reduce this risk by implementing new liquidity requirements for certain funds.
For retail investors, the practical takeaway is this: government money market funds have never broken the buck as a category and carry the lowest structural risk. Prime and municipal funds now operate under tighter liquidity minimums, reducing, though not eliminating, the probability of NAV instability under stress.
Inflation Risk and FDIC Distinction
Because their returns track short-term rates, money market funds can lag inflation when rates are very low, causing a negative real return.
Additionally, money market funds are not FDIC-insured like bank deposits. However, they are protected as securities by SIPC when held at a registered brokerage firm, a key distinction when comparing them to traditional savings accounts.
You May Also Like
- 👉 Cash equivalents: Preserve liquidity and earn modest yield
- 👉 Treasury Bonds: A Practical Guide to Steady Income
How to Access and Evaluate These Funds
Money market funds are available through most brokerage accounts and IRAs. Investors buy and sell shares directly from the fund, and capital is generally accessible on any business day.
When evaluating a specific fund, five dimensions provide a complete picture:
- Fund Type: Compare options based on your risk tolerance and tax situation (e.g., government for lowest risk, prime for a potential yield premium, and municipal for federal tax exemption).
- 7-Day SEC Yield: Examine this metric to compare funds within the same category, as cross-category comparisons require adjusting for risk and tax differences.
- Expense Ratio: Check the fee and confirm whether it includes a temporary waiver or reflects the permanent contractual rate.
- Sweep Default: Verify your brokerage account’s default cash position and determine if a manual change to a money market fund is available.
- After-Tax Yield: Calculate this for any fund producing tax-exempt income, especially if you are in a high tax bracket.
Minimum investment requirements vary. Some funds accessible through brokerage sweep programs have no minimum, while others may require $1,000 or more to open. Institutional share classes, which often have the lowest expense ratios, typically require a much higher initial investment.
Conclusion
A money market fund is a simple and effective tool for managing cash, yet it is often underutilized. The difference between an optimal and a subpar return lies not in the product itself but in the choices an investor makes.
By looking beyond the default sweep account, comparing expense ratios, and considering tax implications, you can ensure your cash is working as hard as possible. Taking a few moments to make a deliberate selection can have a meaningful and lasting impact on your savings.
Watch a clear explanation of money market funds and how they can fit into a low-risk savings strategy.
Frequently Asked Questions
What are the benefits of choosing a money market fund over a traditional savings account?
How often can investors access their funds in a money market account?
Are there specific fees associated with money market funds that investors should be aware of?
What role do money market funds play in an investment portfolio?
How do changes in interest rates affect money market fund returns?
