Glen Bennett
A money market fund is a mutual fund that puts investors' cash into short term, highly liquid debt instruments such as Treasury bills, commercial paper and repurchase agreements, aiming to hold a steady $1 share price while paying out modest interest income.
At a Glance
- Money market funds invest in cash, cash equivalents and short term, high quality debt with maturities under 13 months.
- They target a stable $1 net asset value, though that target has failed only twice in the fund industry's history.
- The Securities Investor Protection Corporation covers these funds, not the Federal Deposit Insurance Corporation.
- They come in prime, government and tax exempt varieties, each with different holdings and tax treatment.
- They work well as a short term parking spot for cash but offer little in the way of long term growth.
How the $1 Share Price Actually Works
A money market fund functions much like any other mutual fund: it pools investor money, buys a portfolio of securities and issues redeemable shares. The twist is that fund managers work to keep each share priced at exactly $1. Any interest the portfolio earns above operating costs gets passed along to shareholders as dividends rather than showing up as price appreciation. That structure makes the math simple. If a fund is paying a certain yield, an investor can generally expect their balance to grow by roughly that amount over a year, since the share price itself isn't supposed to move.
To keep that promise, funds are restricted to short term, high quality debt. Regulators require a weighted average maturity of 60 days or less across the whole portfolio, and no single issuer (other than the government) can make up more than 5% of assets. At least a quarter of holdings must be convertible to cash within a day, and half within a week. These rules exist specifically so redemptions never get stuck behind illiquid assets.
When Funds Have Broken the Buck
Occasionally a fund's NAV slips below $1, a situation known informally as breaking the buck. It has happened only twice in a meaningful way. In 1994, the Community Bankers U.S. Government Money Market Fund liquidated at $0.96 per share after heavy losses tied to derivatives. Far more consequential was the Reserve Primary Fund in 2008, which held large amounts of Lehman Brothers debt. When Lehman collapsed, panicked withdrawals pushed the fund's NAV down to $0.97 and forced its liquidation, rattling money markets broadly.
Those episodes prompted regulators to act. The Securities and Exchange Commission tightened portfolio rules in 2010 and gave fund boards tools like liquidity fees and redemption suspensions. In 2014 it went further, requiring prime institutional funds to let their NAV float instead of staying fixed at $1, while retail and government funds kept the stable pricing model. After investors yanked money out of funds during the 2020 pandemic shock, the SEC issued another round of reforms in 2023, this time removing rules that had let funds impose fees once weekly liquid assets dropped below a certain threshold.
Comparing the Main Types of Money Market Funds
Not all money market funds hold the same things, and the differences affect both yield and tax treatment. Prime funds chase slightly higher returns by holding corporate and agency debt. Government funds stick almost entirely to Treasuries and government backed repurchase agreements, trading some yield for extra safety. Tax exempt, or municipal, funds hold state and local government debt and can shield income from federal and sometimes state taxes.
| Fund Type | Typical Holdings | Relative Yield | Tax Treatment |
|---|---|---|---|
| Prime Money Fund | Commercial paper, floating rate corporate and agency debt | Generally highest among the three | Fully taxable |
| Government Money Fund | At least 99.5% in cash, Treasuries and fully collateralized repos | Lower than prime, considered safer | Fully taxable, though some Treasury income may be state tax exempt |
| Tax Exempt (Municipal) Money Fund | Municipal bonds and short term municipal debt | Lower headline yield, but often competitive after taxes | Exempt from federal income tax, sometimes state tax too |
Some funds also set a high bar for entry, with institutional share classes requiring minimums as large as $1 million, while retail versions are open to ordinary investors with far smaller amounts.
Money Market Fund or Money Market Account: Knowing the Difference
The names sound alike, but the products are not interchangeable. A money market fund is an investment product sold by a fund company or brokerage, carries no principal guarantee, and is covered by SIPC rather than FDIC insurance. A money market account is a bank or credit union savings product, insured by the FDIC or the National Credit Union Administration, and it often comes with check writing or debit card access. Because it isn't exposed to market price swings, a money market account tends to be even safer than a money market fund, though that added safety can mean somewhat lower returns.
Money market funds also compete with other short duration options such as ultrashort bond funds and enhanced cash funds, which may reach for higher yield by taking on a bit more risk or a wider range of holdings.
What Interest Rates Do to Fund Returns
Because a money market fund's income comes directly from the yields on its underlying holdings, broader interest rate policy drives returns almost entirely. Between 2010 and 2015, the Federal Reserve held short term rates near zero, and money market fund yields sank accordingly, disappointing investors used to better payouts in prior decades. Post crisis regulation also shrank the pool of eligible securities, tightening supply further.
Quantitative easing compounded the effect. As the Fed and other central banks bought up government securities to push rates down and expand the money supply after 2008, a wave of cash flowed into money market funds looking for a safe harbor, only to find yields depressed for years by the very policy that sent it there. Investors weighing a money market fund today should keep in mind that returns will rise and fall with the prevailing rate environment, not with anything the fund manager does independently.
Where Money Market Funds Fit and Where They Fall Short
These funds shine as a place to hold cash between investment decisions or ahead of a planned expense. They're inexpensive, usually free of sales loads, highly liquid, and historically low risk. They are not, however, a substitute for long term investments like retirement accounts, since the $1 share price target caps any capital appreciation. Investors chasing growth over decades will need other vehicles; those looking for a safe, liquid, modestly interest bearing home for near term cash have found money market funds useful since they first appeared in the U.S. in the early 1970s.
Will Regulation Keep Pace With the Next Market Shock?
Money market funds have absorbed three rounds of SEC reform since 2008, each aimed at preventing a repeat of the Reserve Primary Fund episode. Whether those safeguards hold up under the next bout of market stress remains an open question, one that regulators, fund managers and investors will only be able to answer when it actually arrives.
Nichole Martinez
