Money Market Funds Explained: How They Work, and Why They're Not a Bank Deposit

When investors park cash they don't need immediately, many turn to money market funds (MMFs) rather than a bank savings account. These funds are a staple of brokerage sweep accounts, 401(k) cash options, and corporate treasury management. But despite feeling like a bank product, an MMF is a regulated mutual fund with its own rulebook, risks, and history of regulatory overhauls.

What a Money Market Fund Actually Holds

An MMF pools investor money to buy a portfolio of short-term, high-quality debt instruments. The goal is capital preservation and daily liquidity, not growth. The U.S. Securities and Exchange Commission (SEC) oversees these funds under the Investment Company Act of 1940, and Rule 2a-7 specifically governs what they can hold.

• Government money market funds: invest almost entirely in U.S. Treasury securities, government agency debt, and repurchase agreements collateralized by those securities.
• Prime money market funds: can also hold commercial paper, certificates of deposit, and other short-term corporate or bank debt, generally offering slightly higher yield potential with slightly more credit exposure.
• Municipal (tax-exempt) money market funds: invest in short-term debt issued by states, cities, and other municipal entities, offering income that may be exempt from federal (and sometimes state) income tax.
• Within prime and municipal categories, funds are further split into retail funds (limited to natural-person investors) and institutional funds (open to organizations).

Stable vs. Floating NAV

Government MMFs and retail prime/municipal MMFs are generally permitted to use amortized cost accounting and round their share price to maintain a stable $1.00 net asset value (NAV), making the fund feel like cash. Institutional prime and institutional municipal funds, by contrast, must use a floating NAV, meaning the share price moves in small increments to reflect the actual market value of the underlying securities, similar to any other mutual fund.

Why the Rules Exist: Lessons From Past Stress

Money market funds have faced two notable periods of acute stress. During the 2008 financial crisis, the Reserve Primary Fund's NAV fell below $1.00 per share — commonly described as 'breaking the buck' — after it wrote down debt issued by Lehman Brothers, triggering a wave of redemptions across the prime fund industry. In March 2020, at the onset of the COVID-19 market disruption, prime and municipal funds again saw heavy, rapid outflows, prompting emergency intervention by the Federal Reserve to support short-term funding markets.

Following each episode, the SEC tightened MMF rules. Reforms adopted in 2010 and 2014 introduced tighter liquidity and credit standards and the floating NAV requirement for institutional prime and municipal funds, along with the ability for fund boards to impose liquidity fees or temporarily halt redemptions ('gates') during stress.

The 2023 SEC Reforms

In July 2023, the SEC adopted further amendments to Rule 2a-7, with compliance phased in during 2024, aimed at making MMFs more resilient and reducing investor incentives to redeem early during market turmoil.

• Discretionary liquidity fees and redemption gates tied to a fund dropping below weekly liquidity thresholds were eliminated for most funds.
• Mandatory liquidity fees were introduced for institutional prime and institutional municipal funds during periods of significant net redemptions, so that redeeming shareholders bear more of the liquidity cost they impose on remaining investors.
• Minimum daily and weekly liquid asset requirements were raised, giving funds a larger buffer of highly liquid holdings.
• Stress testing and reporting requirements to the SEC were enhanced for fund advisers.

Money Market Funds vs. Bank Savings Accounts

Both are used for cash management, but the mechanics differ in important ways.

• Insurance: Bank deposits (checking, savings, CDs) are FDIC-insured up to applicable limits per depositor, per bank, per ownership category. MMFs carry no deposit insurance.
• Yield behavior: Bank savings rates are set by the institution and often lag market rates; MMF yields adjust more quickly to short-term interest rate movements because the fund directly holds short-term securities.
• Access: Both typically offer same-day or next-day liquidity, though MMFs may apply fees during unusual stress for certain institutional fund types under the 2024 rules.
• Tax treatment: Municipal MMFs can offer income that is exempt from federal tax, a feature ordinary savings accounts don't provide.

For most retail investors, the practical takeaway is to read the fund's prospectus, confirm whether it is a government, prime, or municipal fund, and understand that while MMFs have historically been very low-risk, they are market-based investment products rather than insured deposits. This article is for informational and educational purposes only and does not constitute investment, tax, or legal advice.