This is general information, not investment advice.

When markets wobble, money pours into "money market funds." It's worth knowing exactly what they are — and what protects them.

What it is

A money market fund is a type of mutual fund that holds very short-term, high-quality debt — Treasury bills, government securities, commercial paper, repurchase agreements, short-term CDs — aiming to preserve principal, stay liquid, and pay a yield close to short-term interest rates. Funds are built to hold a stable net asset value (NAV) of $1.00 per share, so each dollar in is meant to come back as a dollar plus interest. The SEC regulates them under Rule 2a-7, which limits the credit quality, maturity and liquidity of what they can hold.

Why people use them

They're a place to park cash that earns a real yield while staying accessible. When short-term rates are high — as after the Fed's 2022–24 hiking cycle — money fund yields often beat bank savings accounts. And in risk-off stretches, investors move cash out of stocks into money funds, exactly the pattern behind this week's big money-fund inflows and equity outflows. Total US money fund assets have reached record levels, crossing roughly $7 trillion in recent years, per Investment Company Institute data (figures move weekly).

The distinction that trips people up

This is the most important point. A money market FUND is an investment product from an asset manager (Fidelity, Vanguard, Schwab). It is not a bank deposit and not FDIC-insured — if it lost value, no government guarantee makes you whole. A money market DEPOSIT ACCOUNT (MMDA) at a bank looks similar but is FDIC-insured up to $250,000 per depositor, per institution. The SEC's Investor.gov is explicit that money market funds "are not guaranteed or insured by the FDIC." Same-ish name, fundamentally different products.

Types

  • Government/Treasury funds: only US government securities and repos — lowest credit risk, common in brokerage cash sweeps.
  • Prime funds: also hold short-term corporate/bank debt — slightly higher yield, marginally more risk.
  • Tax-exempt/municipal funds: short-term muni debt; income often federally tax-exempt.

The one risk: "breaking the buck"

Money funds are very low risk, not zero. The danger is breaking the buck — the NAV falling below $1.00. It's rare but real: in September 2008, the Reserve Primary Fund held Lehman Brothers commercial paper; when Lehman failed, its NAV fell to $0.97, triggering a run on money funds and emergency government backstops. The episode led to SEC reforms — tighter maturity and liquidity rules, and a floating NAV for institutional prime and tax-exempt funds.

How yields move

Money fund yields track short-term rates closely: they rise when the Fed hikes and fall when the Fed cuts. Investors who chased 4–5% yields in 2023–24 should expect that to compress as the rate cycle turns — not a flaw, just the short-duration design working as intended.

How it compares

  • Money market fund: market-rate yield, not FDIC-insured, next-day liquidity.
  • High-yield savings / MMDA (bank): FDIC-insured to $250k, easy access.
  • CD: fixed rate locked in, FDIC-insured, penalty for early withdrawal.
  • Treasury bills: backed by the US government, sold at auction, tradable.

What it means for savers

Money market funds are legitimate, well-regulated, useful cash tools, and the practical risk of losing principal in a government fund is very low. But "very low" isn't "zero," and the absence of FDIC insurance is a genuine structural difference worth understanding before you choose between a fund and an insured bank account. Know what you own, know what protects it — and remember the yield advantage narrows when the Fed cuts.