Money Market Funds
Funds holding very short-term debt, designed to maintain a stable value. They are not deposits, they are not insured, and the distinction has mattered twice.
MadStockAlerts Research · Updated August 29, 2026
What to take away
- They hold very short-dated instruments and aim for a stable share price.
- They are securities, not bank deposits, and carry no deposit insurance.
- Government, prime and municipal categories carry different risks and treatments.
- Institutional prime funds have a floating value and can impose fees or gates.
- Their yields track the policy rate closely and with a short lag.
MAD Academy Training Video · 0:45
Nearly Cash, and Not Quite
Money market funds hold very short, very high-quality paper and are close to cash — but they are investments, not deposits.
This lesson is part of a Stock Alerts + Tools plan.
What they hold
A money market fund holds short-dated, high-quality debt: Treasury bills, government agency paper, repurchase agreements, commercial paper and certificates of deposit, subject to strict rules on maturity, credit quality and diversification.
| Category | Holds | Notes |
|---|---|---|
| Government | Treasuries, agency debt, repo backed by them | The most conservative category |
| Treasury | Treasuries and repo on them only | Interest may be state tax exempt |
| Prime | Adds commercial paper and bank obligations | Higher yield, and credit exposure |
| Municipal | Short-dated municipal debt | Interest generally federally exempt |
The differences in yield between these categories are usually small and widen precisely when credit conditions tighten, which is when the differences in risk also matter most.
They are not deposits
A money market fund is a security. It is not a bank deposit, it carries no deposit insurance, and its stable share price is an objective rather than a guarantee.
That distinction has been tested. A prominent fund fell below a dollar a share in 2008 after holding commercial paper from a failed issuer, an event that prompted emergency support for the sector and a subsequent overhaul of the rules.
The reforms that followed introduced a floating value for institutional prime funds and mechanisms allowing fees or redemption gates in stress. Retail government funds retained a stable value, and the categories are not equivalent.
How the yield behaves
Because the holdings are very short-dated, the portfolio turns over quickly and the yield tracks prevailing short-term rates with a lag of weeks rather than years.
- When the policy rate rises, the fund's yield follows within a few weeks.
- When it falls, the yield follows down just as quickly.
- There is essentially no duration, so the share price does not fall when rates rise.
- The yield quoted is net of the fund's expenses, which matter more when rates are low.
The third point is the property that distinguishes them from every other fixed income holding in this pillar. A money market fund's value does not respond to rate changes because there is almost no duration to respond with.
Sweep accounts and the alternative
Brokers automatically place uninvested cash into a sweep vehicle, and the default is frequently a bank deposit programme rather than a money market fund. The two pay materially different rates in some conditions.
| Bank sweep | Money market fund | |
|---|---|---|
| Protection | Deposit insurance, subject to limits | Securities protection, not deposit insurance |
| Rate | Set by the broker, often well below market | Tracks short-term market rates |
| Default | Frequently the default | Usually requires an explicit purchase |
The middle row is where the difference is largest and least noticed. In periods of high short-term rates, the gap between a default sweep rate and a money market fund yield has been several percentage points, on cash the holder may not have thought of as an investment at all.
The alternatives at the short end
Several instruments compete for the same role, and they differ in protection, liquidity and tax treatment rather than materially in yield.
| Instrument | Protection | Liquidity | Note |
|---|---|---|---|
| Government money market fund | Securities protection | Same day, usually | Yield tracks the policy rate |
| Treasury bills held directly | Sovereign credit | Secondary market, or hold to maturity | Interest generally state tax exempt |
| Bank savings or certificates | Deposit insurance to the limits | Varies; certificates carry penalties | Rate set by the bank |
| Ultra-short bond funds | Securities protection | Same day | Some duration and credit risk; the price moves |
The last row is the one that is frequently grouped with the others and does not belong there. An ultra-short bond fund holds duration and credit, so its price moves, and several have declined during stress in a way that money market funds did not.