Reading money market funds: Rule 2a-7, stable NAV and run risk

Where does money go when it is neither in a bank nor invested? A large part sits in money market funds, a reservoir of nearly $8 trillion in the United States that almost no one watches, until the day it empties. These funds finance a major share of the government’s short-term debt, absorb the world’s spare cash, and sit at the heart of every liquidity crisis, from 2008 to March 2020. Reputed to be dull, they are in fact one of the most sensitive nodes of financial plumbing. This guide explains how they work, and how to read them.

A money market fund, in brief

A money market fund is a pooled vehicle that invests only in very short-term, high-quality instruments: Treasury bills, repo, agency debt, commercial paper from solid issuers. Its promise is three words: safety, liquidity, yield. You park cash there as in an account, you can withdraw it any time, and it earns interest close to short-term market rates. It is the waiting vehicle par excellence, where corporate treasurers, managers and households park their cash between two uses.

This convenience rests on a useful fiction: the stable one-dollar net asset value. Where an equity fund’s price moves constantly, a money fund strives to keep each share at exactly one dollar, so the saver feels they hold cash, not a risky investment. The whole regulatory edifice aims to keep that fiction workable. And all the danger arises the day it no longer is.

Government, prime, municipal: three families

Money market funds are not all alike. Three families differ by what they hold, and hence by their risk-return trade-off.

The distinction between government and prime matters most. A government fund can hardly default, since it lends only to the state and against state collateral. A prime fund holds commercial paper and certificates of deposit from companies and banks, which pay more but can lose value, or even become unsellable under stress. It is this prime family that broke the buck in 2008 and suffered the runs of 2020, while government funds saw money pour in seeking safety.

Rule 2a-7: the regulatory corset

For a fund to promise a stable value, the SEC imposes a precise corset, Rule 2a-7. It limits what it can hold and how. Credit quality must be high. The portfolio’s weighted average maturity cannot exceed sixty days, bounding interest-rate risk. Above all, the fund must keep buffers of immediately mobilizable assets: since the 2023 reform, at least 25% in daily liquid assets and 50% in weekly ones. These buffers are its first line of defence against redemptions.

The rule also distinguishes two types of valuation. Government and retail funds keep a stable one-dollar NAV, rounded. Institutional prime funds, since the 2016 reform, must show a floating NAV that varies at the fourth decimal with the portfolio’s real market value. The idea was to wean large investors off the illusion of the fixed dollar, and to make panic withdrawals less mechanical. The result was mixed, as we will see.

Breaking the buck: the dread

A money fund’s nightmare has a name, breaking the buck: seeing its NAV fall below one dollar, precisely below 0.995, which materializes a loss. It is exceedingly rare, but devastating, because the fund’s very structure encourages flight. The first to leave is repaid at par, at one dollar; those who stay inherit the losses. This first-mover advantage turns the slightest doubt into a run, a dynamic identical to that which threatens a bank, but on a vehicle meant to be risk-free.

History has two dates. In September 2008, the Reserve Primary Fund, the oldest U.S. money fund, held $785 million of Lehman Brothers commercial paper. The bank’s collapse dropped its NAV to $0.97, triggering a general run on prime funds that only an exceptional federal guarantee stopped. In March 2020, the scenario nearly repeated: the dash for cash drained prime funds, and the Federal Reserve urgently created a support facility, the MMLF, to buy their assets and halt the panic. Twice in twelve years, a product sold as near-cash required the state’s rescue.

The 2023 reform: goodbye gates

These episodes fed two waves of reform. The 2016 one had imposed the floating NAV on institutional prime funds and introduced redemption gates, along with liquidity fees, which funds could trigger under stress. The problem is that these gates, far from calming holders, encouraged them to flee even earlier, before the barrier fell. The remedy worsened the ill.

The 2023 reform learned the lesson. It removed the gates, raised the liquidity buffers, and imposed mandatory liquidity fees on institutional prime funds as soon as one day’s net redemptions exceed 5% of assets, to make leavers pay the cost they impose on others. The effect was radical: rather than comply, most managers preferred to close or convert their institutional prime funds, whose number fell from about twenty-five to nine. The regulator made prime funds safer mainly by making them nearly disappear, which the industry held against it.

The systemic role: a tap of T-bills and repo

Beyond their internal mechanics, money funds matter because they are huge and concentrated on a precise segment. With nearly $8 trillion in assets, a record reached in May 2026 amid a flight to safety, they are one of the leading buyers of Treasury bills and a pillar of the repo market. What they do with their cash irrigates the whole plumbing of short-term funding.

Their behaviour carries two signals. First, the Fed’s reverse repo facility, the RRP, where they parked spare cash, went from over $2.5 trillion in 2023 to near zero in 2025: that money went to seek yield elsewhere, notably in T-bills, absorbing part of the record issuance we described in our article on Treasury auctions. Second, the size of money funds is a barometer of risk aversion: when it swells fast, as in 2026, it often means investors prefer the paid safety of cash funds to the uncertainty of risk assets. Finally, a cousin was born of the same logic, payment stablecoins, whose short Treasury-bill reserves make them money funds that dare not speak their name, as our stablecoins guide shows.

Reading money market funds in practice

Reading a money fund means, first, looking at what it holds: a government fund and a prime fund do not carry the same risk, and the latter’s extra yield is paid for in fragility at the worst moment. It means, next, checking its liquidity buffers, the share of assets mobilizable within a week, which says its ability to absorb redemptions without selling at a loss. It also means watching its weighted average maturity, a gauge of its rate sensitivity. At the aggregate level, two series deserve the eye: total assets, a thermometer of risk appetite, and RRP usage, which reveals where cash finds a home in the plumbing.

One caveat, finally. A money fund’s advertised stability is a contract of trust, not a law of nature. The one-dollar value holds as long as the assets are safe and the holders calm; it can break fast if either gives way. The 2023 reform strengthened the defences, but the original sin remains: promising immediate liquidity on assets that, in a general stress, are no longer quite liquid. That is why these funds, reputed the dullest in finance, remain among the most closely watched by those who track systemic risk.

Sources and further reading

• U.S. Securities and Exchange Commission, 2023 money market fund reform (fact sheet): mandatory liquidity fees, removal of gates, raised buffers.
• Investment Company Institute, weekly money market fund statistics: assets by fund family.
• U.S. SEC, money market fund statistics: portfolio composition and maturity.
• Bloomberg, money market fund assets at a record $8.3 trillion in May 2026.
• l0g, Record auctions: the weekly referendum on U.S. debt.
• Related guides: Reading net liquidity: reserves, TGA, RRP, Reading the repo market and SOFR and Reading stablecoins and the GENIUS Act.