What Does It Mean When a Money Market Fund 'Breaks the Buck'?
Money market funds are designed around a simple promise: each share stays priced at a fixed, stable value. “Breaking the buck” is the phrase for the rare moment that promise doesn’t hold.
The short answer
Breaking the buck refers to a money market fund’s net asset value falling below its target stable price, most commonly one dollar per share, usually because losses on the fund’s underlying holdings outweigh the interest it has earned. It’s considered a rare and notable event precisely because these funds are structured to avoid it.
How a fund is supposed to stay stable
A money market fund holds a basket of short-term debt, and under normal conditions the interest earned on those holdings is what generates return, while the share price itself is meant to stay essentially flat. That structure works as intended as long as the underlying securities are paid back as expected and market conditions stay orderly. Problems arise when one or more of a fund’s holdings loses significant value or defaults, and the loss is large enough that spreading it across all shareholders pushes the price below the stable target.
Why it has happened before
In moments of severe financial-market stress, some funds holding corporate or bank debt have seen the value of specific holdings drop sharply, sometimes tied to a single issuer running into serious trouble. When that loss is big enough relative to the fund’s size, it can outweigh the accumulated interest cushion and push the share price down. This risk is generally more associated with funds that hold corporate and bank debt, discussed further in a comparison of prime versus government money market funds, than with funds holding only government securities.
What changed afterward
Episodes like this prompted meaningful changes to how money market funds are overseen. Regulators tightened requirements around the credit quality, maturity, and liquidity of what funds are allowed to hold, a shift covered in more detail in a look at how money market funds are regulated. Funds also gained, and in some cases were required to adopt, tools like liquidity fees and temporary redemption limits to manage a sudden rush of withdrawals without forcing a fire sale of holdings, which is explored in a piece on money market fund liquidity fees and gates.
What this means for someone holding a fund
- It remains uncommon. The vast majority of money market funds, across long stretches of market history, have maintained their stable share price.
- Fund type matters. Government-only funds carry less exposure to this risk than funds holding corporate or bank debt.
- No backstop applies. Unlike an insured bank deposit, a money market fund’s stability is a design goal, not a promise, which is why understanding whether these funds carry deposit insurance is worth doing before treating one as equivalent to cash in the bank.
- Disclosures explain the holdings. A fund’s prospectus and periodic filings describe what it holds and how it manages risk, which is the most direct way to see what’s actually inside.
What to weigh
Breaking the buck is a low-probability event, but understanding why it can happen, and which fund types are more or less exposed to it, is useful context for anyone treating a money market fund as a substitute for a bank account. The stability is real, engineered, and generally reliable, but it is not the same thing as a government-backed promise.