While money market funds have attracted regulatory attention on both sides of the Atlantic since the financial crisis, in Europe, the industry has had a more fundamental issue to tackle: defining for the first time what can and cannot be called a money market fund. In the United States, the legal framework for the money fund industry is relatively straightforward: Such funds are defined and regulated by Rule 2a-7 of the Investment Company Act of 1940. That said, Rule 2a-7 has had to keep up with the times.

When Lehman Brothers fell in September 2008, the Reserve Primary Fund's exposure to the bank led the money market fund to write off $785 million in Lehman debt—and consequently to "break the buck," as its net asset value (NAV) fell below the crucial $1 per share to 97 cents. A brief run on U.S. money market funds followed, and many other funds had to be supported by their sponsors.

In order to make money market funds more robust, the Securities and Exchange Commission has made a number of changes to Rule 2a-7 since 2008, including reducing the maximum weighted average maturity (WAM) from 90 days to 60 days and introducing a weighted average life (WAL) restriction of 120 days, as well as minimum liquidity requirements.