The first U.S. exchange-traded fund (ETF)—the SPY based on the S&P500—began trading in 1993. Since then, the number of such funds has grown dramatically, so that by mid-2016 there were more than 1,600 ETFs on U.S. exchanges valued at roughly $2.2 trillion. This means that ETFs are now roughly one-sixth the size of open-end mutual funds. And, with this ETF growth has come a broadening in their scope and character. Today, there are ETFs that include less liquid assets such as corporate bonds and emerging market equities, and there are funds that provide inverse or leveraged exposure to the underlying assets.

Given these trends, it is no surprise that ETFs have attracted regulators’ attention (see, for example, here and here). Should they be concerned? Is this a consumer protection issue? Do ETFs contribute to systemic risk? Or, is their design stabilizing? Might financial stability even be served by the conversion of all open-end mutual funds into ETFs? ...