Federal Reserve chairman Jerome Powell has told everyone from Princeton professor and former Fed vice chair Alan Blinder to Congressional committees to the public at large via press conferences that negative interest rates are not an “appropriate tool” for the United States.

In a recent interview, Powell underscored the point by saying, “the evidence whether it actually works is mixed.”

So is Powell’s stance the Fed’s version of American exceptionalism? That is, it may have had some positive results in Japan and Europe but it won’t work here?

Or, perhaps negative rates will be too disruptive to money markets or detrimental to banks? Or, impossible, of course, for the apolitical Fed chair, they may even have unwanted political consequences in an election year?