I explained why a few months ago in my free weekly column, Thoughts from the Frontline, at Mauldin Economics.

In that regard, something important happened recently. And not many people noticed. I'll do a quick review to explain.

In Congressional testimony last February, a member of Congress asked Janet Yellen if the Fed had legal authority to use negative interest rates. Her answer was this:

In the spirit of prudent planning we always try to look at what options we would have available to us, either if we needed to tighten policy more rapidly than we expect or the opposite. So we would take a look at [negative rates]. The legal issues I'm not prepared to tell you have been thoroughly examined at this point. I am not aware of anything that would prevent [the Fed from taking interest rates into negative territory]. But I am saying we have not fully investigated the legal issues.

So as of then, Yellen had no firm answer either way.

A few weeks later, she sent a letter to Rep. Brad Sherman (D-CA). He had asked what the Fed intended to do in the next recession and whether it had authority to implement negative rates.