Of course, there’s always the chance that asset prices react to QE announcements in the short term, but that these reactions fade over time. Lewis found that the effect persists to the end of the day, but months or years are a different matter. Swanson found that changes in asset prices persist for as long as four months after an asset-purchase announcement, though this requires making various additional assumptions.

These exercises show how hard it is to get solid, conclusive proof in macroeconomics. Even if all of these researchers’ assumptions hold, certainty is elusive; lower borrowing costs might not always make companies invest more. And changes in the economy since the early 2010s might make a future recession a different story. For example, if QE worked mainly by taking toxic housing-backed assets off of shaky banks’ balance sheets, it won’t be very effective in a future recession that doesn’t involve banks loading up on garbage bonds.

Although these papers (and others that found similar results) don’t make a slam-dunk case for QE’s effectiveness, they represent the slow accumulation of evidence in favor of the policy. In the next recession, QE will be — and should be — the Fed’s main line of defense.

This opinion piece was provided by Bloomberg News. 

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