"[4](registration required) In 2019 The Cato Institute published his fifth book, Gold, the Real Bills Doctrine, and the Fed: Sources of Monetary Disorder – 1922–1938, a collaboration between Humphrey and his co-author Richard H. Timberlake.

[22] Humphrey's works on monetary theory are cited in David Laidler's book Fabricating the Keynesian Revolution: Studies of the Inter-War Literature on Money, the Cycle, and Unemployment.

[23] In 2006, Federal Reserve Chairman Ben S. Bernanke at the Fourth ECB Central Banking Conference, Frankfurt, Germany, cited Humphrey's article on the real bills doctrine.

[24][25] In 2008 Humphrey gave the Fourth Annual Ranlett Lecture in Economics at California State University, Sacramento, entitled Lender of Last Resort: The Concept in History.

In 2009 Humphrey participated in the Adam Smith Program, Jepson School of Leadership Studies at the University of Richmond, where he presented The Fed's Deviation from Classical Thornton-Bagehot Lender-of-Last-Resort Policy, a paper co-authored with Richard Timberlake.