Voluntary Testimony by Executive Branch Officials: An Introduction [November 27, 2019] [open pdf - 980KB]
From the Introduction: "In recent presidential Administrations, there have been several high-profile disputes between Congress and the White House regarding access to executive branch officials. This has included attempts by Congress to enforce subpoenas issued to Harriet Miers, White House Counsel to President George W. Bush; Eric Holder, Attorney General to President Barack Obama; and Wilbur Ross and William Barr, Secretary of Commerce and Attorney General to President Donald Trump, respectively. [...] This report focuses on one facet of inter-branch interaction: testimony before congressional committees. The report outlines the origins of voluntary testimony by the executive branch, identifies some notable incentives for voluntary participation, and covers some key dimensions of the practice of voluntary participation."
CRS Report for Congress, R46061
Congressional Research Service: https://crsreports.congress.gov/