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President Trump’s lockdown on information provisions by federal agencies includes a blockade of communication with Congress, which already struggles to understand complex policy issues because staffing levels on its committees and Congressional Research Service have been dwindling for 30 years. A less noticed, but important contribution to Congressional ignorance is the virtual disappearance of a particular type of witness at hearings: investigative journalists.

The scrappy reporter heroically spilling big secrets at a high-profile Congressional hearing — typically with a corrupt official or sleazy plutocrat looking on while sputtering with impotent rage — is a staple of novels, TV shows and movies about Washington politics. These fictional portrayals resonated with the public because they used to be common in real life. To cite one of many examples, investigative reporter Lea Thompson’s testimony describing how infants had suffered permanent brain damage from inadequate baby formula help spur Congress to eliminate such products by passing the Infant Formula Act of 1980.

Thompson’s Congressional hearing appearance was one of hundreds of times investigative journalists brought critical information to federal lawmakers. But research in a new book by media economist James T. Hamilton indicates that such consequential interactions between the press and the Congress are almost a thing of the past.

Of the 644 Congressional hearings with journalist witnesses in the 1950-2009 period, almost a third occurred in the 1970s, with a steadily decreasing proportion ever since. Although Congress holds fewer hearings than it used to, this does not explain the decline: the number of journalist witnesses per 1000 hearings dropped 85 percent from 1970s to the 2000s.

Hamilton attributes the near disappearance of journalist witnesses to “declining revenues and staff at newspapers translating into less investigative activity.” Local and regional investigative reporting were particularly hard hit. Hamilton notes that “while Freedom of Information Act requests by media outlets at federal agencies dropped by 25% between 2005 and 2010, such records requests made by local newspapers dropped by nearly 50%.”

The withering of regional and local investigative journalism in markets in and around Washington, D.C. eliminated from the pool of potential witnesses many ink-stained wretches with deep knowledge of federal government operations. But the loss of local and regional journalists whom members of Congress could invite to attest how federal policy was working (or more likely, not working) in the member’s district also contributed to the disappearance of journalist witnesses at hearings.

Harry Truman famously railed against the “do-nothing” Congress. Combined with Trump blocking federal agency communication with members, and the slashing of Congress’s once formidable investigative research capacity, the disappearance of investigative journalists from hearings will help create a “know nothing” Congress. Members may understandably feel they are stumbling in the dark as they attempt to navigate complex policy issues and vexing social problems with only lobbyists and partisan think tanks on hand to guide them.