incestuous amplification
n. The reinforcement of set beliefs among like-minded people, leading to miscalculations and errors in judgment.
Back in Washington, around the water coolers at the Pentagon, they talk about this idea called "incestuous amplification" as a bad thing.

In the echo chamber of Hollywood, however, incestuous amplification … is what they call a movie studio.

This town embraces incestuous amplification! Which leads us to this year's nominations for the Oscars. People, it's groupthink run amok.
—William Booth, “A Quest for Gold,” The Washington Post, February 22, 2004
Without knowing it, the Columbia investigators were identifying a pervasive social problem, one that unites these examples and that leads to many failures in the public and private sectors. In military circles, this process is called "incestuous amplification." Among psychologists, it is known as "group polarization."

In a nutshell: Like-minded people, talking only with one another, usually end up believing a more extreme version of what they thought before they started to talk.
—Cass R. Sunstein, “The Power of Dissent,” Los Angeles Times, September 17, 2003
1984 (earliest)
One immutable law of Pentagon force planning is that the Joint Chiefs of Staff will generate requirements greater than the existing force, irrespective of the strategy in vogue at the time. It is the inevitable result of a cycle that a former officer calls ''incestuous amplification.''
—David Evans, “A runaway Pentagon,” The New York Times, October 03, 1984
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