Newspapers and TV often report controversial risk warnings over technological innovations, scientific developments, or environmental hazards that have at their core a dispute between experts who contradict one another not only on preferred policy but also on the scientific facts that underlie decisions about public policy. Recent examples are the disputes about climate change, fracking, vaccination and autism, and genetically modified organisms (GMOs). Adversaries mobilize constituents with similar interests or ideologies, form opposing coalitions, and compete for media exposure. They...