It's mostly because the news came from TV writers, Brendan Nyhan writes for CJR: "Predictably, some of them resorted to 'he said,' 'she said' style coverage that failed to make clear just how extreme and scientifically discredited [Jenny] McCarthy’s views are." Nyhan praises two exceptions — Meredith Blake of the Los Angeles Times, and Bill Carter of the New York Times, who said McCarthy's anti-vaccination crusade was based on a "widely disproved theory."
July 22, 2013