Partisan Echo Chambers of the Past: The Evolution and Electoral Drivers of U.S. Media Slant

This paper examines the historical evolution of media slant in U.S. local newspapers from 1878 to 1977, employing a novel dataset of newswire articles and employing modern textual analysis methods. Using a Bayesian approach, I quantify partisan media slant by comparing newspapers’ language to partisan phrases used by members of Congress. The findings reveal significant shifts in media slant over this time period, including both growing Democratic leanings among local media and an increasing polarization between Democratic and Republican newspapers. Utilizing a regression discontinuity design around close U.S. Congressional elections, I find that electoral outcomes cause newspapers to shift their slant toward the winning party, likely in an effort to align with an audience perceived to have moved in the winning party’s political direction. This research provides novel insights into historical media-politics dynamics, enhances methodologies for measuring ideological bias with textual data, and contributes to understanding feedback loops between media and electoral politics.