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Walter Cronkite was often cited as "the most trusted man in America" as he delivered the news on CBS in the 1960s and '70s—a time when fewer news options created a "shared reality" that scholars argue fostered civic engagement, empathy, and shared national identity. The situation looks quite different in today's disparate media landscape.

Much scholarship has focused on how the decline in a common baseline of facts has increased polarization and decreased trust in institutions, but less attention has been paid to whether—or in what manner—separate realities have become more common. Additionally, analyses have largely detailed online news, whereas television accounts for five times as much news consumption for average Americans.

A team from the University of Pennsylvania's Computational Social Science Lab (CSSLab)—a joint venture of the School of Engineering and Applied Science, Annenberg School for Communication, and Wharton School—has spent years analyzing bias in TV news produced between December 2012 and October 2022. They coded more than 13.4 million hard news and talk/opinion segments from three broadcast networks—ABC, CBS, and NBC—and three cable stations—CNN, Fox News, and MSNBC.

The findings show that while the broadcast channels continued to cover similar topics with similar language over the decade, cable stations increasingly diverge from each other and from broadcast news in the topics they cover and the language used. Meanwhile, "viewers of broadcast news receive largely interchangeable news regardless of which station they watched or when in time they watched it," the authors write. Their findings are in Scientific Reports.

"To our knowledge, this is the most comprehensive study to date of topical focus and polarization across major U.S. TV news networks," says senior author Duncan Watts, director of the CSSLab and a Penn Integrates Knowledge University Professor.

The percentage of U.S. adults turning to broadcast news as their primary news source fell from 35% in 2016 to 25% in 2023. "More and more people are getting different information, and a smaller group of people are getting similar information, which really will contribute to having a different understanding of the world around us," says first author Homa Hosseinmardi, a senior research associate in the CSSLab at the time of this research and a visiting scholar at the CSSLab.

The researchers calculated the proportion of airtime each station dedicated to 24 topics, grouped into socially polarizing issues (e.g., abortion and immigration), issues made salient by a specific event (e.g., vaccines and Russia), and issues of perennial relevance (e.g., health care and the economy).

While Fox News was the most distinct from broadcast news overall at the beginning and end of the 10-year period, MSNBC and CNN demonstrated greater increases in the difference in topic selection during this period, the researchers found. But while MSNBC and CNN diverged from broadcast news, the two stations grew more alike in topic selection and language.

The researchers found that Fox News gave greater airtime to the economy, taxes, immigration, and terrorism; MSNBC provided more coverage of abortion and ethnicity; and broadcast networks aired a higher proportion of segments on topics that are less overtly partisan, such as technology and education.

The researchers also measured polarization in TV news production, which they define as the average probability that a listener could correctly identify a station from a one- or two-word utterance. The findings show that polarization increased leading up to the 2016 and 2020 elections, and that MSNBC had the highest polarization during Donald Trump's first term, while Fox News had the highest polarization during the Biden Administration.

"News sources aren't fixed, they're not monoliths, and they do change over time," says co-author Sam Wolken, a joint Ph.D. student in communication and political science. "They are companies that are changing in response to audience behavior and economic conditions in the political environment, and so when we think about these outlets, they're always reinventing themselves and trying to meet the time, and our paper really draws that out."

Hosseinmardi and Wolken say that they are surprised to find that increased polarization in hard-news programming contributed more to the growing differences in cable news than opinion content, despite opinion shows hosting the most polarizing content overall.

This study builds on previous research from Watts and Hosseinmardi showing that TV news is a larger driver of echo chambers than the internet. Looking forward, Hosseinmardi says she is interested in applying these findings to issues other than news and politics—such as health topics—and in studying news from streaming services.