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Machine learning analysis of 1.8 million headlines shows bias in the media has been getting worse.

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A new study analyzed 1.8 million news headlines from major US news outlets from 2014 to 2022. The findings show that news stories about domestic politics and social issues are becoming more polarized along ideological lines. The study was led by a professor of computer science and engineering from the University of Rochester and presented at the MEDIATE workshop of the International AAAI Conference on Web and Social Media.

Previous studies on media bias were limited in scope and used small sample sizes. This study used machine learning techniques to analyze a vast sample of headlines over an eight-year period across nine representative media outlets, including the New York Times, Bloomberg, CNN, NBC, the Wall Street Journal, Christian Science Monitor, the Federalist, Reason, and the Washington Times.

The researchers grouped the stories into four categories (domestic politics, economic issues, social issues, and foreign affairs) and analyzed how left, right, and central media outlets differed in the language they used in their headlines. They used a technique called multiple correspondence analysis to measure the fine-grained thematic discrepancies among headlines.

The team observed that US media outlets across the political spectrum were consistent and similar in covering economic issues. While they found discrepancies in reporting foreign affairs, they attributed that to diversity in individual journalistic styles. For example, the authors say the Wall Street Journal and Bloomberg primarily concentrate on the economic and financial implications of geopolitical tensions, resulting in differing perspectives compared to other media outlets. But headlines in the domestic politics and social issues categories showed important differences.

Hanjia Lyu, a computer science PhD student who was the lead author of the study, provided an excellent example of news bias:

“We observed a lot of subtle differences in the words they choose when they cover the same high-level topics. For example, when covering abortion issues, Reason tends to use the term ‘abortion law,’ while CNN underscores its ideological position by using the term ‘abortion rights.’ On a higher level they are both talking about abortion issues, but you can feel the subtle difference in the words that they choose.”

The research team hopes to dig deeper to better understand how and why media outlets use different words to cover the same kind of topics. They say understanding these discrepancies, and when they may indicate media bias, is important for both media outlets and readers alike.

Says Luo: “For consumers, it’s useful to know this information because the echo chamber effect is very strong and people are used to only listening to things they like to hear. Showing the divergence and the increased partisanship may make them aware that they need to be more conscious consumers of news.”

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