This study, conducted in the 1980s, helps to explain a lot of the heat and light that gets produced by those commenting on media bias across the political spectrum, including the remarkably vitriolic outpourings often seen in the comment sections of newspaper websites and across the internet.
hese groups watched the same news and came to opposite conclusions as to which way it was biased. And each side thought it was biased against their side.
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The study demonstrates what the authors call the 'hostile media phenomenon': people's tendency to view news coverage about which they hold strong beliefs as biased against their own position.
There were two mechanisms at work here:
1. The truth is black and white: partisans generally thought that the truth about the Arab-Israeli debate was black and white. Any hint of shades of grey in the news reports was interpreted by partisans as bias towards the other side. In other words: any balanced report will seem biased to partisan viewers.
2. The news report was too grey: as well as thinking the Arab-Israeli issue was either black or white, partisans also perceived that the specific news report they watched was too grey.
Put simply: when we care about an issue, we tend not to notice all the points we agree with, and focus on the ones we don't.
http://www.spring.org.uk/2010/02/why-the-media-seems-biased-when-you-care-about-the-issue.php