  Wikipedia is the largest online encyclopedia: its open contribution policy
allows everyone to edit and share their knowledge. A challenge of radical
openness is that it facilitates introducing biased contents or perspectives in
Wikipedia. Wikipedia relies on numerous external sources such as journal
articles, books, news media, and more. News media sources, in particular, take
up nearly third of all citations from Wikipedia. However, despite their
importance for providing up-to-date and factual contents, there is still a
limited understanding on which news media sources are cited from Wikipedia.
Relying on a large-scale open dataset of nearly 30M citations from English
Wikipedia, we find a moderate yet systematic liberal polarization in the
selection of news media sources. We also show that this effect is not mitigated
by controlling for news media factual reliability. Our results contribute to
Wikipedia's knowledge integrity agenda in suggesting that a systematic effort
would help to better map potential biases in Wikipedia and find means to
strengthen its neutral point of view policy.

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