(TELEGRAPH) — Over 600,000 Facebook users have taken part in a psychological experiment organised by the social media company, without their knowledge.
Facebook altered the tone of the users' news feed to highlight either positive or negative posts from their friends, which were seen on their news feed.
They then monitored the users' response, to see whether their friends' attitude had an impact on their own.
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"The results show emotional contagion," wrote a team of Facebook scientists, in a paper published by the PNAS journal - Proceedings of the National Academy of Scientists of the United States.