A Facebook researcher has apologised after admitting he changed the news feeds of around 700,000 users.
Adam Kramer changed the way stories are shown on the News Feed as part of a social experiment.
His work led to a study published last month which showed that people who saw negative updates from their Facebook friends were more likely to write negative updates themselves.
In an update on his own Facebook page, the researcher says he's "very sorry" for not telling users their news feeds were being experimented with.
