Facebook is in hot water following a report that it secretly manipulated the newsfeed of almost 700,000 users.

Unbeknownst to the world, Facebook data scientists, in collaboration with Cornell University and the University of California, ran an experiment in 2012 to test how emotions can be transmitted through social media. They did this by manipulating the newsfeed of 689,003 English-speaking Facebook users, so it would show low numbers of positive or negative posts, and observed how this influenced their posts.

The results of the study were published late last week and have since gone viral. They concluded that emotional states could be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness.

Reaction was negative and swift, with people predictably angry to find out Facebook had tweaked user feeds without permission. Critics questioned the ethics of the study; the researchers were criticised for not seeking consent; and the social networking giant was deemed creepy by angry users.

Adam Kramer, a Facebook employee and one of the authors of the study, apologised for the emotional contagion, saying, in hindsight, the research benefits of the paper may not have justified all the anxiety caused.

He defended the goal of the experiment, saying the social network felt that it was important to investigate the common worry that seeing friends post positive content leads to people feeling negative or left out. At the same time, he said Facebook was concerned that exposure to friends’ negativity might lead people to avoid visiting the site.

Facebook may have been concerned about users’ exposure to negativity, but it is unlikely it thought about how users would feel after discovering they were lab rats for the social network.

Either way, the study once again highlights Facebook’s unclear boundaries with its membership.