The head researcher of a controversial Facebook experiment has apologised, via the social network, after a global media outcry questioning the study’s ethics.

Adam Kramer of Facebook’s core data science team took to the social network to explain researchers’ point of view and apologise for any anxiety caused by the experiment.

The study, titled ‘Experimental evidence of massive-scale emotional contagion through social networks’ was published in scientific journal, Proceedings of the National Academy of Sciences (PNAS) in June.

Between 11 and 18 January 2012, the Facebook research used a program to manipulate the news feeds of 689,003 users to reduce the number of either negative or positive comments that appeared in a bid to determine ’emotional contagion’.

“I can understand why some people have concerns about it, and my coauthors and I are very sorry for the way the paper described the research and any anxiety it caused,” Kramer posted.

Debate around Facebook and the study’s ethical issues has surfaced among Facebook users and global media criticising the study, with some users threatening to leave the social network due to a loss of trust.

University of Maryland Professor of Law James Grimmelmann criticised the study in a blog post for failing to allow participants to give informed consent, and argued that its method was harmful.

“The unwitting participants in the Facebook study were told (seemingly by their friends) for a week either that the world was a dark and cheerless place or that it was a saccharine paradise. That’s psychological manipulation, even when it’s carried out automatically,” Grimmelmann wrote.

The paper points out that the research was consistent with the social network’s data use policy but many commentators have said that is irrelevant to Facebook ethical issues.

Cornell University, a partner in the Facebook research, issued a statement explaining that it determined an ethics review of the study was not required as its staff-member, co-author Professor Jeffrey Hancock, was “not directly engaged in human research”.

“The research was conducted independently by Facebook and Professor Hancock had access only to results – and not to any data at any time.”

Kramer’s Facebook apology said: “The actual impact on people in the experiment was the minimal amount to statistically detect it”.

“Nobody’s posts were ‘hidden’, they just didn’t show up on some loads of feed. Those posts were always visible on friends’ timelines, and could have shown up on subsequent news feed loads.”

Kramer admitted that the paper may have failed to make clear the study’s motivations: “The reason we did this research is because we care about the emotional impact of Facebook and the people that use our product. We felt that it was important to investigate the common worry that seeing friends post positive content leads to people feeling negative or left out.”

The study ‘s results contrasted with prevailing assumptions, finding that seeing lots of positive posts actually improved Facebook users’ moods rather than creating negativity through emotions such as jealousy (and vice versa).

“When positive expressions were reduced, people produced fewer positive posts and more negative posts; when negative expressions were reduced, the opposite pattern occurred,” it said.