The second most powerful executive at the company, Sheryl Sandberg, says experiments were ‘poorly communicated’

Facebook’s second most powerful executive, Sheryl Sandberg, has apologised for the conduct of secret psychological tests on nearly 700,000 users in 2012, which prompted outrage from users and experts alike.

The experiment, revealed by a scientific paper published in the March issue of Proceedings of National Academy of Sciences, hid "a small percentage" of emotional words from peoples' news feeds, without their knowledge, to test what effect that had on the statuses or "likes" that they then posted or reacted to.

“This was part of ongoing research companies do to test different products, and that was what it was; it was poorly communicated,” said Sandberg, Facebook’s chief operating officer while in New Delhi. “And for that communication we apologise. We never meant to upset you.”

The statement by Sandberg, deputy to chief executive Mark Zuckerberg, is a marked climbdown from its insistence on Tuesday that the experiment was covered by its terms of service. The secret tests mean that the company faces an inquiry from the UK's information commissioner, while the publishers of the paper have said they will investigate whether any ethics breach took place. Psychological tests on human subjects have to have "informed consent" from participants - but independent researchers and Facebook have disagreed on whether its terms of service implicitly cover such use.

Facebook’s first public comment on the experiments came as the social network attempted to woo Indian advertisers as part of its efforts to tailor adverts to users outside of the US. The aim of the government-sponsored study was to see whether positive or negative words in messages would lead to positive or negative content in status updates.

The company's researchers decided after tweaking the content of peoples' "news feeds" that there was "emotional contagion" across the social network, by which people who saw one emotion being expressed would themselves express similar emotions.

“We take privacy and security at Facebook really seriously because that is something that allows people to share” opinions and emotions, said Sandberg.

The UK data protection watchdog, the Information Commissioner’s Office (ICO), is investigating the experiment to determine whether Facebook has infringed on UK law.

The social network also faced stern criticism from commentators and researchers, over is handling of the experiment which was not explained to users and therefore was performed without their permission, with some referring to it as "creepy", "evil", "terrifying" and "super disturbing”.

Facebook later added a “research” policy to its terms and conditions, four months after the experiment took place.

• If Facebook can tweak our emotions and make us vote, what else can it do?