For a week in 2012, researchers affiliated with Facebook, Cornell, and the University of California–San Francisco, used algorithms to manipulate the news feeds of almost 700,000 people in order to study “emotional contagion through social networks.”

They published the results of the study this month.

“We show… that emotional states can be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness,” the researchers wrote in the study’s abstract.

Using a program to analyze whether a Facebook post’s text contained positive or negative words, researchers provided the unknowing test subjects either with primarily neutral to happy information from their friends, or primarily neutral to sad information. All the subjects’ subsequent posts were then evaluated to see whether their emotional status changed.

“When positive expressions were reduced, people produced fewer positive posts and more negative posts,” the researchers wrote. “When negative expressions were reduced, the opposite pattern occurred. These results indicate that emotions expressed by others on Facebook influence our own emotions, constituting experimental evidence for massive-scale contagion via social networks.”

Understandably, the study has stoked much controversy. Even disregarding the ethical questions raised to intentionally make thousands of its users sad, Facebook may have actually violated the law. University of Maryland’s James Grimmelmann notes that federal law requires informed consent for federally funded studies, which users arguably never provided here. The study treated users’ acquiescence to the Facebook privacy policy as informed consent.

The researchers themselves appear to have some regrets. One of the study’s authors, Facebook data scientist, Adam Kramer, posted a long explanation / apology on his Facebook page.

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