Facebook manipulated the news feeds of nearly 700,000 users in an experiment to see if the content posted in the feeds would have an effect on the users' future posts.

A study recently published in the Proceedings of the National Academy of Sciences suggests that emotional states can be transferred to others via a process known as "emotional contagion."

Emotional contagion is a well-studied phenomenon where people are able to transfer both positive and negative emotional states to others through social contact and networks.

For one week in January 2012, Facebook altered the news feeds of 689,003 people by reducing either the number of positive or negative emotional posts in their feed.

The participants in the study were randomly selected, and the company used a software program that analyzes language to determine if the posts that were being swept into the users’ feeds were positive or negative in tone.

After being exposed to the manipulated feeds, the software then analyzed the status updates of the users to determine the tone of their posts.

None of the researchers involved in the study saw any of analyzed texts.

The researchers – who are affiliated with Facebook, the University of California, San Francisco and Cornell University -- found that when positive posts were reduced in people's news feeds, the percentage of positive words in their subsequent status updates decreased.

Conversely, when negative posts were reduced in people's news feeds, the percentage of negative words in their subsequent posts decreased.

"These results suggest that the emotions expressed by friends, via online social networks, influence our own moods, constituting, to our knowledge, the first experimental evidence for massive-scale emotional contagion via social networks," the authors of the study conclude.

The study also observed a withdrawal effect, where people who were exposed to fewer emotional posts (either positive or negative) in their news feed were less expressive on Facebook overall on the following days.

The authors acknowledge that while the observed effect was small (d=0.001), "given the massive scale of social networks such as Facebook, even small effects can have large aggregated consequences."

They conclude: "Emotional states can be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness."

Study ethics questioned

The study was quickly slammed by critics and Facebook users who questioned the ethics of using account holders as subjects in an experiment without their knowledge.

While other studies have used Facebook data to examine emotional contagion, this is the first known study that manipulated the company’s algorithms for the purposes of research.

According to the researchers, the study was consistent with Facebook's Data Use policy, which all users must agree to before receiving a Facebook account.

Under the terms of the policy, data collected from the vast social network can be used by the company for "internal operations," including data analysis and research.

The study authors say this constitutes "informed consent" to participate in this experiment.

However, many people are shocked that the company purposely manipulated the user experience to test a hypothesis.

Here's a sampling of some of the online reaction:

In the wake of both the Snowden stuff and the Cuba twitter stuff, the Facebook "transmission of anger" experiment is terrifying. — Clay Johnson (@cjoh) June 28, 2014

Time to update my Internet-safety policies: Read Terms of Service, just in case they include organ harvesting next time. #Facebook — Katie Mack (@AstroKatie) June 29, 2014

The editor of the study, Prof. Susan Fiske, told The Atlantic that when she first read the research she had concerns about the ethics involved.

"People are supposed to be, under most circumstances, told that they're going to be participants in research and then agree to it and have the option not to agree to it without penalty," Fiske said.

However, when she spoke with the authors about it, they told her that their local institutional review boards – the committees that approve or deny studies involving human subjects – had approved the project.

She added that because Facebook is a private company, it doesn’t have to agree to the same ethical standards as federal agencies and universities.