A recently published study that manipulated Facebook News Feeds has sparked outrage among users who are criticizing the ethics behind the experiment, which was conducted by Facebook and several universities.

Researchers tweaked the feeds of 689,003 users to show a disproportionate number of positive or negative statuses for one week in January 2012. They found that the emotions of others on your News Feed can affect your mood, and published the results in the journal PNAS (Proceedings of the National Academy of Sciences). However, the researchers did not inform users that they were manipulating News Feeds, and many questioned the study's ethics.

Legally, Facebook is allowed to do this. As soon as users sign up for the social network, they agree to give up their data for analysis, testing and research. In this case, however, it's not the research people are criticizing — it's the manipulation of data without users' prior consent or knowledge.

Psychologists often follow the practice of obtaining "informed consent" from study participants before conducting experiments, according to The Atlantic. One could argue that Facebook's user agreement provides enough informed consent, but it doesn't mention data manipulation, and many are perturbed at the idea that the social network can mess with their News Feeds without their knowledge.

The study has troubling implications for Facebook's ability to manipulate the user experience for a variety of ends http://t.co/6RRw65aa4g — Greg McNeal (@GregoryMcNeal) June 28, 2014

May be time to start looking for an alternative to Facebook......what else can they manipulate? http://t.co/JpOaZwQbYK — Rob Cerroni (@realretroguy) June 29, 2014

Shame on @facebook for cynically misinterpreting informed consent. Shame on PNAS for publishing the study. http://t.co/3Q3SrnMdRI @PNASNews — Chris Patil (@DoNotGoGently) June 29, 2014

The American Psychological Association, a scientific organization that represents U.S. psychologists, has several definitions for "informed consent" in its code of conduct, depending on what kind of research professionals are doing.

Standard procedure is as follows: "When psychologists conduct research ... they obtain the informed consent of the individual or individuals using language that is reasonably understandable to that person."

Facebook's data-use policy runs nearly 10,000 words, but it does mention that user information can be studied.

However, the APA has a different definition of informed consent when studies involve trickery. When conducting "deceptive" experiments, psychologists must "explain any deception that is an integral feature of the design and conduct of an experiment to participants as early as is feasible."

The Facebook study does not mention telling the 689,003 users that they were, in fact, participants — even after the experiment was over.

For its part, the social network defended the study.

“None of the data used was associated with a specific person’s Facebook account,” a Facebook spokesperson told Forbes. “We do research to improve our services and to make the content people see on Facebook as relevant and engaging as possible. A big part of this is understanding how people respond to different types of content, whether it’s positive or negative in tone, news from friends, or information from pages they follow. We carefully consider what research we do, and have a strong internal review process."

But not everyone involved is as unconcerned.

Susan Fiske, a Princeton University psychology professor who edited the study for publication, told The Atlantic, "I'm still thinking about it and I'm a little creeped out." Facebook reportedly manipulates user's News Feeds all the time — something Fiske discovered when she asked the study's authors about the ethics behind what they had done.

That knowledge has some people worried about what other liberties Facebook has been taking with user data.