Ever wonder why your Facebook feed got screwy? And maybe you noticed that “themes” sometimes seem to arise. There’s a reason for that. Facebook has been  conducting a secret massive psychology experiment on its users.

It was conducted to find out how Facebook users respond to positive and negative messages – without telling anyone. In the end, they involved over 600,000 of us in their "little study."

How could they get away with such a thing? Because all users – even you - have to tick a box agreeing to their terms and conditions. It’s covered under the guise of "internal operations, including troubleshooting, data analysis, testing, research and service improvement.” Yeah, we didn’t feel better either.

How’d they do it? Facebook altered the tone of the users' news feed to highlight either positive or negative posts from their friends, which were seen on their news feed.  They then monitored the users' response, to see whether their friends' attitude had an impact on their own.

"The results show emotional contagion," wrote a team of Facebook researchers, in a study with results published in PNAS Journal.

In other words, they spread positive, people were positive. When they spread negative, people got snarktastic.

As the white coats put it: “These results indicate that emotions expressed by others on Facebook influence our own emotions, constituting experimental evidence for massive-scale contagion via social networks."