Facebook experiments with "emotion contagion" in social media. Which emotion triggers buying?

In this paper, facebook scientists reveal some of what they've learned while experimenting with "emotion contagion", a well known phenomena where positive or negative emotions are transferred to people that come in contact with the positive, or negative, person. At Facebook they tested this in the realm of social media, by messing with peoples feeds.

In an experiment with people who use Facebook, we test whether emotional contagion occurs outside of in-person interaction between individuals by reducing the amount of emotional content in the News Feed. When positive expressions were reduced, people produced fewer positive posts and more negative posts; 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. This work also suggests that, in contrast to prevailing assumptions, in-person interaction and nonverbal cues are not strictly necessary for emotional contagion, and that the observation of others’ positive experiences constitutes a positive experience for people.

Facebook manipulated the feeds of over 600,000 users to conduct this experiment. Facebooks users have unwittingly had their emotions secretly manipulated. Did you sign up for this? Yes you did, it's in the TOS, right under the line that says "zuck can do whatever he wants, nyah". In all seriousness, it's in the line that says you agree to let Facebook do whatever in conjunction with: “internal operations, including troubleshooting, data analysis, testing, research and service improvement.” You did read that terms of service, didn't you?

Adam Kramer is the data scientist who lead the research, and in his welcome to FB interview he says: Facebook data constitutes the largest field study in the history of the world. . Indeed. And this is human research on a large scale where the papers authors freely admit to manipulating users emotions. Isn't that swell?

So, while the ethics are a bit problematic here, as the randomly selected users never gave informed consent, this exposes another conundrum: is Facebook an entertainment service, or a information utility? Does it help you keep tabs with your actual friends real lives, or is it there so you can check out the latest fun news & be entertained?

So while you little lab rats post images of kittens, or depressing news stories about dead cats, we'll see how fast this information can be used by advertising. Which emotion triggers buying?