Emotional contagion through social networks

I have frequently noticed that simply reading the barrage of negative news in the daily New York Times about the myriad things in our world that aren't working can unconsciously tilt me into a more negative or depressed mood that requires active countermeasures. Kramer et al. now use the newsfeed of a social network to demonstrate and quantify such a phenomenon. (The collaboration of Facebook with researchers that used a random selection of 500,000 Facebook users as lab rats has drawn a storm of comment.. I'm posting this earlier than I planned, because now I'm watching the NBC evening news do a segment on the issue. And, here is Jaron Lanier weighing in on the debate.) The article's abstract:

Emotional states can be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness. Emotional contagion is well established in laboratory experiments, with people transferring positive and negative emotions to others. Data from a large real-world social network, collected over a 20-y period suggests that longer-lasting moods (e.g., depression, happiness) can be transferred through networks [Fowler JH, Christakis NA (2008) BMJ 337:a2338], although the results are controversial. 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.