Editorial Expression of Concern and CorrectionPSYCHOLOGICAL AND COGNITIVE SCIENCESPNAS is publishing an Editorial Expression of Concern re-garding the following article: “Experimental evidence of massive-scale emotional contagion t hrough social networks,”by Adam D. I.Kramer, Jamie E. Guillory, and Jeffrey T. Hancock, whichappeared in issue 24, June 17, 2014, of Proc Natl Acad SciUSA (111:8788– 8790; first published June 2, 2014; 10.1073/pnas.1320040111). This paper rep resents an important and emerg-ing area of social science research that needs to be approachedwith sensitivity and with vigilance regarding personal privacy issues. Questions have been raised about the principles of informedconsent and opportunity to opt out in connection with the re-search in this paper. The authors noted in their paper, “[Thework] was consistent with Facebook ’s Data Use Policy, to whichall users agree prior to creating an account on Facebook, con-stituting informed consent for this research. ”When the authorsprepared their paper for publication in PNAS, they stated that:“ Because this experiment was conducted by Facebook, Inc. forinternal purposes, the Cornell University IRB [Institutional Re-view Board] determined that the project did not fall under Cor-nell ’s Human Research Protection Program. ”This statement hassince been confirmed by Cornell University .Obtaining informed consent and allowing participants to optout are best practices in most instances under the US Departmentof Health and Human Services Policy for the Protection of HumanResearch Subjects (the “Common Rule” ). Adherence to the Com-mon Rule is PNAS policy, but as a private company Facebook wasunder no obligation to conform to the provisions of the CommonRule when it collected the data used by the authors, and theCommon Rule does not preclude their use of the data. Based onthe information provided by the authors, PNAS editors deemedit appropriate to publish the paper. It is nevertheless a matter ofconcern that the collection of the data by Facebook may haveinvolved practices that were not fully consistent with the prin-ciples of obtaining informed consent and allowing participantsto opt out.Inder M. VermaEditor-in-Chiefwww.pnas.org/cgi/doi/10.1073/pnas.1412469111PSYCHOLOGICAL AND COGNITIVE SCIENCESCorrection for “Experimental evidence of massive-scale emotionalcontagion through social networks, ”by Adam D. I. Kramer,Jamie E. Guillory, and Jeffrey T. Hancock, which appeared inissue 24, June 17, 2014, of Proc Natl Acad Sci USA(111:8788–8790; first published June 2, 2014; 10.1073/pnas.1320040111). The authors note that, “At the time of the study, the middleauthor, Jamie E. Guillory, was a graduate student at CornellUniversity under the tutelage of senior author Jeffrey T. Hancock,also of Cornell University (Guillory is now a postdoctoral fellowat Center for Tobacco Control Research and Education, Universityof California, San Francisco, CA 94143). ”The author and af-filiation lines have been updated to reflect the above changesand a present address footnote has been added. The online versionhas been corrected. The corrected author and affiliation lines appear below.Adam D. I. Kramera,1, Jamie E. Guilloryb,2,and Jeffrey T. Hancockb,caCore Data Science Team, Facebook, Inc., Menlo Park, CA 94025; andDepartments ofbCommunication andcInformation Science, CornellUniversity, Ithaca, NY 148531To whom correspondence should be addressed. Email: akramer@fb.com.2Present address: Center for Tobacco Contro l Research and Education, University ofCalifornia, San Francisco, CA 94143.www.pnas.org/cgi/doi/10.1073/pnas.1412583111www.pnas.org PNAS|July 22, 2014|vol. 111|no. 29|10779CORRECTION
Editorial Expression of Concern and CorrectionPSYCHOLOGICAL AND COGNITIVE SCIENCESPNAS is publishing an Editorial Expression of Concern re- garding the following article: “Experimental evidence of massive- scale emotional contagion t hrough social networks,”by Adam D. I. Kramer, Jamie E. Guillory, and Jeffrey T. Hancock, whichappeared in issue 24, June 17, 2014, of Proc Natl Acad Sci USA (111:8788–8790; first published June 2, 2014; 10.1073/ pnas.1320040111). This paper rep resents an important and emerg- ing area of social science research that needs to be approached with sensitivity and with vigilance regarding personal privacy issues. Questions have been raised about the principles of informedconsent and opportunity to opt out in connection with the re-search in this paper. The authors noted in their paper, “[The work] was consistent with Facebook ’s Data Use Policy, to which all users agree prior to creating an account on Facebook, con-stituting informed consent for this research. ”When the authors prepared their paper for publication in PNAS, they stated that: “ Because this experiment was conducted by Facebook, Inc. for internal purposes, the Cornell University IRB [Institutional Re-view Board] determined that the project did not fall under Cor- nell ’s Human Research Protection Program.”This statement has since been confirmed by Cornell University. Obtaining informed consent and allowing participants to optout are best practices in most instances under the US Department of Health and Human Services Policy for the Protection of Human Research Subjects (the “Common Rule”). Adherence to the Com- mon Rule is PNAS policy, but as a private company Facebook was under no obligation to conform to the provisions of the Common Rule when it collected the data used by the authors, and the Common Rule does not preclude their use of the data. Based onthe information provided by the authors, PNAS editors deemedit appropriate to publish the paper. It is nevertheless a matter of concern that the collection of the data by Facebook may have involved practices that were not fully consistent with the prin-ciples of obtaining informed consent and allowing participants to opt out. Inder M. Verma Editor-in-Chiefwww.pnas.org/cgi/doi/10.1073/pnas.1412469111PSYCHOLOGICAL AND COGNITIVE SCIENCESCorrection for “Experimental evidence of massive-scale emotional contagion through social networks, ”by Adam D. I. Kramer, Jamie E. Guillory, and Jeffrey T. Hancock, which appeared inissue 24, June 17, 2014, of Proc Natl Acad Sci USA(111:8788– 8790; first published June 2, 2014; 10.1073/pnas.1320040111). The authors note that, “At the time of the study, the middle author, Jamie E. Guillory, was a graduate student at CornellUniversity under the tutelage of senior author Jeffrey T. Hancock, also of Cornell University (Guillory is now a postdoctoral fellow at Center for Tobacco Control Research and Education, Universityof California, San Francisco, CA 94143). ”The author and af- filiation lines have been updated to reflect the above changes and a present address footnote has been added. The online version has been corrected.The corrected author and affiliation lines appear below. Adam D. I. Kramera,1, Jamie E. Guilloryb,2, and Jeffrey T. Hancockb,caCore Data Science Team, Facebook, Inc., Menlo Park, CA 94025; and Departments ofbCommunication andcInformation Science, Cornell University, Ithaca, NY 148531To whom correspondence should be addressed. Email: akramer@fb.com.2Present address: Center for Tobacco Contro l Research and Education, University of California, San Francisco, CA 94143. www.pnas.org/cgi/doi/10.1073/pnas.1412583111www.pnas.org PNAS|July 22, 2014|vol. 111|no. 29|10779CORRECTION
Experimental evidence of massive-scale emotional contagion through social networksAdam D. I. Kramera,1, Jamie E. Guilloryb,2, and Jeffrey T. Hancockb,caCore Data Science Team, Facebook, Inc., Menlo Park, CA 94025; and Departments ofbCommunication andcInformation Science, Cornell University, Ithaca, NY 14853 Edited by Susan T. Fiske, Princeton University, Princeton, NJ, and approved March 25, 2014 (received for review October 23, 2013)Emotional states can be transf erredtoothersviaemotional contagion, leading people to experience the same emotions without their awareness. Emotional contagion is well established in laboratory experiments, with people transferring positive andnegative emotions to others. Data from a large real-world socialnetwork, 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) BMJ337:a2338], al- though the results are controversial. In an experiment with peoplewho 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 positiveexpressions were reduced, people produced fewer positive posts and more negative posts; when negative expressions were re- duced, the opposite pattern occurred. These results indicate thatemotions expressed by others on Facebook influence our ownemotions, constituting experimental evidence for massive-scale contagion via social networks. This work also suggests that, in contrast to prevailing assumptions, in-person interaction and non-verbal cues are not strictly necessary for emotional contagion, andthat the observation of others ’positive experiences constitutes a positive experience for people.computer-mediated communication|social media|big dataEmotional states can be transferred to others via emotional contagion, leading them to experience the same emotions as those around them. Emotional contagion is well established inlaboratory experiments (1), in which people transfer positive andnegative moods and emotions to others. Similarly, data froma large, real-world social network collected over a 20-y periodsuggests that longer-lasting moods (e.g., depression, happiness)can be transferred through networks as well (2, 3). The interpretation of this network effect as contagion of mood has come under scrutiny due to the study ’s correlational nature, including concerns over misspecification of contextual variablesor failure to account for shared experiences (4, 5), raising im-portant questions regarding contagion processes in networks. Anexperimental approach can address this scrutiny directly; how-ever, methods used in controlled experiments have been criti-cized for examining emotions after social interactions. Interacting with a happy person is pleasant (and an unhappy person, un- pleasant). As such, contagion may result from experiencing aninteraction rather than exposure to a partner ’semotion.Prior studies have also failed to address whether nonverbal cues arenecessary for contagion to occur, or if verbal cues alone suffice.Evidence that positive and neg ative moods are correlated in networks (2, 3) suggests that this is possible, but the causalquestion of whether contagion processes occur for emotions inmassive social networks remains elusive in the absence of ex-perimental evidence. Further, others have suggested that inonline social networks, exposure to the happiness of othersmay actually be depressing to us, producing an “alone together” social comparison effect (6). Three studies have laid the groundwork for testing these pro- cesses via Facebook, the largest online social network. This research demonstrated that ( i) emotional contagion occurs via text-based computer-mediated communication (7); ( ii) contagion of psy- chological and physiological qualities has been suggested basedon correlational data for social networks generally (7, 8); and( iii) people’s emotional expressions on Facebook predict friends’ emotional expressions, even days later (7) (although some sharedexperiences may in fact last several days). To date, however, there is no experimental evidence that emotions or moods are contagious in the absence of direct interaction between experiencer and target. On Facebook, people frequently express emotions, which are later seen by their friends via Facebook ’s“News Feed”product (8). Because people ’s friends frequently produce much more content than one person can view, the News Feed filters posts, stories, and activities undertaken by friends. News Feed is theprimary manner by which people see content that friends share.Which content is shown or omitted in the News Feed is de-termined via a ranking algorithm that Facebook continuallydevelops and tests in the interest of showing viewers the contentthey will find most relevant and engaging. One such test isreported in this study: A test of whether posts with emotionalcontent are more engaging. The experiment manipulated the extent to which people ( N= 689,003) were exposed to emotional expressions in their NewsFeed. This tested whether exposure to emotions led people tochange their own posting behaviors, in particular whether ex-posure to emotional content led people to post content that wasconsistent with the exposure —thereby testing whether exposure to verbal affective expressions leads to similar verbal expressions,a form of emotional contagion. People who viewed Facebook inEnglish were qualified for selection into the experiment. Two parallel experiments were conducted for positive and negative emotion: One in which exposure to friends ’positive emotional content in their News Feed was reduced, and one in which ex-posure to negative emotional content in their News Feed wasreduced. In these conditions, when a person loaded their NewsFeed, posts that contained emotional content of the relevantemotional valence, each emotional post had between a 10% and SignificanceWe show, via a massive ( N=689,003) experiment on Facebook, that emotional states can be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness. We provide experimental evidencethat emotional contagion occurs without direct interaction be-tween people (exposure to a friend expressing an emotion is sufficient), and in the complete absence of nonverbal cues.Author contributions: A.D.I.K., J.E.G., and J.T.H. designed research; A.D.I.K. performed research; A.D.I.K. analyzed data; and A.D.I.K., J.E.G., and J.T.H. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission.Freely available online through the PNAS open access option.1To whom correspondence should be addressed. Email: akramer@fb.com.2Present address: Center for Tobacco Control Research and Education, University of California, San Francisco, CA 94143.8788 –8790|PNAS|June 17, 2014|vol. 111|no. 24 www.pnas.org/cgi/doi/10.1073/pnas.1320040111
90% chance (based on their User ID) of being omitted from their News Feed for that specific viewing. It is important to note that this content was always available by viewing a friend ’s con- tent directly by going to that friend ’s“wall”or“timeline,”rather than via the News Feed. Further, the omitted content may have appeared on prior or subsequent views of the News Feed. Fi-nally, the experiment did not affect any direct messages sent from one user to another. Posts were determined to be positive or negative if they con- tained at least one positive or negative word, as defined by Linguistic Inquiry and Word Count software (LIWC2007) (9) word counting system, which correlates with self-reported and physiological measures of well-being, and has been used in prior research on emotional expression (7, 8, 10). LIWC was adaptedto run on the Hadoop Map/Reduce system (11) and in the News Feed filtering system, such that no text was seen by the researchers. As such, it was consistent with Facebook ’s Data Use Policy, to which all users agree prior to creating an account on Facebook, constituting informed consent for this research. Bothexperiments had a control condition, in which a similar pro- portion of posts in their News Feed were omitted entirely at random (i.e., without respect to emotional content). Separatecontrol conditions were necessary as 22.4% of posts contained negative words, whereas 46.8% of posts contained positive words. So for a person for whom 10% of posts containing posi- tive content were omitted, an appropriate control would with- hold 10% of 46.8% (i.e., 4.68%) of posts at random, comparedwith omitting only 2.24% of the News Feed in the negativity- reduced control. The experiments took place for 1 wk (January 11 –18, 2012). Participants were randomly selected based on their User ID, resulting in a total of ∼155,000 participants per condition who posted at least one status update during the experimental period. For each experiment, two dependent variables were examined pertaining to emotionality expressed in people ’s own status updates: the percentage of all words produced by a given person that was either positive or negative during the experimental period (as in ref. 7). In total, over 3 million posts were analyzed,containing over 122 million words, 4 million of which were positive (3.6%) and 1.8 million negative (1.6%). If affective states are contagious via verbal expressions on Facebook (our operationalization of emotional contagion), peo- ple in the positivity-reduced condition should be less positive compared with their control, an d people in the negativity- reduced condition should be less negative. As a secondary mea- sure, we tested for cross-emotional contagion in which theopposite emotion should be inversely affected: People in the positivity-reduced condition should express increased negativity, whereas people in the negativity-reduced condition should ex-press increased positivity. Emotional expression was modeled, on a per-person basis, as the percentage of words produced by that person during the experimental period that were either positive or negative. Positivity and negativity were evaluated separately given evidence that they are not simply opposite ends of thesame spectrum (8, 10). Indeed, negative and positive word use scarcely correlated [ r=−0.04,t(620,587)=−38.01,P