That Facebook Study

Let’s do a test of selective memory. Do you remember ‘that Facebook study’ from a few years ago? It was really creepy and ethically dubious right? Do you remember what the study was actually about? No?

Emotional Contagion

Well let me tell you. It was about ‘emotional contagion’. This is the (very old) theory that the spread of emotions in a social network (on or off-line) is essentially replicative, like the spread of a virus. This ‘epidemiological’ approach can be traced back to Gustav Le Bon’s 1896 work ‘Psychologie des Foules’ or ‘The Psychology of Crowds’. Le Bon’s work was motivated by the French elite who were becoming increasingly afraid of emotional contagion in rioting masses and its potential effects on social order. Le Bon believed that the spread of emotions in crowds could be seen like the spread of germs and that this effect deprived them of their capacity to act individually and rationally.

The emotional spread in the modern contagion model (see Hatfield et al, 1994) is thought to occur not directly, but through two steps:

Gustav Le Bon

1841 - 1931

The eighteenth century ruling french elite feared emotional contagion in rioting mobs.

1. The observer mimics the behaviour of the individual experiencing the emotion (not necessarily in its entirety, but e.g. by tensing one’s stomach in response to fear, screwing up one’s face in response to disgust etc.)

2. The mimicked behaviour causes the observer to experience the same emotion.

If your brain is immediately coming up with counter examples to this model, don’t worry, you aren’t alone (see the recent paper by Dezecache et al. [2015] for a discussion of its limitations). The model holds fairly well for things like disgust and fear or anger at an out-group (like the ruling French elite), but what about interpersonal emotions like envy? This doesn’t usually trigger envy in an onlooker, and certainly not in the person being envied ... The title of Ben Goldacre's semi-recent book 'I think you'll find it's a bit more complicated than that' comes to mind.

The Facebook Study

Moving onto the Facebook study itself, what were they actually trying to do? The study was titled ‘Experimental Evidence of Massive-Scale Emotional Contagion through Social Networks’. So they were trying to test this emotional contagion theory on positive emotions in the biggest data set ever, with all the power of technology at their fingertips. They classified posts as 'emotionally positive’ if they had at least one positive word in them and no negative, (I don’t blame your brain if it is again coming up with issues with this method, but it is at least in accordance with previous work). They then reduced the amount of these positive posts in some people’s news feeds and not others, selected randomly (you remembered that bit didn’t you?).

So what did they find? By reducing the frequency of these ‘positive’ posts in a person’s news feed they were able to decrease the amount of positive posts that person then produced themselves. This was good enough to prove the emotional contagion theory for these authors. To quote the paper: “The results show emotional contagion.”

Confounds - always confounds

But hold on there Core Data Science Team, Facebook, Inc., are you entirely sure you have thoroughly examined your reasoning process here? In psychology we like to think about ‘confounds’ when interpreting our findings. These are things which explain a finding other than what you are claiming is the explanation. So, is there anything that could explain this change other than ‘emotional contagion’? Well, I can think of a few. What if the original ‘positive’ post provides information of some event which affects the person who sees the post e.g. “Oh my God I am so Happy ­-Insert Popular Band- is coming to town!” which would make someone in the same town who likes said popular band but didn’t know they were coming more happy and more likely to post positive things, possibly about the same band. Or what if the post directly mentions other individuals e.g.“I can’t wait to see Jim, Bob and Frank this weekend, I hope you are ready for me, it’s going to be great fun!!!”. It would probably make Jim, Bob or indeed Frank quite happy to know their friend was looking forward to coming to see them.

In both of these cases positive emotions are spreading through the social network, but it has nothing to do with behavioural mimicry and isn’t behaving like the spread of germs. It is spreading through revelation of a mutually happy event in the first example, and social bonding in the second. The basic problem is that positive emotions aren't thrown out randomly on facebook, they are written for a reason, and that reason could well be the source of the increased happiness in the reader of the post. So while the Facebook study was able to show that positive emotions spread, it really wasn’t able to say anything about why and unfortunately emotional contagion is a ‘why’ theory. This is a good example of one of the problems with big data experiments. No data set in the world can make up for a flawed experimental method (not even if it’s REALLY BIG). And methodology tends to get more sloppy with really big samples. Here is a link to a nice article by Tim Harford, of BBC R4’s ‘More or Less’ on this topic.

Tiny effect sizes - always tiny effect sizes

Now for our second test of selective memory. If you were one of the people who did actually remember what the study was about, do you remember what was the difference between the groups who got the ‘less happy news feed’ and those in the control group? What would your guess be? 10% more positive posts? 20%? You can see the answer in the graph below from their paper. Looks pretty impressive huh? Now look at the scale. Yep. 0.1%. When positives posts were reduced in the person’s news feed, they produced, on average, 0.1% less positive words in their own posts. In the Psychology field we call this … a very, very, very small difference.

So, the next time someone asks you what you think about ‘that Facebook study’ you can reply “Yea, that was so dodgy! Emotional Contagion is an overly simplistic model, their method was confounded and anyway they only found a 0.1% change in positive post frequency”.


Dezecache, G., Jacob, P., & Grèzes, J. (2015). Emotional contagion: its scope and limits. Trends in Cognitive Sciences, (APRIL). doi:10.1016/j.tics.2015.03.011

Goldacre, B. (2014). I think you’ll find it’s a bit more complicated than that. Harper Collins.

Hatfield, E. et al. (1994). Emotional Contagion. Cambridge University Press.

Kramer, D. I., Guillory, J. E. & Hancock, J. T. (2014). Experimental evidence of massive scale emotional contagion through social networks. Proceedings of the National Academy of Sciences of the United States of America, 111(29), 1073. doi:10.1073/pnas.1412469111

Le Bon, G. (1896). Psychologie des Foules, Macmillan.