For one week in January 2012, Facebook altered the news feeds of 689,003 users.
For one group, posts containing positive emotional words were reduced in frequency. For another, posts containing negative words were reduced. The researchers then measured the emotional content of what those users subsequently posted themselves.
The effect was small and it was there. Users who saw fewer positive posts produced fewer positive words and slightly more negative ones, and vice versa. The paper — by Adam Kramer of Facebook with Jamie Guillory and Jeffrey Hancock — was published in the Proceedings of the National Academy of Sciences on 17 June 2014, titled Experimental evidence of massive-scale emotional contagion through social networks.
The reaction was not about the effect size. It was about the fact that it had been done.
None of the 689,003 people had been asked. The paper's ethics statement asserted that the research was consistent with Facebook's Data Use Policy, to which all users agree on account creation. PNAS published an editorial expression of concern noting that the collection may not have conformed to the principles of obtaining informed consent, while declining to retract.
Two things are worth separating.
The finding itself is a demonstration of emotional contagion — the process Hatfield, Cacioppo and Rapson described, in which emotional states transfer between people — operating through text at scale. The mechanism is straightforward: feed composition sets the affective baseline against which a person evaluates their own circumstances, and mood-congruent processing means an established mood shapes what is subsequently noticed, recalled and expressed.
The more important point is not scientific. It is that the study is not evidence that a platform can affect a population's mood. It is evidence that doing so is a routine product decision.
Every change to a ranking algorithm is an experiment of this kind conducted without publication. The reason this one attracted attention is that it was written up. A/B testing of feed composition, at populations in the hundreds of thousands, is the ordinary practice of the industry, and the only unusual feature of the emotional contagion study is that the outcome measured was mood rather than engagement.
Which is the observation to carry into the next six chapters. The instruments described there are not scandals. They are the normal operation of systems that optimize something, and the something is chosen by a small number of people.
Cross-ref: Defense — Auditing Your Information Diet