[thisrod]

Experimenters have tell fibs in order to discover the truth about your behavior: the only way to prevent you from thinking of the word "elephant" is say the experiment is about cats. This is generally considered OK, provided that you won't be upset when you find out what was really going on.

And, without the benefit of hindsight, it is hard to see why people would get upset about this experiment. I reckon Facebook users are more upset by the truth, that their "personal" feelings are so influenced by trivial aspects of their environment, than by the way Facebook demonstrated it. The truth sometimes hurts, but science isn't to blame.

[nness]

That's not quite right; Even in circumstances where participants are mislead for other purposes, they are still informed that they are taking part in an experiment.

The issue here is that Facebook conducted behavioural experiments on participants whom were not informed that they were part of a study. It is unethical. Whilst the outcomes are tame for those involved, the shear number of those involved and Facebook's influence and presence in everyday life makes it all the more alarming that they attempted it in the first place.

[amirmc]

Those people generally get to decide for themselves if they even want to be part of the experiment. They also have the right to opt-out at any given time during the experiment. Neither of those happened here.