Ignoring the fact that you probably consent to be studied in the Twitter ToS (and perhaps fairly since nobody reads the ToS so I’m not sure it constitutes a contract in spirit), why is it okay for Twitter to run an experiment on their user-base in this manner but not for say a research group to do the same without some sort if ethics review? E.g. is Twitter screening the users they opt in to make sure they don’t include people especially susceptible to seeing negative sentiment who’s day might be ruined by the presence of new downvotes? Not trying to be hyperbolic just thinking of a rhetorical example. What if this introduces more stress into people’s lives and they don't want/can’t/shouldn’t be expected to tolerate it?