we differentiated between opinion change and selective turnout as drivers of social influence bias. we analyzed changes in turnout (the likelihood of rating) and changes in positivity (the proportion of positive ratings) to identify variance explained by selection effects and opinion change respectively. this analysis ruled out selection effecs (differential turnout by different types of raters. such as positive or negative raters) and suggested that a combiantion of opinion change and greater turnout combined with a natural tendency to up-vote on the togeter created the herding effects we observed