The topic of moral judgment and social norms has been widely addressed to understand human behaviour. Since nowadays the majority of interpersonal interactions happen on online platforms, researchers need to examine the new social spaces constituted by social platforms. Social media can be in fact precious data sources: together with a sociological interpretation of the results, they can provide new insights into the understanding of the perception of social norms. This thesis investigates social norms in the context of Reddit: a social news aggregation, web content rating, and discussion website. The Reddit community of interest for the present study is r/AITA, a community dedicated to asking and providing feedbacks about social behaviour.