A method for determining a personality profile of an online user is disclosed. Social speech content data associated with an online user is stored. A machine learning model is used to determine a first personality profile of the online user based at least in part on the social speech content data associated with the online user. A second personality profile of the online user is determined based on the social speech content data using a scientific personality model encoded in an ontology, wherein the ontology encodes statistical relationships between a plurality of words and a plurality of personality traits based on one or more scientific research studies. An ensemble model is applied to determine a third personality profile of the online user based at least in part on the first personality profile and the second personality profile.