Hostile media perception (HMP) is a well-documented perceptual media effect. Yet, an understanding of its causal mechanisms has been difficult to ascertain. This study examined two promising, inter-disciplinary theoretical explanations – self-categorization and involvement. An experiment tested the effect of various facets of self-categorization and involvement on personal and perceived media positions regarding two issues that are salient to both political and religious identities – same-sex marriage and poverty. Political and religious self-categorization predicted personal positions on the issues, as well as perceptions of relevant media messages. Both HMP and message assimilation were related to self-categorization. Political self-categorization emerged as the strongest predictor of personal beliefs and media perceptions. Value involvement increased position extremity and message assimilation. Findings suggest particular usefulness of self-categorization in predicting HMP, while raising further questions about how readily HMP occurs, particularly among weaker partisans, and which audience or message factors predict message assimilation or contrast. Theoretical and methodological implications for future research are discussed.