This paper explores the psychological origins of politically motivated reasoning and differences in openness to new information between liberals and conservatives. Decades of research in political psychology suggest that ideological differences in motivated reasoning are driven by needs for epistemic closure. This asymmetry hypothesis suggests that conservatives should be more politically closed-minded than liberals. An opposing theory argues that political bias emerges from political identity and predicts symmetry across ideology. In a series of national survey experiments we evaluate the extent of ideologically asymmetric politically-motivated reasoning using multiple issues and measures of political orientation, and provide what is to our knowledge the first direct test of the theorized mechanisms underpinning the asymmetry and symmetry hypotheses. Specifically, we compare the moderating role of personality traits to measures of identity and psychological investment in political attitudes. While we find substantial amounts of motivated reasoning, our results clearly indicate no ideological difference in openness. Further, we find that traits commonly associated with closed-mindedness have no consistent moderating effect on openness. Perhaps surprisingly, we find no consistent moderating role of investment in one’s opinions and identity. Our paper suggests that political scientists know less than we thought about the bases of political closed-mindedness.