Authoritarianism has been the subject of scientific inquiry for nearly a century, yet the vast majority of authoritarianism research has focused on right-wing authoritarianism. In the present studies, we investigate the nature, structure, and nomological network of left-wing authoritarianism (LWA), a construct famously known as “the Loch Ness Monster” of political psychology. We iteratively construct a measure and data-driven conceptualization of LWA across six samples (N= 7,258) and conduct quantitative tests of LWA’s relations with more than 60 authoritarianism-related variables. We find that LWA, right-wing authoritarianism, and social dominance orientation reflect a shared constellation of personality traits, cognitive features, beliefs, and motivational values that might be considered the “heart” of authoritarianism. Relative to right-wing authoritarians, left-wing authoritarians were lower in dogmatism and cognitive rigidity, higher in negative emotionality, and expressed stronger support for a political system with substantial centralized state control. Our results also indicate that LWA powerfully predicts behavioral aggression and is strongly correlated with participation in political violence. We conclude that a movement away from exclusively right-wing conceptualizations of authoritarianism may be required to illuminate authoritarianism’s central features, conceptual breadth, and psychological appeal.