Two facets of automaticity: motor automaticity opposes habit formation

Repeated practice produces both motor automaticity (action fluency) and inflexible habitual responding (non-deliberate actions). Although traditionally assumed to reflect a unified automaticity process, their relationship remains largely untested. Using a novel dual-task paradigm designed to jointly capture motor automaticity and habit formation, and three generalization datasets, we found a counterintuitive inverse relationship: individuals with greater motor automaticity showed reduced habit expression. Additionally, we provide a window into habit consolidation, showing that motor automaticity during habitual errors is preserved after extensive training but disrupted after limited training. Our findings dissociate execution from selection automaticity, challenging prevailing assumptions in neurocognitive accounts of learning. They revise our understanding of how practice shapes behavior, with critical implications for distinguishing adaptive skill from maladaptive rigidity in healthy and clinical populations.