Taken together, our findings demonstrate some support for the “deficit-matching” hypothesis (e.g., Miller et al., 2005); that is, emotion regulation deficits predict better outcome because in- terventions can target these deficits. Low heart rate variability - an index of emotion dysregulation - predicted better outcome, which is consistent with neuroimaging studies of pre-treatment amyg- dala hyperactivation as a positive predictor of outcome (e.g., McClure et al., 2007). These findings suggest that behavioral treatments for anxiety serve to “correct” biologically-linked defi- cits in emotion dysregulation rather than enhance or capitalize on individuals' prior emotion regulation capacities (e.g., Rude &Rehm, 1991). In addition, the treatment-specific effects of behav- ioral avoidance suggest that this emotion regulation deficit is uniquely addressed in ACT. Though CBT purports to address behavioral avoidance of sensations through interoceptive expo- sure, it is possible that highly behaviorally avoidant individuals first need training in how to participate or engage in interoceptive exposures, such as through willingness and acceptance strategies in ACT.