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8 DISCUSSION 193 Hout, & Hermans, 2013). Furthermore, in recent a study by Duits et al. (2017), patients with anxiety disorders more often showed impaired extinction learning than healthy controls. Notably, impaired extinction learning predicted poorer treatment outcomes. There are numerous individual difference factors that impact fear conditioning processes (see Lonsdorf et al., 2017), and that may affect avoidance learning (see Krypotos et al., 2015). Future research that investigates individual differences in the interference of safety behavior with extinction learning in humans is needed. Finally, the findings presented in chapter 3, in which cleaning safety behavior did not prevent the extinction of contamination, fear of contamination, danger, disgust, and contamination-related threat beliefs, suggests that it may be relevant to distinguish between stimuli that give rise to disgust, and stimuli that evoke fear (van Uijen, van den Hout, Klein Schiphorst, et al., 2017). In the case of fear, a stimulus (e.g., dizziness in panic disorder) activates the representation of a future catastrophe (e.g., fainting). A contaminating object (e.g., doorknobs in obsessive-compulsive disorder [OCD]), however, may not (only) activate the representation of a future catastrophe (e.g., getting contaminated and becoming ill), but may directly evoke disliking and motivate avoidance of the stimulus. Disgust can be conditioned (Borg, Bosman, Engelhard, Olatunji, & de Jong, 2016) and can be decreased by habituation (Mason & Richardson, 2012). Habituation is a non-associative mechanism of response reduction that plays a role in extinction in addition to inhibitory learning (Myers & Davis, 2007). It entails a reduction in responding to the stimulus, and can be achieved by repeated physical contact with the disgust-eliciting stimulus (Bosman, Borg, & de Jong, 2016). Safety behavior that prevents the occurrence of a feared future catastrophe, but allows confrontation with the disgust-eliciting stimulus (e.g., cleaning safety behavior) may allow habituation. This may explain the positive effects of cleaning safety behavior during exposure to a contaminant in the studies by Rachman, Shafran, Radomsky, and Zysk (2011), van den Hout, Engelhard, Toffolo, and van Uijen (2011), and van den Hout, Reininghaus, van der Stap, and Engelhard