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8 DISCUSSION 189 (safety behavior) after each instance of exposure, or (3) no exposure or safety behavior. Participants rated their threat belief associated with the contaminant before and after the experimental manipulation. Cleaning safety behavior, which may preclude the occurrence of contamination and illness (i.e., threat), did not prevent a decrease in threat beliefs about contamination and illness (van Uijen, van den Hout, Klein Schiphorst, et al., 2017). Together, the findings of chapter 2 and 3 suggest that the negative effects of safety behavior on exposure outcomes may not depend critically on whether safety behavior prevents the occurrence of threat: Safety behavior that allows threat occurrences may prevent extinction learning, and safety behavior that precludes threat occurrences can allow extinction learning. How can we explain these findings? It may be helpful to explain the finding that safety behavior that precluded the occurrence of threat and safety behavior that minimized the severity of threat prevented extinction learning (chapter 2, van Uijen, Dalmaijer, et al., 2017) in terms of learning theory (Blakey & Abramowitz, 2016; Treanor & Barry, 2017). Exposure therapy derives its positive effects from extinction (Vervliet, Craske, & Hermans, 2013), during which a conditional stimulus (CS) acquires a second, inhibitory association with an unconditional stimulus (US; i.e., CS – no US), which is then available alongside the first, excitatory association (i.e., CS – US; Bouton, 2002, 2004, 2016; Craske, Treanor, Conway, Zbozinek, & Vervliet, 2014). The extent to which inhibitory learning occurs depends on the discrepancy between the expected outcome (i.e., the feared catastrophe) and the actual outcome (i.e., the nonoccurrence of this catastrophe; Bouton, 2004). A larger violation of negative expectancies results in more inhibitory learning (Baker et al., 2010). When safety behavior reduces the perceived likelihood or severity of the expected catastrophe, it decreases this discrepancy, which may hamper inhibitory learning (Blakey & Abramowitz, 2016; Craske et al., 2014). First, safety behavior that precluded the occurrence of threat prevented extinction learning in chapter 2 (van Uijen, Dalmaijer, et al., 2017), and in the study