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5 SAFETY BEHAVIOR AFTER EXTINCTION TRIGGERS A RETURN OF THREAT EXPECTANCY 123 INTRODUCTION Cognitive behavioral therapy (CBT), in particular, exposure-based therapy is the treatment of choice for anxiety disorders (Hofmann, Asnaani, Vonk, Sawyer, & Fang, 2012). Patients are exposed to the feared, but innocuous stimulus (e.g., going to the supermarket) to learn that the threat they expect (e.g., getting a heart attack) does not follow. In classical, or Pavlovian, conditioning terms, the feared, but innocuous stimulus is a conditional stimulus (CS), and the expected threat is an unconditional stimulus (US). Threat expectancy is used as an index of fear in fear conditioning research (Boddez, Baeyens, Luyten, Vansteenwegen, & Hermans, 2013). The decrease in fear (conditional response or CR) following repeated exposure to the CS is called extinction, a process that is widely used as a model for exposure therapy (e.g., Milad & Quirk, 2012). During extinction learning, the original C S - US relationship is not unlearned, but the CS acquires an additional inhibitory association with the US (i.e., CS - no US). This inhibitory association exists alongside the original excitatory association (Bouton, 2016). Thus, after fear extinction, the CS has an ambiguous status. The context in which the CS is presented determines whether or not it evokes fear (Bouton, 2002), which poses a continuous risk for a return of fear. Although exposure-based therapy is the treatment of choice, a substantial minority shows insufficient improvement from it (Barlow, Allen, & Choate, 2004) or relapses after initial recovery (e.g., Vervliet, Craske, & Hermans, 2013). Fear conditioning research has indicated several conditions that may trigger a return of fear after extinction, and which are likely involved in relapse (see Bouton, 2002; 2016; Vervliet et al., 2013). Extinguished fears can return with a change in the external context or person’s internal state (that is different from the extinction context; renewal), with the passage of time (spontaneous recovery), and following an unsignaled US (reinstatement). Investigating other conditions that may trigger a return of experimentally conditioned fear can improve our understanding of the processes underlying return of fear in clinical practice.