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5 SAFETY BEHAVIOR AFTER EXTINCTION TRIGGERS A RETURN OF THREAT EXPECTANCY 127 METHOD Participants Participants were 45 student volunteers ( M age = 22.38, SD = 2.38; 36 women, 9 men). They gave written informed consent and received €7 or course credit for their cooperation. The study was approved by the Ethics Review Board of the Faculty of Social and Behavioural Sciences of Utrecht University. Apparatus and stimulus materials The task was programmed in Python (Python Software Foundation) and presented on a 19-inch monitor. The US was 1-s 100 dB white noise presented through headphones (cf. Leer & Engelhard, 2015) that were connected to the computer with a sound amplifier. CSs were a black square, triangle, and circle. Experimental task Each trial consisted of the presentation of a CS for 5 s, followed by a 5-s waiting period during which participants rated threat expectancy, immediately followed by either the US or no US. The inter-trial interval was 3 s. A picture of a plug was visible in the upper right corner of the screen during each CS presentation. The color of this plug indicated safety behavior availability. Safety behavior was unavailable to the participant if the plug was grey and available if the plug was green. During safety behavior available trials, participants could unplug the headphones from the sound amplifier, which prevented them from hearing the US. At the end of each safety behavior available trial, an instruction screen told participants to plug the headphones back into the sound amplifier. The design of the study is shown in Table 1. A, B, and C were randomly allocated to the different shapes for each participant. In the Pavlovian acquisition phase, A and C were followed by the US, and B was not. In the Safety behavior acquisition phase, safety behavior was available during presentation of A, B, and C. In this phase A, B, and C were also presented without