Stress broadly affects the ability to regulate emotions and may contribute to generalization of threat-related behaviors to harmless stimuli. Behavioral generalization also tends to increase over time as memory precision for recent events gives way to more gist-like representations. Thus, acute stress coupled with a delay in time from a negative experience may be a strong predictor of the transition from normal to generalized fear expression. Here, we investigated the effect of a single-episode acute stressor on generalization of aversive learning when stress is administered either immediately after an aversive learning event or following a delay. In a betweensubjects design, healthy adult volunteers underwent threat (fear) conditioning using a tone-conditioned stimulus paired with an electric shock to the wrist and another tone not paired with shock. Behavioral generalization was tested to a range of novel tones either on the same day (experiment 1) or 24 h later (experiment 2) and was preceded by either an acute stress induction or a control task. Anticipatory sympathetic arousal [i.e., skin conductance responses (SCRs)] and explicit measures of shock expectancy served as dependent measures. Stress administered shortly after threat conditioning did not affect behavioral generalization. In contrast, stress administered following a delay led to heightened arousal and increased generalization of SCRs and explicit measures of shock expectancy. These findings show that acute stress increases generalization of older but not recent threat memories and have clinical relevance to understanding overgeneralization characteristics of anxiety and stress-related disorders.stress | Pavlovian conditioning | associative learning | generalization | memory A number of anxiety and stress-related disorders can be characterized by an inability to discriminate threat from safety. For instance, a core symptom of trauma-and stressrelated disorders [e.g., posttraumatic stress disorder (PTSD)] is persistent and widespread fear and avoidance of myriad harmless cues that act as reminders of the trauma, often referred to as overgeneralization (1). A key component of overgeneralization in anxiety and stress-related disorders may hinge on stress-induced changes in neural circuitry underlying the ability to discriminate threat from safety and regulate emotional responses (2-4). Behavioral generalization also tends to increase over time, as memory precision for recent events gives way to more gist-like representations (5, 6). It then follows that stress-induced impairments in discriminating threat from safety coupled with a loss of memory precision might jointly influence the transition away from normal fear (i.e., highly specific to a known threat) toward overgeneralized fear. Here, we investigate how acute stress and the time between learning and test impacts fear generalization in humans.The effects of stress on the ability to regulate defensive behaviors have been detailed using Pavlovian threat (fear) conditioning tasks in humans and labo...