A method and system is provided for determining a discrimination index in a subject that may be suffering from or at risk for a stress-induced psychiatric disorder. The discrimination index may be equal to a ratio of a subject's cued fear response and non-cued fear response measured during a fear-potentiated startle (FPS) paradigm. Such a value may allow a physician or researcher to quantify how well a subject discriminates between signaled (cued) fear and un-signaled (non-cued) fear, which may be a biomarker for psychiatric disorders like post-traumatic stress disorder, panic disorder, phobias, and/or generalized anxiety disorder. The determined discrimination index may provide a standardized way of diagnosing and evaluating mental illnesses, more uniform treatment of patients, and/or more precise monitoring and evaluation of treatment efficacy.