Post-Traumatic Stress Disorder (PTSD) is a complex psychiatric condition, the effects of which can be seriously debilitating. As it originates from a specific traumatic event, it often impacts soldiers and victims of violent crime. It is currently one of the most frequently litigated mental diseases. Neuroscience is slowly discovering the neural bases of PTSD and other psychiatric ailments and is building tests to distinguish actual patients from non-suffering individuals. Even extremely sensitive tests (> 95%) can have insufficient accuracy if the prevalence of a condition in the tested population is low. Therefore, the law ought to take into account the prevalence of PTSD and other psychiatric conditions when the decision whether to admit neuro-evidence in courts or not is made.