A behavioral experiment is a technique often used in cognitive behavioral therapy for PTSD and other disorders. Following cognitive restructuring, a therapist may ask the patient to test out new beliefs or thoughts by going into situations that were once avoided.
For example, someone with PTSD may have the belief that going to the grocery store is dangerous and should be avoided. A therapist will help the patient challenge that belief and then test out a new, more accurate belief by going to a grocery store. By encountering something that was once feared and not experiencing any negative outcomes, the person will have even more evidence that their previous thoughts were not so accurate.