A 30-year-old woman is referred for evaluation after her therapist observed marked shifts in her behavior, voice, and mannerisms during sessions, with the patient sometimes unable to recall what she said minutes earlier. The patient reports finding unfamiliar clothes in her closet, discovering journal entries in handwriting she does not recognize as her own, and being told by coworkers about conversations she cannot remember having. She reports a history of severe and prolonged childhood abuse. She denies substance use, and her medical workup including EEG and brain MRI is unremarkable. Which of the following assessment findings is most essential for establishing a diagnosis of dissociative identity disorder?