Mirrored-self misidentification is the delusional belief that one's reflection in the mirror is another person – typically a younger or second version of one's self, a stranger, or a relative. This delusion occurs most frequently in patients with dementia and an affected patient maintains the ability to recognize others' reflections in the mirror. It is caused by right hemisphere cranial dysfunction that results from traumatic brain injury, stroke, or general neurological illness. It is an example of a monothematic delusion, a condition in which all abnormal beliefs have one common theme, as opposed to a polythematic delusion, in which a variety of unrelated delusional beliefs exist. This delusion is also classified as one of the delusional misidentification syndromes (DMS). A patient with a DMS condition consistently misidentifies places, objects, persons, or events. DMS patients are not aware of their psychological condition, are resistant to correction and their conditions are associated with brain disease – particularly right hemisphere brain damage and dysfunction.

Delusional misidentification syndromes (DMS) can occur in patients with a wide variety of cranial dysfunctions. Mirrored-self misidentification, a type of DMS, occurs most typically in patients with dementia, especially Alzheimer's disease. Approximately 2% to 10% of all patients with Alzheimer's disease have mirrored-self misidentification. Patients with schizophrenia, right frontal ischemic stroke, and rarely patients with Parkinson's disease have also reported being affected by this delusion. The exact prevalence of patients with this delusion is relatively unknown because the typical patient has many comorbidities; this makes it difficult to separate the symptoms of mirrored-self misidentification from other existing psychological conditions. Furthermore, a standard neurological or neuropsychological workup tends to overlook the existence of this delusion because affected patients have extensive cognitive degeneration that is the main focus of medical attention. As such, it can be incumbent upon the patient's family to recognize symptoms of the delusion, mainly that the patient is unable to recognize him or herself in the mirror but has an intact ability to recognize the reflections of others.

All patients with mirrored-self misidentification have some type of right hemisphere dysfunction. The right hemisphere, particularly frontal right hemisphere circuits, is involved in processing self-related stimuli and helps one recognize a picture or reflection of oneself. An impairment in the right hemisphere, the likely source of the "self" in the brain, can inhibit one's ability to recognize faces, especially one's own.

Patients tend to experience a distortion of the right dorsolateral prefrontal cortex, which impairs the patient's belief evaluation system. Patients can no longer logically reject delusional beliefs. Injury to the right frontal lobe is found in 35% of cases and can also inhibit one's ability to reject delusional beliefs on the basis of implausibility. Patients with this delusion also tend to have larger right anterior horns than the typical person. These cranial distortions point to right frontal atrophy and general right hemisphere dysfunction.

While approximately 50% of all patients exhibit left hemisphere damage, all patients with this hemispheric dysfunction also have cranial lesions in the right hemisphere. As no patients with solely left hemisphere damage have been reported to experience this delusion, this points to the delusion being strongly associated with right hemisphere dysfunction.

In a few select case studies, patients with this delusion have such extensive right-hemisphere cranial damage that one is also unable to identify close relatives and others with whom the patient has close association. Such extensive damage in mirrored-self misidentification patients is rare; typically patients retain the ability to recognize others’ reflections in the mirror. When such extensive neurological damage occurs, the affected patient relies on non-facial cues to identify relatives. When looking in a mirror, the patient can only use facial cues to recognize one self. Therefore, in these rare cases, despite damage to the entire facial recognition area of the brain, the patient is still able to recognize relatives but unable to recognize the self in the mirror.

There are two famous case studies of patients with mirrored-self misidentification that have contributed to the overall understanding of this delusion.

Patient TH was in the early stages of dementia and was affected by mirror agnosia. While TH was looking into a mirror, the researcher held an object behind TH in such a way so it was reflected in the mirror. Due to his belief that mirrors represent a separate place, TH tried to reach into the mirror to retrieve the object rather than reaching over his shoulder. TH's mirror agnosia accounts for the development of the delusional idea. Because not all patients with mirror agnosia develop the delusion, there needs to be the presence of a second factor that explains why TH does not reject his delusional belief that the object is inside the mirror. Neuropsychological testing showed TH had an impaired right hemisphere. He displayed poor visual memory and visuoconstructional problems but still had basic visuoperceptual skills, a normal intelligence, typical vocabulary, and average semantic ability. These issues are indicative of significant right hemisphere dysfunction, specifically in the right dorsolateral prefrontal cortex. Because of such cranial impairments, TH was unable to use logic to reject his delusional beliefs based on implausibility. From this case study, researchers concluded that while not all patients with mirror agnosia develop mirrored-self misidentification, when mirror agnosia is paired with right hemisphere damage of the belief evaluation system, the delusion will develop.