Neurodegeneration and Identity.

There is a widespread notion, both within the sciences and among the general public, that mental deterioration can rob individuals of their identity. Yet there have been no systematic investigations of what types of cognitive damage lead people to appear to no longer be themselves. We measured perceived identity change in patients with three kinds of neurodegenerative disease: frontotemporal dementia, Alzheimer's disease, and amyotrophic lateral sclerosis. Structural equation models revealed that injury to the moral faculty plays the primary role in identity discontinuity. Other cognitive deficits, including amnesia, have no measurable impact on identity persistence. Accordingly, frontotemporal dementia has the greatest effect on perceived identity, and amyotrophic lateral sclerosis has the least. We further demonstrated that perceived identity change fully mediates the impact of neurodegenerative disease on relationship deterioration between patient and caregiver. Our results mark a departure from theories that ground personal identity in memory, distinctiveness, dispositional emotion, or global mental function.