Roughly 65% of patients who developed psychosis after age 40 showed abnormal tau buildup on PET imaging, compared with about 15% of healthy older controls, according to a study published online in Molecular Psychiatry on August 2, 2026. Amyloid was also more common in the patient group: 35% tested positive, against 2% of controls. Tau and amyloid are the two proteins most closely tied to Alzheimer's disease and related dementias.
That matters because hallucinations and delusions appearing for the first time in midlife or later are usually handled as psychiatric problems with no identified biological cause. The findings suggest that in a substantial share of these patients, the symptoms sit on top of the same brain changes that drive neurodegenerative disease - which may be part of why two people with seemingly identical symptoms can follow very different clinical courses and respond differently to the same treatment.
The research team describes this as the first in vivo evidence that varied tau-related brain changes may be involved in late-onset psychosis. Earlier work pointing in this direction came from epidemiological studies and postmortem brain examinations; direct measurement in living patients had been scarce.
What the study does not establish is how often these protein changes go on to produce dementia, or whether identifying them would change how late-life psychosis is treated. For now, the practical takeaway is narrower but still significant: when psychosis begins late in life, a neurodegenerative process may be part of the picture, and imaging can reveal it while the patient is alive.