A long term study of 392 elderly people in Sweden looked at how many of them developed psychosis and whether this was linked to them going on to develop dementia. The subjects of the study were assessed between the ages of 70 and 90. In the whole sample 4.8% developed psychosis but of those people who survived to 85 19.8% showed symptoms of psychosis. 64% of people with hallucinations developed dementia, compared to 30% of people with delusions and 25% of people with no psychotic symptoms.
Ostling, S., Palsson, S.P. and Skoog, I. - The incidence of first-onset psychotic symptoms and paranoid ideation in a representative population sample followed from age 70-90 years. Relation to mortality and later development of dementia International Journal of Geriatric Psychiatry June 2006, 22(6), 520-528