The American study included families with two or more members diagnosed with definite, probable, or possible Alzheimer's disease. The 371 proband patients, individuals diagnosed with Alzheimer's disease and psychotic behaviors, had a total of 461 siblings who had also been diagnosed with Alzheimer's disease.

"There was a significant association between family members' psychosis and the occurrence of Alzheimer's disease plus psychosis of their siblings," said Robert A. Sweet, M.D., study author. "Among the siblings, the odds of exhibiting psychosis were more than double among siblings of probands who themselves exhibited psychosis."

Sibling age and patient age at onset of disease were considered as part of the study but were ruled out as factors in the development of psychosis.

The presence of psychotic symptoms in patients with the disease is important because such symptoms are also linked to more aggressive behavior, more rapid functional decline, and early institutionalization, said Sweet.

The finding of the combination of Alzheimer's disease and psychosis among siblings is an important first step in evaluating the genetics that may play a role, Sweet said. "There may be a set of genes that each contribute a modest risk to psychosis across neurodevelopmental and neurodegenerative conditions," he said.

The researchers did not rule out the possibility that environmental factors in the patients' early lives might make these patients more susceptible to psychosis.

Subjects were assessed through semi-structured interview questions, and some were rated on the Brief Psychiatric Rating Scale. Because validated behavioral rating scales were not used in all assessments, there may have been an overestimate of the frequency of psychosis, according to Sweet.