"I don't know" responses in elderly depressives and in dementia.

"I don't know" responses were recorded during administration of the Mini-Mental State Examination in elderly patients with primary degenerative dementia and primary major depression. More "I don't know" responses were given by patients with degenerative dementia; however, demented patients did not differ overall from depressives in the proportion of Not Correct responses consisting of "I don't know" responses. These responses were positively correlated with age and with Hollingshead education and social class scores in depressives but not in demented patients. A higher proportion of Not Correct responses consisting of "I don't know" responses needs to be demonstrated in depressive pseudomentia than in degenerative dementia if this clinical sign is to be used in helping to differentiate the two disorders.