Alzheimer's disease usually shows up late in life, but researchers said that signs may appear decades in advance.

One of the signs could be poor school performance, University of South Florida researchers said.

The researchers based their conclusion on a study of nuns. Those who had a smaller head size were more likely to have had dementia or showed signs of Alzheimer's during an autopsy.

The people with smaller heads -- and, thus, it was assumed, smaller brains -- also were more likely to have done poorly in school.

A news release said that previous studies have found that Alzheimer’s disease is related to head size, with people having smaller heads more likely to show symptoms.

"If brain damage related to Alzheimer's disease begins earlier in adult life, then having less reserve due to a smaller brain could compromise intellectual ability in those destined to get Alzheimer's and lead to them getting less education," said lead author James Mortimer.

Other studies have shown that those who develop Alzheimer's disease have problems with memory and thinking decades before the disease is diagnosed, the researcher said.

The Nun Study has been following 678 Catholic sisters since 1992.

Their research is published online this month in the journal Alzheimer's Disease and Associated Disorders.