One important factor is that conscientiousness tends to grow with age, as people take on more responsibilities, Wilson said. "If in old age you have been unable to show that kind of growth, if people are unable to learn from experience, that might have implications for brain structure."
There could also be a physical mechanism related to the cardiovascular system, said Brent Roberts, a professor of psychology at the University of Illinois. "Conscientiousness deletes risk factors that cause premature death, such as smoking and drinking and driving," he said. "One possibility is that if you look at what doctors call cognitive decline, it is related to cardiovascular disease. It is completely speculative, but people who are less conscientious are more likely to smoke more and exercise less. But it could be something completely different, something we have yet to identify."
Understanding the relationship between conscientiousness and Alzheimer's risk may suggest novel strategies for delaying the symptoms of Alzheimer's disease, the journal report said.
In a related study, people who don't complete high school are more likely to develop dementia and Alzheimer's disease than those with more education, Finnish researchers report in the Oct. 2 issue of the journal Neurology.
The finding, from a study of almost 1,400 people, found that this association was independent of lifestyle factors such as occupation, income, physical activity and smoking.
The study participants were divided into three groups -- five or less years of education (low), six to eight years (medium), and nine or more years (high). The researchers tracked their outcomes through middle age and late life, for an average of 21 years.
Compared to people with a low education level, those with a medium level were 40 percent less likely to develop dementia, and people
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