A lower IQ increases a person's risk of developing a common form of dementia
, a study found on Wednesday.
The study published in the journal Neurology examined Scottish children who took a test of their mental ability in 1932 and found that those with lower IQs were more likely to develop vascular dementia decades later.
"In this case these people had the same backgrounds and exposure to environmental factors compared to their counterparts with higher IQs who did not get vascular dementia later in life," said John Starr, a geriatrician at the University of Edinburgh, who led the study.
"There is something about your mental ability that adds further to your risk of vascular dementia."
Vascular dementia, the second most common form of dementia after Alzheimer's disease, is often associated with stroke. High blood pressure and smoking are risk factors.
The team also looked at Alzheimer's but found no link between mental ability and that disease, suggesting there is no evidence for the idea that people with higher IQs have a "cognitive reserve" to draw on to delay onset of the condition.
The researchers do not know why IQ may lead to vascular dementia but Starr said mental ability could indicate how well the brain functions in doing things like controlling blood pressure.
"It may be that IQ is a measure of system integrity," Starr said in a telephone interview. "The IQ may reflect something about the brain and the brain's control over vascular disease later on."
The researchers studied 172 people with dementia who took the IQ test when they were 11 years old and compared them to people of the same age and gender of similar neighbourhoods and backgrounds.
To control the study even more closely, they compared the people with dementia to people whose fathers had similar occupations and found that overall those with vascular dementia were 40 percent more likely to have low test scores.