Age is the single biggest risk factor for dementia, with the odds doubling about every five years after age 65. But many things influence those odds for a given individual. . . . Certain mental conditions, particularly depression and schizophrenia, have also been linked to dementia. But because depression can itself be a sign of cognitive decline, the causality has been a bit muddy. Earlier this year an analysis of data from New Zealand provided the most convincing evidence to date linking many kinds of mental illness with dementia. That study raises important questions about the reasons for this increased risk and what could be done to reduce it.
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The study did not examine biological, social or other reasons for the increased risk, but research on dementia points to several possible explanations. “There might be shared genetic risk factors,” suggests psychologist Leah Richmond-Rakerd of the University of Michigan, lead author of the study. Recent studies have found some overlap in genetic markers associated with Alzheimer's disease and those linked to bipolar disorder and to major depression. Long-term use of psychiatric medications could also be playing a role in dementia, but Richmond-Rakerd and her co-authors do not think it is a major contributor.