" We conducted systematic literature review by looking across different research databases and a set of criteria to identify studies addressing this important question. We identified 34 studies all together that either measured depression as a baseline or the symptoms as a baseline. We also looked at the outcomes of the studies reported on. The outcomes included decline in cognitive state, including memory, concentration, decision making, and attention. All this function can decline with age. And to answer your question, what we found is, by combining the evidence from 34 studies and using the statistical tools, that if the person experiences depression across (on average) six years, there is a decrease in cognitive state and it’s a faster decrease than for a person without depression as this baseline. It’s not just the clinical diagnosis of depression, but also people with symptoms of depression, in which for both it represents a risk factor for faster cognitive decline. "