People with depression often have problems thinking - something psychologists call cognitive deficit. However, it is unclear whether these cognitive deficits are due to depression itself or other mental-health problems that can go alongside depression. A team of Finnish researchers looked into this issue in a study of 197 people. 69 of them had depression with no other mental-health problems, 57 had other mental-health problems as well as depression and 71 were healthy controls. The study found no differences between the participants with depression and those who had depression and another mental-health problem. The depressed participants had 'mildly compromised verbal learning' but there were no other cognitive deficits among the participants. People who had received treatment for their depression had more impaired verbal memory and decision making abilities (executive functioning) while those who had become ill at a younger age also had impaired executive functioning.
Castaneda, A.E. ... [et al] - The effect of psychiatric co-morbidity on cognitive functioning in a population-based sample of depressed young adults Psychological Medicine January 2010, 40(1), 29-39