One common problem for those with depression is the inability to think clearly. However, the research shows that understanding this problem is complex. In fact, with certain types of tasks people with clinical depression exceed the abilities of those without depression.
How is that possible? It may be that people with depression are more greatly impacted by problems with motivation than with an inability to reason. In particular, it is possible that the depressive's pessimism and desire for control actually improves analytical reasoning and decision-making under certain conditions.
When researchers compared those with depression against those without depression on a decision-making task requiring an evaluation of job candidates they found that people with depression were better at selecting the more qualified candidates. It seems that those without depression tend to be more positively biased which leads to less realistic assessments and a tendency to more quickly accept less qualified candidates. The type of task used in this study is described as sequential decision-making (von Helversen et al., 2011).
So, in other words, people with depression are able to evaluate and make better decisions when the task requires more realistic analysis. This is consistent with the theory that people who don't tend to be depressed are more unrealistically optimistic.