The causes of<unk> -type major depressive disorder are believed to be mostly biological factors; some may have inherited the disorder from their parents. Sometimes stressful situations can trigger episodes of<unk> , though this is a contributing cause rather than a necessary or sufficient cause. People with psychotic symptoms are also thought to be more susceptible to this disorder. It is frequent in old age and often unnoticed by some physicians who perceive the symptoms to be a part of dementia. Major depressive disorder,<unk>  or otherwise, is a separate condition that can be comorbid with dementia in the elderly.
Melancholic depression
Collective depression
Masked depression
Double depression
Depression 
Psychotic depression
Retarded depression
Late life depression