There are no known biological causes that apply consistently to all cases of<unk> , which suggests diverse origin of the disorder. However, there are some indications that there is a genetic predisposition to<unk> : "The rate of depression in the families of people with<unk>  is as high as fifty percent for the early-onset form of the disorder". Other factors linked with<unk>  include stress, social isolation, and lack of social support. In a study using identical and fraternal twins, results indicated that there is a stronger likelihood of identical twins both having depression than fraternal twins. This provides support for the idea that<unk>  is in part caused by heredity means. Co-occurring conditions<unk>  often co-occurs with other mental disorders. A "double depression" is the occurrence of episodes of major depression in addition to<unk> . Switching between periods of<unk>  moods and periods of hypomanic moods is indicative of cyclothymia, which is a mild variant of bipolar disorder. "At least three-quarters of patients with<unk>  also have a chronic physical illness or another psychiatric disorder such as one of the anxiety disorders, cyclothymia, drug addiction, or alcoholism". Common co-occurring conditions include major depression (up to 75%), anxiety disorders (up to 50%), personality disorders (up to 40%), somatoform disorders (up to 45%) and substance use disorders (up to 50%). People with<unk>  have a higher-than-average chance of developing major depression. A 10-year follow-up study found that 95% of<unk>  patients had an episode of major depression. When an intense episode of depression occurs on top of<unk> , the state is called "double depression." Double depression==== Double depression occurs when a person experiences a major depressive episode on top of the already-existing condition of<unk> . It is difficult to treat, as sufferers accept these major depressive symptoms as a natural part of their personality or as a part of their life that is outside of their control. The fact that people with<unk>  may accept these worsening symptoms as inevitable can delay treatment. When and if such people seek out treatment, the treatment may not be very effective if only the symptoms of the major depression are addressed, but not the<unk>  symptoms. Patients with double depression tend to report significantly higher levels of hopelessness than is normal. This can be a useful symptom for mental health services providers to focus on when working with patients to treat the condition. Additionally, cognitive therapies can be effective for working with people with double depression in order to help change negative thinking patterns and give individuals a new way of seeing themselves and their environment. It has been suggested that the best way to prevent double depression is by treating the<unk> . A combination of antidepressants and cognitive therapies can be helpful in preventing major depressive symptoms from occurring. Additionally, exercise and good sleep hygiene (e.g., improving sleep patterns) are thought to have an additive effect on treating<unk>  symptoms and preventing them from worsening.
Dysthymia
Euthymia 
Cyclothymia
Myokymia
Congenital athymia
Minor depressive disorder
Atypical depression
Panic disorder