What are some of the causes of unipolar depression?

In terms of biological causes of depression, family studies are used with the idea that depression runs in the family and you are more likely to develop it if a close relative has it. Gerson found that if close relatives had depression then they would be more likely to get it themselves, supporting the biological cause of depression. In terms of adoption studies, Wedler found support for adopted children and their biological parents to both have depression. Adopted children were 8x more likely to have depression if their biological parents did, suggesting genetic causes. Also, twin studies are used in support of the cause of depression. A concordance rate of 40% for MZ twins and only 11% for DZ twins suggests that depression is genetic. Bernstein supports this and claims that depression has a substantial inheritable component. However, it is likely that other alternative factors influence the cause of depression and twin studies are less reliable as twins are raised in the same or very similar environment suggesting a reason for the results. 
In terms of biochemical causes of depression the permissive amine theory shows how noradernaline, serotonin and dopamine are all neurotransmitters in the monoamine group. Depressed people have low levels of serotonin which affect the adequate control of the other neurotransmitters. This is supported by Teuting et al who compared urine samples in depressed and non-depressed patients and found depressed patients had lower levels of the by-product of the neurotransmitters, suggesting that low levels of serotonin cause depression. However, it is unknown whether that depression causes low serotonin or whether low serotonin causes depression. The use of antidepressants increase levels of serotonin and dopamine in patients and reduce the effects of depression, suggesting that these chemicals cause depression. However, although the antidepressants increase the levels of neurotransmitters very quickly in the brain, the symptoms of depression take much longer to go suggesting that there must be other factors involved causing the depression other than chemicals.