... MZ twins are more likely than DZ twins to both receive a diagnosis of depression. However it was 100% so it is also indicates environmental influences that can be seen as causing the onset of depression and it also fails to explain why the other percentage of MZ twins do not develop the disorder. The adoption studies looked at people with depression who had been adopted before the age of 4 and found that the biological parents were the most likely to share the same diagnosis as the adopted child. One biological theory came from the use of anti-depressant drugs, which helped with the symptoms of uni-polar depression (the emotional, physical, behavioural, cognitive, motivational symptoms) by increasing the levels of the neurotransmitters seratonin and nor-adrenaline. So it was supposed that depression was the result of lower levels of seratonin and nor-adrenaline levels, this theory was strengthened by the cognitive theory ...

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