Evidence from this can come from twin studies; MZ twins share 100% of their genes whereas DZ twins only have 50%. If genes are to be a facto in depression we would expect a higher number of MZ twins to share the disorder. One study based on nearly 200 pairs of twins found that when an MZ twin was diagnosed with unipolar disorder, there was a 46% chance that the other twin would receive a similar diagnosis. The figure for DZ twins was 20%. The study is credible with over 200 sample cases which make the findings relatively accurate and findings seem to prove that genetics to play a role in the disorder to an extent. However, there are number problems with these findings. Firstly; the link between genetics and the disorder is below 50% and so undermines the extent into the affect it has over the disorder. Secondly, the fact that DZ twins have a concordance rate not much less than MZ twins highlights the fact that psychological factors could be involved. Ultimately the twins share the same environment and so it is difficult for these findings to account for solely biological factors.
One way of eliminating environmental factors is adoption studies. Since they were raised apart from their biological relatives, similarities with those relatives would indicate genetic influences. Most studies of adoptees diagnosed with depression show that their biological relatives are more likely than their biological relatives compared with a rate of 20% whereas adoptive relatives have a rate of 5-10%. This suggests that biological factors are more important than psychological in the development of the condition.
One other biological explanation for depression can be biochemistry which argues that people do not inherit the condition as such. Nor do they directly inherit a tendency for depression or a risk of depression. Instead what they inherit are genes, and genes make proteins. Genetic differences could therefore present themselves as differences in biochemistry and thus influence depression. One example of biochemistry is neurotransmitters.
Evidence for this can be seen from antidepressant drugs. There are often affective in reducing symptoms of unipolar disorder. One group of antidepressants increases the levels of the neurotransmitters serotonin and noradrenalin. These findings suggest that Unipolar is, at least in part, caused by low levels of serotonin and noradrenalin. However, the findings of other studies have mixed results; some findings have suggested that low levels of either serotonin and noradrenalin can cause depression. However, there are implications to this explanation as it is difficult to identify whether the disorder led to the decrease in serotonin and noradrenalin or whether this was a bi-product of being depressed. Finally, there are individual differences, not everyone reacts in the same way to anti-depressant drugs.
Another example of biochemistry is hormones. This explanation is built around the basis that people who have unipolar disorder have high levels of the hormone cortisol. This hormone is produced in times of stress which is a characteristic often associated with the disorder. However, this explanation is limited in explaning biological causes. There is no evidence to suggest that cortisol causes stress and the development of unipolar depression or stress has caused the increased production of cortisol. If the latter is the case then the hormone explanation could be more down to environmental factors rather biochemical.
To conclude the above evidence seems indicate that biological factors influence depression. However, how and why they influence depression is far from clear. Although genes appear to be involved, the role of genetic factors in depression is enormously complex. We do not know which genes are involved fro which people. Furthermore we do not know what these genes do or how they are inherited. Similarly for biochemistry, we know very little about the way in which neurotransmitters like serotonin actually work. Ultimately we do not know that biochemical changes cause depression. Finally, the diathesis-stress model argues all psychological disorders are caused by diathesis (predisposition) and an environmental stressor. We may have the predisposition for depression. However, unless environmental stressors interact with the predisposition the condition will not be developed.
