Depression is sometimes categorized as a mental, rather than a physical illness…[but d]epression does run in families, which could mean depression is genetic, an inherited condition. But families don’t just share genes – they also experience a similar environment. So how can we separate genetic influences on depression from environmental factors?
Analysis in 2005 of a large number of twin studies indicated that around 40% of the risk of developing depression is genetic, and the remaining 60% associated with non-shared environmental factors. It is therefore too simplistic to say…depression [is] either genetic or environmental.
Given that depression, like cancer, is not one single illness, it is perhaps not surprising that a single gene for depression has not been found.
…[The psychiatric genetics consortium group] concluded in 2015 that most of the genetic effects found so far…are not very reliable or robust. But this…just means we haven’t really done the right studies yet….
In the same way that we think about heart disease, cancer, obesity as having multiple “susceptibility genes”, the same is probably true for depression.
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