The LA Times has an interesting article on evolutionary theories of depression that also discusses how these might lead to new and improved treatments for the condition.
The fact that mental illness is both widespread and disabling is a puzzle in evolutionary terms, if you believe that a vulnerability to psychological disorder is strongly inherited.
Indeed, the evidence suggests that there is a significant inherited component in mental illness, although the extent of this influence is debated.
If this is the case, the question arises ‘why do we still have mental illness if inheriting the risk for it makes you much less likely to reproduce?’. Surely it should have been ‘bred out’ of the population?
Some use this as an argument to suggest that the role of genetics in mental illness has been overstated, and that the majority of risk arises from environmental factors, particularly those that cause stress and trauma.
Others suggest that the same inherited attributes that increase risk for mental illness can be beneficial when they don’t result in serious impairment.
For example, research has suggested that people who are at high risk for schizophrenia, or have slight or fleeting psychosis-like thoughts, are more likely to be creative or original thinkers [pdf].
This might explain why genes that increase these tendencies are still in the gene pool, and only when too many of these traits are inherited is the person very likely to suffer ill-effects when confronted by severe life stresses.
A similar theory was put forward by the late Dr David Horrobin is his book The Madness of Adam and Eve: How Schizophrenia Shaped Humanity (ISBN 0593046498).
As an aside, Horrobin was famously the subject of a controversy after a critical obituary was published in the British Medical Journal, leading to an angry reaction and the journal publishing an apology.
The LA Times article is a great overview of evolutionary theories of depression that might help answer questions about why someone might inherit a tendency to be depressed.
If this tendency is understood as an exaggerated form of something that might be beneficial in small doses, it may give clues to new treatments, and the article looks at what treatments researchers are considering with this in mind.
Link to LA Times article ‘The mind, as it evolves’.