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Does natural selection favour depression?

Depressed person stands under a bridge.

Does depression survive because it favours survival? We explore the hypothesis that the existence of depression today is explained by its adaptive value during evolution.

Man is an aggressive species but has developed strategies to ensure that aggression is kept in check. Such strategies include the development of social hierarchies.1 However, though dominant status is a driving force for survival, what happens once a former top dog is demoted?

 

Top dog or whipped pup?

 

John Price argued that for hierarchies to be stable, particular, strongly-selected behavioural patterns developed – irritability towards inferiors, anxiety towards superiors, elation on going up the hierarchy and depression on going down.2 Depression, in particular, would be advantageous as it prevents the demoted group member fighting back, thereby aiding their survival.

Depression, in the past, may have been selected for since it served as a survival mechanism following ousting from a higher social position. However, the environment in which we now exist does not generally involve such strict social structures. Thus depression may persist merely as an evolutionary remnant.

Rumination and anhedonia may reflect our predecessors’ need to solve problems while under stress

 

Sinking through thinking

 

There have been a number of more recent suggestions for the persistence of depression. Paul Andrews and Anderson Thomas have put forward the ‘analytical rumination hypothesis’. They argue that depression is an evolutionary adaptation for analysing complex problems and evolved as a stress response mechanism.3

In the past, to ensure sufficient time and processing resources for complex and challenging problems, a mechanism evolved to sustain concentration by reducing exposure to distraction. Thus, they suggest, the rumination and anhedonia associated with depression today may be an evolutionarily adaptive relic of our predecessors’ need to solve problems while under stress.

 

Mood monitors goal attainment

 

Reducing risk to the self and enhancing analytical skills are two potential and not mutually exclusive benefits that could explain the persistence of depression. At the root of the problem is mood, and how mood may sculpt behaviours to enhance fitness.4

Johnathan Rottenberg suggests that differences in mood arise from unconscious monitoring of progress towards the attainment of a goal and result in behaviours that favour accomplishment of that aim. Minor obstacles motivate further effort. But the response to major problems is a shutting down of activity and lowering of mood. After all, pursuing unattainable goals is non-adaptive, perhaps fatal, in survival terms.

Depressed people may be over committed to goals that are unachievable

Rottenberg also argues that depressed people don’t lie in bed because they are underachievers – but because they are over-committed to goals that are unachievable. If so, setting more realistic goals should be part of the therapy. Might rising rates of depression in the young be accounted for by the setting of increasingly unrealistic goals?

 

Neanderthal blues

 

A recent paper traced the presence of genes for depression triggered by disruption of circadian rhythms to our Neanderthal ancestors.5,6 Following our predecessors’ exodus from Africa, interbreeding with other archaic hominid species occurred to the extent that 1.5% of Western and Asiatic DNA is of Neanderthal origin.

Intriguingly, sub-Saharan Africans have not inherited any Neanderthal genome – it would be interesting to see whether this type of depression occurs in sub-Saharan Africans and, if so, what its origin might be.  

Our correspondent’s highlights from the symposium are meant as a fair representation of the scientific content presented. The views and opinions expressed on this page do not necessarily reflect those of Lundbeck.

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