An animal that learns optimally may still be prone to inactivity when conditions are good, which is a symptom of depression


Evolutionary explanations for why people suffer from clinical depression tend to focus on the idea that depressive behaviour helps avoid costly effort when any benefits are unlikely. This might explain why people get depressed in hard or poor situations, but not why they don’t stop being depressed when things improve. The only explanations for this tend to focus on the idea that the brain is somehow malfunctioning, meaning that people take drugs that ‘correct’ brain chemistry. We explored whether a brain has to be malfunctioning to be inactive when conditions are good.


Our computer program finds the best decisions for an animal that lives in a world that is sometimes good and sometimes bad, but the animal has to learn which. The animal decides whether to invest in costly effort that may give a benefit, and will give the animal information about current conditions. We show that even if all individuals follow use a set of decisions that is perfect, some unlucky individuals happen to fail to learn and so don’t put in effort when they should. Individuals are more likely to make this mistake if they first experience good conditions for a long time.


Our results show that a major symptom of depression (being inactive in seemingly good conditions) can arise even if the brain is functioning perfectly. It can happen because a certain history leads the person to (subconsciously) learn that the world is bad and will remain so, when this is not the case. Our work suggests that the causes of depression can be further back in a person’s history than is currently appreciated. Effective treatments for depression may lie in trying to alter what the sufferer has learnt about the world, rather than directly manipulating brain chemistry.


Evolutionary medicine

Subject Group

Zoology and Ecology


optimal foraging





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on Fri Oct 27 2017

Article ID


Details of original research article:

Trimmer PC, Higginson AD, Fawcett TW, McNamara JM, Houston AI. Adaptive learning can result in a failure to profit from good conditions: Implications for understanding depression.Evolution Medicine and Public Health. 2015;123-135.

Preceded by:

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