Is there an evolutionary explanation for depression?

Medicine has difficulties in explaining why mental illnesses are not decreasing, especially in developed societies, despite improved health care and living conditions. Researchers with a focus on evolutionary biology therefore advocate not only researching the genesis of depression and schizophrenia on individual patients and their genes, but also taking the ancestral context into account. A leading exponent of this approach in Germany is the psychiatrist Martin Brüne from the Bochum University Clinic.

SZ:According to epidemiological studies, every second person will develop a mental disorder at least once in their life. Why are these diseases so persistent and so widespread?

Brüne: The question is explosive because, after all, evolution would have had enough time to eradicate depression through selective processes. I suspect that mental disorders are subject to the same evolutionary principles as physical illnesses. These arise in part from behaviors, such as eating habits, that were optimized for hunter-gatherer societies. There, looking for sweet and fatty foods was a good survival strategy. In modern affluent societies, however, this behavior leads to obesity and diabetes. These diseases exist because biological evolution is not as fast as human social and cultural development. Our phylogenetically acquired adaptations are no longer optimal for today's environments.

SZ: Do you think that our psyche is also more adapted to life in the primeval savannah?

Brüne: Yes, there are indications, for example, that the size of the social environment and the large number of contacts with strangers in modern societies overwhelm some people and put them under chronic stress, which ultimately also causes depression.

SZ:In epidemiological studies, however, there are hardly any indications that the number of mental illnesses has increased dramatically in the past few decades.

Brüne: This is about completely different temporal dimensions, namely the comparison of modern and Stone Age societies.

SZ:Are indigenous societies free from mental illness? Other authors point out that depression is also found among Ache Indians in Paraguay and Bushmen in South Africa.

Brüne: There are certainly people in every culture who meet our diagnostic criteria for mental disorders, the question is: how many? In addition, dealing with such diseases seems to be different. In tribal societies, for example, extreme grief reactions can often be found, which then quickly disappear - chronification is less common. Even in developing countries, schizophrenics have even better prognoses than ours. Incidentally, it does not contradict an evolutionary theory of psychiatry that people have always suffered from mental disorders - on the contrary; great apes probably already suffer from mental disorders.

SZ:Can chimpanzees have delusions?

Brüne: A very interesting question. I do believe, however, that apes lack the cognitive apparatus to do this. It is part of a delusion to make assumptions about what others think, feel or plan to do, so you need a so-called theory of mind. It is very controversial whether chimpanzees have these. So I don't think chimpanzees can hallucinate. However, it is undisputed that they can suffer from symptoms that resemble human depression, post-traumatic disorders and fears. They express themselves in severe stereotypes and social withdrawal.

SZ: Could this ubiquity of mental disorders even in non-human primates be an indication of hidden positive effects caused by them?

Brüne: In fact, some researchers believe that depression in particular can be understood as an evolutionary adaptation, that is, as a selection advantage: the constant brooding helps, for example, with decision-making in complex social situations; the sadness generates helpfulness in the social environment. I am very skeptical about this: this may be the case with very mild disorders, but with clinical depression the behaviors are so extreme that they cannot be considered helpful in any way for the person affected, they cannot be adaptive. Most evolutionary psychiatrists see mental disorders as extreme variants outside of adjustment - as statistical outliers. There is a continuum between healthy and sick behavior; if a threshold is exceeded, an experience is no longer helpful: Normal fear promotes survival, an anxiety disorder only paralyzes.

SZ:Popular authors also construct a connection between genius and madness, for example in the case of artists and scientists.

Brüne: Yes, and then cite the bipolar disorder of the composer Robert Schumann as evidence, but today we know that it was triggered by syphilis. These are anecdotes that don't prove much. However, there are indications that schizophrenia and manic-depressive illnesses occur more frequently in families with gifted and creative people; this is indicated by some studies from genetically closed populations such as in Iceland. But in these families it is not those affected by the disease who achieve a lot. Rather, these studies show that there can be different phenotypic developments with similar genetic makeup.

SZ:Give an exampleiel.

Brüne: In the case of the so-called serotonin transporter gene, for example, it has been shown that people with a certain variant of this gene have an increased risk of depression, suicidal and antisocial behavior. But if people with the same vulnerability gene grow up under particularly favorable environmental conditions, they can even become more empathetic and socially more successful than carriers of the normal variant! That could explain why these genes still exist despite evolution.

SZ:So an adjustment after all?

Brüne: Yes, but based on the genotype, not the phenotype, the appearance. Depression is not adaptive, but genes that predispose to depression under unfavorable conditions are perhaps adaptive because they cause a different phenotype under good conditions. This ultimately evolutionary explanation is an advance on the usual diathesis-stress model of psychiatry, which assumes that a person has a certain vulnerability - vulnerability - to an illness, which then breaks out under certain stressors. Evolutionary psychiatrists know that the same genes can, under certain environmental conditions, be favorable or unfavorable for the person who carries them.

SZ:What do evolutionary approaches bring for everyday clinical practice?

Brüne: Even the generally better understanding of the disease through the theory of evolution helps - for example, to the insight that we have to differentiate between men and women better, which current diagnostic catalogs do not sufficiently do. If, for example, the therapist knows that depressed men are often dismissive because it was not genetically meaningful to present oneself as a weakling

, then he doesn't have to search long for other causes.

SZ:Are there also differences between male and female schizophrenias?

Brüne: Yes. When mad, men usually feel persecuted by groups of other men: mafia, police, agents. Women, on the other hand, feel threatened by people from their personal environment, neighbors, family members. This is understandable in the evolutionary context, because this is how the early dangers looked: Men were threatened by groups of foreign men, women by violations from the ingroup. Or take the delusions of love, in which someone is firmly convinced that they are loved by another person. With women, the delusion is mostly directed towards high-ranking, somewhat older men. In men - where the disorder rarely occurs anyway - more attractive, younger women. This reflects the different reproductive strategies known from evolutionary psychology: women seek security for long-term brood care, men want to spread their genes among as many young and healthy women as possible.

SZ:If mild mental disorder symptoms are sometimes helpful, would it make sense to wait with therapy?

Brüne: Of course, it depends on the severity of the illness, but I also believe that one should not simply treat away every symptom without hesitation: Mild depressive symptoms can indeed be an indication that something is going wrong in their life and perhaps give the impetus to make important biographical decisions.

SZ:So shouldn't man always strive for happiness?

Brüne: Some researchers argue that there is no selective benefit in just being glad and happy all the time. Rather, it could be that a certain dissatisfaction drives people to take action and improve their situation.

SZ:So evolution condemns us to eternal dissatisfaction?

Brüne: Perhaps it is not just dissatisfaction per se, but man’s constant striving for something new.