Why do so many contemporary humans suffer from a mental illness? The global, collective prevalence of chronic depression, Attention-Deficit Disorder (ADD), Obsessive-Compulsive Disorder (OCD), autism, and other disorders of the mind is today fairly high, which is very concerning, seeing as these types of conditions have a devastating impact on measures of health and quality of life.

Given that so many people in the world today don’t operate at peak capacity, mental health wise, it’s not surprising that a substantial amount of time, money, and resources are put into developing effective treatments for various psychiatric illnesses. In this very moment, researchers are hard at work trying to figure out which receptors and genes that play a critical role in the development of Alzheimer’s disease, what types of pharmaceutical compounds that are useful in the treatment of autism, and how gut bacteria may contribute to the development of Parkinson’s disease, among other things.

This type of research can certainly be useful, in the sense that it may help aid our progress towards developing therapeutic solutions to various mental health disorders. It has its limitations though. One of the things it won’t tell us is why mental disorders such as the ones mentioned in the first paragraph exist in the first place. In order to answer that question, we need to bring out our Darwinian toolkits, which can help us determine what the ultimate causes of mental illness is; an understanding that can potentially be turned into effective treatments for OCD, ADD, autism, and other similar conditions.

Evolutionary Mismatch: The Ultimate Cause of Mental Illness?

The fact that conditions such as autism and major depression exist is interesting, from a Darwinian point of view, considering that these conditions severely impair Darwinian fitness. This is particularly true if the affected organism lives in a natural environment. A severely depressed hunter-gatherer would obviously be at a disadvantage in the struggle for existence when compared with a healthy, thriving hunter-gatherer. Not only would the former likely be less motivated to optimize his food-seeking behavior, but his affliction would probably compromise his ability to develop tight social bonds and attract a healthy mate. In other words, he would less likely to survive and reproduce.

The fact that disorders such as major depression compromise organismal fitness begs the question: Why hasn’t natural selection eliminated – or at least severely reduced the frequency of – well-known risk alleles for major depression and other mental health conditions that compromise organismal fitness?

As I see it, there is only one good answer to this question, and that is that the relevant risk alleles only become problematic under conditions that differ from the conditions under which we evolved. Otherwise, they wouldn’t have been as common as they are. This idea is supported by a large body of research, including reports from explorers and researchers which indicate that hunter-gatherers and other non-westernized, traditional people tend to be healthy and vibrant;2, 4, 8 studies showing that the prevalence of many mental health conditions have increased as of late in countries that have become increasingly more westernized;4, 6 and a large body of evidence implicating diet and lifestyle practices in the pathogenesis of various mental illnesses.1, 3, 5, 6, 11

The Adaptive Value of Ancient Brain Features

Darwinian logic suggests that the bodily features that leave us susceptible to develop mental illness in a modern environment initially evolved because they serve an adaptive function. Let’s take chronic depression for example. Recent research has made it clear that inflammation is a fundamental cause of many cases of depression.1, 7, 9 Not only do depressed people tend to have elevated levels of proinflammatory cytokines, acute phase proteins, chemokines, and cellular adhesion molecules circulating in the bloodstream, but it has been shown that moody states can be induced by injections of proinflammatory cytokines.9

This moody response to inflammatory stimuli likely serves an adaptive purpose.9, 10 If a Paleolithic hunter-gatherer came into contact with a pathogen that caused his immune system to light up, it would likely be of benefit to him to seek isolation and rest, which is promoted by moody feelings. Not just because that would limit his risk of coming into contact with additional proinflammatory exposures, but also because it would conserve energy for his immune system and most likely aid recovery and repair processes. Not only that, but it would keep him for infecting closely genetically related individuals, which is relevant, as it has Darwinian implications.

The problem we have today is that our genomes operate in an environment that differs markedly from the type of milieu they were “designed” for. Whereas the immune insults our primal ancestors faced were acute in nature, many of the immune insults the modern man faces are of a chronic nature. These days, a lot of people eat a species-inappropriate diet, don’t exercise much, don’t get sufficient amounts of high-quality sleep, and harbor an imbalanced microbiota; hence, it’s not surprising that a lot of people are chronically inflamed and suffer from inflammation-related health problems such as depression.

Key Takeaways

Through Darwinian thinking, we can determine what the adaptive function of various human behaviors and brain responses are. This, in turn, can help us combat mental illness and construct a healthy human brain. Hopefully, this is something the conventional medical and psychiatric establishment will soon recognize.

Read the full Evolutionary Mismatch series:

  1. Introduction: Evolutionary Mismatch and What To Do About It by David Sloan Wilson
  2. Functional Frivolity: The Evolution and Development of the Human Brain Through Play by Aaron Blaisdell
  3. A Mother’s Mismatch: Why Cancer Has Deep Evolutionary Roots by Amy M. Boddy
  4. It’s Time To See the Light (Another Example of Evolutionary Mismatch) by Dan Pardi
  5. Generating Testable Hypotheses of Evolutionary Mismatch by Sudhindra Rao
  6. (Mis-) Communication in Medicine: A Preventive Way for Doctors to Preserve Effective Communication in Technologically-Evolved Healthcare Environments by Brent C. Pottenger
  7. The Darwinian Causes of Mental Illness by Eirik Garnas
  8. Is Cancer a Disease of Civilization? by Athena Aktipis
  9. The Potential Evolutionary Mismatches of Germicidal Ambient Lighting by Marcel Harmon
  10. Do We Sleep Better Than Our Ancestors? How Natural Selection and Modern Life Have Shaped Human Sleep by Charles Nunn and David Samson
  11. The Future of the Ancestral Health Movement by Hamilton M. Stapell
  12. Humans: Smart Enough to Create Processed Foods, Daft Enough to Eat Them by Ian Spreadbury
  13. Mismatch Between Our Biologically Evolved Educative Instincts and Culturally Evolved Schools by Peter Gray
  14. How to Eliminate Going to the Dentist by John Sorrentino
  15. Public Health and Evolutionary Mismatch: The Tragedy of Unnecessary Suffering and Death by George Diggs
  16. Is Shame a Bug or a Feature? An Applied Evolutionary Approach by Nando Pelusi
  17. The “Benefits,” Risks, and Costs of Routine Infant Circumcision by Stephanie Welch
  18. An Evolutionary Perspective on the Real Problem with Increased Screen Time by Glenn Geher
  19. Did Paleolithic People Suffer From Kidney Disease? by Lynda Frassetto
  20. The Physical Activity Mismatch: Can Evolutionary Perspectives Inform Exercise Recommendations? by James Steele


  1. M. Berk, L. J. Williams, F. N. Jacka, A. O’Neil, J. A. Pasco, S. Moylan, N. B. Allen, A. L. Stuart, A. C. Hayley, M. L. Byrne, and M. Maes, ‘So Depression Is an Inflammatory Disease, but Where Does the Inflammation Come From?’, BMC Med, 11 (2013), 200.
  2. Pedro Carrera-Bastos, Maelan Fontes-Villalba, James H O’Keefe, Staffan Lindeberg, and Loren Cordain, ‘The Western Diet and Lifestyle and Diseases of Civilization’, DovePress, 2011 (2011).
  3. M. Guszkowska, ‘[Effects of Exercise on Anxiety, Depression and Mood]’, Psychiatr Pol, 38 (2004), 611-20.
  4. Brandon H. Hidaka, ‘Depression as a Disease of Modernity: Explanations for Increasing Prevalence’, Journal of Affective Disorders, 140 (2012), 205-14.
  5. Shia T. Kent, Leslie A. McClure, William L. Crosson, Donna K. Arnett, Virginia G. Wadley, and Nalini Sathiakumar, ‘Effect of Sunlight Exposure on Cognitive Function among Depressed and Non-Depressed Participants: A Regards Cross-Sectional Study’, Environmental Health, 8 (2009), 34-34.
  6. Alan C. Logan, and Felice N. Jacka, ‘Nutritional Psychiatry Research: An Emerging Discipline and Its Intersection with Global Urbanization, Environmental Challenges and the Evolutionary Mismatch’, Journal of Physiological Anthropology, 33 (2014), 22-22.
  7. A. H. Miller, and C. L. Raison, ‘The Role of Inflammation in Depression: From Evolutionary Imperative to Modern Treatment Target’, Nat Rev Immunol, 16 (2016), 22-34.
  8. W. A. Price, Nutrition and Physical Degeneration 8edn (Price-Pottenger Nutrition Foundation, 2008).
  9. C. L. Raison, L. Capuron, and A. H. Miller, ‘Cytokines Sing the Blues: Inflammation and the Pathogenesis of Depression’, Trends Immunol, 27 (2006), 24-31.
  10. C. L. Raison, and A. H. Miller, ‘The Evolutionary Significance of Depression in Pathogen Host Defense (Pathos-D)’, Mol Psychiatry, 18 (2013), 15-37.
  11. E. M. Selhub, A. C. Logan, and A. C. Bested, ‘Fermented Foods, Microbiota, and Mental Health: Ancient Practice Meets Nutritional Psychiatry’, J Physiol Anthropol, 33 (2014), 2.

Header image by Eric Michael Johnson, from an original published in Vanity Fair (1891).

Published On: March 22, 2019

Eirik Garnas

Eirik Garnas

Eirik Garnas is the creator and owner of Darwinian-Medicine.com; a website dedicated to evolutionary nutrition and medicine. He’s a nutritionist (B.Sc. in Public Nutrition, M.Sc. in Clinical Nutrition), science writer, health coach, and personal trainer schooled at the Norwegian School of Sport Sciences. Throughout the years Eirik has been involved in the health and medical industry, he has worked with a variety of clients and written for many health and fitness magazines and websites.


  • M. Makuye says:

    We can note that some epigenetic introductions (certainly related to stress avoidance and diminution) cascade into significant maladaptation when pursued repetitively. Alcohol use is an outstanding maladaptive behavior in a great part of the world. Associations momentarily blocked, lead to more global and persistent failure of ability to associate, to use the evolved capacity of neocortical connectivity.
    Yet, massive societies persist and teach the use of this debilitating maladaptive exogenous toxin.
    Surely, social isolation from learned or genetic adaptive goals can lead to further maladaptations. In this time when such a high proportion of offspring have survived to compete for social niches, combined with our species’ social-status coalition formation, so many very likely completely functional individuals have been lost and suppressed, leading to persistent anxiety difficultiies and the more tragic maladaptations.
    It remains for individuals and formal psychological groups to distinguish between maladaptation of individuals and that of local and larger societies and cultures.

  • Jakob says:

    What about cultural evolution and multilevel selection?

    Maybe certain mental illnesses are not immediately selected against when a society reaches a certain level of complexity?

    Autism for example, heavily selected against in small scale hunter gatherer tribes so don’t reproduce and spread.

    When a societies reaches a certain level of complexity and devision of labour occurs indivuals who are socially impaired and would have been left to die in hunter gatherer times, can in complex societies because of devision of labour and pro social norms like monogamy more likely reproduce and produce the necessary resources to do so because of specialisation.

    Basically a place in society emerges for a person to reproduce who cant interact socially but is good at one particular thing, only because of social norms and division of labour.

    Also this makes sense when you consider autism as a spectrum disorder, the impairment is gradual.

    This also explains the levels of bullying such individuals have to endure usually, only when certain pro social norms exists which make it not OK to bully is it regulated.

    So in hunter gatherer tribes without such norms, indivuals with heavy social impairment would probably be severely bullied and completely left alone, which means slim chance of survival, none for reproduction.

  • ishi says:

    The ‘darwinian causes of mental illness’ are similar to the ‘darwinian causes of slavery’ . Thanks to this Dr. of Sports Medicine for pointing this out. Probably has a PhD in CTE 2. I think it well known that playing NFL football, getting a brain concussion, and practicing mindfulness meditation both increases your iq and your mental health—mental health is measure in units of $. The bigger your bank account, the more mentally healthy you are; though one can also use mindfulness mediation (more minutes you can stare at the floor, or write a redundant article , the mor mentally stable you are—some such as me call it mindlessness meditation–mentally healthy people are known for ‘seeing no evi, speaking no evil, and hearing no evil’ ).

  • Randy Nesse says:

    The question is spot on: Why didn’t natural selection provide better protection from mental disorders?
    My new book “Good Reasons for Bad Feelings: Insights from the Frontier of Evolutionary Medicine” is devoted to answering the question. http://goodreasons.info
    Mismatch is part of it, but only for substance abuse and eating disorders, the rest seem to be present everywhere.
    The same 6 kinds of explanations evolutionary medicine offers for diseases in general apply to mental disorders.

  • Bryan Atkins says:

    Nice work Mr. Garnas.
    Makes sense to me.

    Peeked at a few of the other mismatch articles.
    I’ve been banging on mismatch for years, mostly in a coding context.

    I submit that the dominant phenomenon of our era is exponentially accelerating complexity.
    Complexity increases weaken the efficacy of code, whether genetic, religious, legal, monetary, software, etc.
    I submit that the ongoing increases in complexity continue to create ever-greater mismatches with both our biological and cultural coding.
    Our coding is increasingly non-selectable.
    Verily, if your culture’s relationships with the sky and ocean are deadly, your cultural genome sucks. (Lethal mismatch)

    I further contend that most economists don’t understand code, including monetary code, in a physics, evolution and complexity context.
    Hint to Economists: Ya can’t do multilevel selection in-&-across Geo Eco Bio Cultural & Tech networks, and across Time, with world culture’s dominant app — humans deploying monetary code.
    App lacks information processing Reach Speed Accuracy Power and is often used to throttle Creativity — all of which are information-processing fundaments for passing selection tests.

  • An evolutionary background for mental illness is an interesting subject. A possible nature of self-consciousness can be part of it.
    If we look at what may have been the dawn of mankind in primate evolution, we can identify a specific human anxiety that came up with self-consciousness.
    The performance of identification with conspecifics that was amplified with the emergence of self-consciousness has probably been the source of a huge anxiety increase resulting from identifications with suffering or endangered conspecifics. And the tools developed by our ancestors to limit that anxiety (cooperation, imitation ToM, …) have also procured evolutionary advantages that have accelerated the evolution toward our human self-consciousness, while linking its nature to anxiety management.
    Much of that ancestral anxiety is probably still active in our today human minds where she conditions many of our mental states, including mental illnesses.
    Bottom line, that evolutionary sourced anxiety management has created an evolutionary engine that still links our human nature to unconscious anxiety management processes.
    The corresponding evolutionary scenario can be accessed at http://crmenant.free.fr/Neurex.Strasbourg.2018.Poster-C.Menant.pdf
    or in my home page http://crmenant.free.fr/Home-Page/index.HTM

  • I agree with Randy Nesse’s comments, and his book is good too. There are a lot of epigenetic factors that make us more vulnerable to mental health problems, including language, technology and assortative mating over many generations.

  • Catherine M. Rakow says:

    If the family as a natural system were considered, would there be a different understanding? All of the research focuses on the individual without the context of the family system.

    • James H. Amodio, LMHC says:

      See the works of Murray Bowen, MD, Michael Kerr, MD, et alia “Bowen Family Systems Theory”; “Family Evaluation” et alia.
      The thinking you suggest has been ongoing since the original study at NIMH in the fifties.

  • Peter van den Engel says:

    Very interesting/ but you are, like the world of chemical medicine as a whole, completely off track.
    These type of mental diseases might also have some genetic relation or with food or simple stupid physical exercise/ but they are predominantly for 80% created by the relation between the subject and his environment, including both stages of typical evolution. Which can be understood as timespace consciousness.

    It is not the biological evolution has not prepared us for these type of conditions/ but our current world condition never existed before, so it is adapting within its current state of affairs. Which is much broader than just the body.

    The cause for it in general terms is the rapid change in family conditions, in relation to the economy, which dissolves jobs much faster and hardens living conditions because of cost inflation and high levels of improductive bureaucracy, next to failing communication by mass media in general.
    The evironment itself; in terms of timespace; has become poisonous and certainly cannot be treated by chemical medicine (another job dissolving) or by believing our genes are fully responsible for this.
    What you need is linguistic understanding and definition, as well as overlapping communication about it (cognitive repair modes). So consciousness becomes parallel to behaviour again.

  • Kristopher says:

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  • jobie steppe says:

    Darwin’s survival of the fittest is alive and well. Imagine earth as a molten rock 4/5B years ago giving birth to todays 7.5B brains many who can imagine and see all this!? Imagine a time when modern man & Neanderthals mingled & one didn’t survive!? Imagine a hot molten rock producing the human brain!? Imagine hunters & gathers, caveman, war, disease, civilization and the few human brains that grasp atoms, mapped the human genomes/DNA & are now phenotyping (phenome & phenomics) to explain things like health, disease & evolutionary fitness, Darwin’s survival of the fittest.

    I.E., a very few in humanity take the Georgia Guide Stones to heart envisioning a spaceship earth where humanity is nothing more than virus who out of survival of the fittest created covid-19, which can easily be modified @ any moment to reduce earth’s population as they see fit, as in survival of the fittest!?

    It’s been noted Floy’s death has united blacks & whites in protest, namely by the young – – – many who do not wear protective mask. This article refers to humanity as having contracted all sorts of mental illnesses over vast periods of time; one being misanthropy, or perhaps the disappearance of Neanderthals, when mixing with modern man several hundred thousand years past!?

    This notion/reality could turn into fact faster than blinking of an eye by phenotyping and would give credibility to Darwin’s survival of the fittest!?

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