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).