“Seen in the light of evolution, biology is, perhaps, intellectually the most satisfying and inspiring science. Without that light it becomes a pile of sundry facts, some of them interesting or curious but making no meaningful picture as a whole.”
Theodosius Dobzhanski
For most of history, nature was inexplicable. The flower, the bee’s stinger, birdsong, and the instincts that guide us through our days, seem arbitrary and curious until we view them through Darwin’s lens. Suddenly, their purpose is clarified. Once their evolutionary purpose is resolved, natural phenomena can be thoroughly interrogated. Over the past one hundred and fifty years, theorists have reconciled thousands of biological traits with natural selection. This reconciliation has even been successfully extended to neurological and psychological phenomena. However, the purposes of some of the most common mental conditions have completely eluded explanation.
In this book, I attempt to hold Darwin’s lens to forms of psychological illness, including the stress cascade, schizophrenia, intellectual disability, schizophrenia, and Alzheimer’s disease to bring them into clearer focus. I believe that doing so reveals that some of the most common psychological maladies are not maladies at all. They are alternate ways of being with their own valid perspectives on reality and their own place in humanity’s evolutionary story. Let us start by taking a look at human neurological diversity and how the way it is recognized by society is changing.
Neurodiversity: Different Ways of Thinking and Behaving
Neurodiversity is a term that refers to the natural diversity of human brains and the various ways in which they function. It recognizes that there is not one “normal” way of behaving, and that individuals with conditions such as Autism and ADHD should be valued for their unique abilities and perspectives. Neurodiversity is a form of biodiversity that includes neurological as well as psychological differences such as attention, learning, mood, and sociability.
The perceived costs of many such conditions have already been documented by clinical science. The intention here is to discuss the benefits and how modern social and occupational practices may cloud them. As with anything else, the outcomes of neurodivergent conditions depend on environmental context. In some scenarios a condition might be a detriment, in another it could offer a distinct advantage. This book will emphasize that some forms of neurodiversity may have absolutely flourished in the prehistoric past.
All healthy biological systems have diversity. In fact, variation is key to the survival of species. When a species shows great genetic variability it is thought to indicate multiple evolutionary pressures. The neurocognitive diversity found in humans indicates that we have adaptations to a variety of different environments. It can manifest in subtle ways that are unnoticeable or so pervasively that it is inseparable from the person’s sense of self. Moreover, the diversity can confer strengths and weaknesses that cannot be dissociated.
But it does involve acceptance and integration. Often people with mental disorders are told they need to conform to societal norms to be accepted. Placing rigid expectations on them to adopt a certain way of behaving can be damaging to their well-being. Moreover, requiring that people “fit in” is not only harmful to those who do not fit the mold, but also limits the potential of their contributions to society. By offering different viewpoints and different solutions to problems, neurodiversity is valuable to humanity as a whole. The multiformity of individual differences seen in human populations is a large part of our collective strength. However, it is important to point out that honoring neurodiversity does not mean rejecting medical treatment or turning down educational accommodations.
The neurodiversity movement challenges the idea that neurodevelopmental disorders are diseases that should be fixed. Instead, it advocates that,
- Disability arises when individual differences interact with societal barriers
- Neurodiverse people should not be expected to mask their traits to fit in
- Conceptions of autonomy and independent living should be broadened
- Diversity in thought and neurological makeup is highly valuable
Neurodiversity is not a medical term. It is not synonymous with the more scientific “neurological diversity” but in most of this book I will use it as if it were. I believe using the word in this way is appropriate because neurodiversity is concerned with finding what’s right with conditions that were previously thought of as the brain gone wrong. I will also use neurodiversity as an umbrella term. The term neurodiversity is traditionally associated with the following diagnoses: ADHD, autism, dyspraxia, dyslexia, dyscalculia, and Tourette’s syndrome. In other words, presently, most mental health diagnoses, especially those considered “diseases,” are excluded from the neurodiversity umbrella. But I believe all neurodevelopmental and neuropsychiatric conditions could be categorized as forms of neurodiversity. Not just those that have or may have had adaptive properties. I hope this book convinces you that we should broaden the classification of neurodiversity to include many other types beyond the half dozen diagnoses typically associated with it.
Neurotypical (not affected) individuals could also be included under the umbrella of neurodiversity. All humans are neurologically different in fundamental ways. Because we all sit at different points on numerous cognitive spectra, and we all have unique learning histories, every single human is a case of neurodiversity.
This book will avoid making subjective or political claims about neurodiversity. Instead, it will discuss the forms that it takes and try to understand how they may have been naturally selected. Nearly all psychiatric conditions have a genetic component, have been passed down through countless generations, are still widely shared in the general population, and will continue to be in the future. They are a natural part of humanity’s genetic legacy.
People with discrepant brain wiring pick and choose the content allowed into their cycle of thought in a different fashion from the rest of us. This is why they experience and analyze their environments differently. Neurodivergent thinkers are uniquely positioned to make contributions requiring innovative, out-of-the-box and creative thinking. Their differing perspectives can enrich our communities and workplaces. By providing support to those who think differently, we can create a more inclusive and progressive society. We owe it to ourselves to discover how best to play to their strengths and give them every opportunity to flourish.
This book will attempt to extol the importance of neurodiversity. But because of its scientific and evolutionary perspective, will also be forced to face some uncomfortable facts. Namely, some forms of neurodiversity seem to be coincident with disease and neuropathology, where a person’s brain and mind can be adversely affected. I will do my best to be scientific in the way I represent these conditions while being respectful to the people that must live with them every day.
There are many mental and neurological differences that may not have obvious compensatory advantages. Some may have never conferred any adaptive benefits (i.e., epilepsy, anencephaly, or hydrocephalus). However, each has a unique window on the world, unique challenges and opportunities, and there may be much to be learned from them. The fact is, every human brain is a fantastic information processing machine, unparalleled by any supercomputer. All mental health diagnoses have value, and contributions to make to the world at large.
Here are some of the commonly cited advantages of neurodiversity.
| Advantage | Condition | Description |
| Unique Perspectives | Various | Ability to see the world differently, offering fresh, creative, and innovative viewpoints |
| Persistence | Intellectual Disability | Overcoming the challenges associated with learning disabilities often builds resilience and determination |
| Non-Linear Thinking | Various | Ability to see the bigger picture and make connections between seemingly unrelated concepts |
| Pattern Recognition | Dyslexia and others | Enhanced abilities in identifying patterns and spatial relationships |
| Memory Skills | Autism, ADHD | Exceptional memory skills, for details, facts, emotions or feelings. |
| Honesty and Integrity | Intellectual Disability | A tendency towards directness and honesty can be valuable in fields where integrity is paramount |
Table 1.1. A list of commonly accepted advantages of neurodiverse conditions. These advantages help in personal, social, and occupational contexts.
Evolutionary Medicine: An Evolutionary View on Medical Disorders
Many traits recognized as disorders or diseases in modern times had adaptive qualities in our prehistoric past. They represent a type of mismatch between the environment our body is expecting and the artificial environment it finds itself within (Williams & Nesse, 1995). Much if not most preventable disease arises from a mismatch between our expected environment and the one we have manufactured in the last few thousand years. The growing field of “evolutionary medicine” attempts to identify and explain such mismatches. Research has shown that many “pathological” conditions have compensating benefits and over time the literature has come to accept a large proportion of these as adaptive responses.
The accomplishments of evolutionary medicine in explaining disease in terms of adaptation are extensive. Clinical states associated with adaptive properties include diabetes mellitus, diarrhea, fever, inflammation, obesity, sneezing, sickle cell anemia, and vomiting (Nesse, 1999). It is even thought that evolutionary perspectives on disease can elucidate what is going wrong within the body and ultimately inform how it can be treated (Gluckman, Beedle & Hanson, 2009).
Margie Profit is an evolutionary biologist with no formal biology training that formulated a well-received hypothesis in evolutionary medicine. She explained how morning sickness protects the fetus from unsafe or toxic foods. She carefully detailed how the nausea, vomiting, and food aversions that often accompany early pregnancy likely evolved to protect the developing fetus from toxins in the mother’s diet. She first proposed that expectant mothers more sensitive to certain foods do a better job of protecting their babies from the harsh chemicals contained within. Next, she substantiated this proposal with evidence showing that the taste aversions begin when the fetus is most susceptible (during fetal tissue differentiation), decrease as the growing fetus becomes less vulnerable, and cause avoidance of foods that contain substances known to interfere with fetal development (including many fruits and vegetables). Her novice, armchair hypothesizing led to real medical insight and better methods for preventing birth defects in the first trimester. Her effort and many examples like it demonstrate that work in evolutionary medicine can be tremendously informative and have real implications for understanding diseases, treating them, and creating policy.
The following is a list of medical issues and their purported adaptive functions. Disorders with evolutionary components work within physical constraints, and often involve functional compromises and tradeoffs. As with the features of any structure, adaptations molded by natural selection have costs and benefits and are subject to design compromises. For example, walking on two feet allowed us to carry food and babies but predisposes us to back injury and pain.
| Medical Conditions and Symptoms | Underlying Adaptive Function |
| Pain | Influences the animal to avoid further damage |
| Itching | Initiates scratching to remove external parasites like fleas, tics, lice, and mites |
| Aches | Causes immobility of muscles that need to heal |
| Calluses | Reduces the likelihood of a cut, break, or tear in the skin |
| Malaise | Encourages inactivity, rest, and healing, often when needed |
| Diarrhea | Flushes the digestive tract of pathogenic or infectious waste |
| Sneezing | Expels germs and unwanted particles from the throat and nose |
| Coughing | Clears germs, mucous, and dust, protecting the lungs |
| Allergies | Over-reactive immunological responses, meant to protect the body |
| Fever | Raises body temperature to deactivate or destroy pathogens |
| Nausea | Causes avoidance of foods that may be contaminated by bacteria or fungus |
| Vomiting | Helps to clear pathogenic substances from the digestive tract |
| Inflammation | Aids in healing and immunological processes unless over-reactive |
| Menopause | Women are much more likely to die of childbirth in older age, menopause reduces this risk and allows them to grandmother |
| Menstruation | When ovulation does not result in a healthy pregnancy, the womb gets rid of its lining, and any eggs or damaged embryos |
| Tanning | Melanin pigment proteins shade and protect underlying tissues from the sun |
| Myopia (nearsightedness) | Reading, schooling, close work, and time indoors cause asymmetrical growth in the eye interfering with long-range vision |
| Ulcers | High levels of stomach acids break down foods, destroy bacteria and viruses, and protect against gastrointestinal infections |
| Lactose Intolerance | Many humans lose their ability to break down milk sugars after weaning because this ability is unnecessary for adult foragers |
| Cavities | Increased frequency and duration of exposure to processed sugar leads to tooth damage |
| Diabetes mellitus | A tendency for elevated levels of blood sugars helps to protect animals from starvation |
| Osteoporosis | Without the heavy loads coming from the activity of foraging, our bones can become weak and brittle |
| Sickle Cell Anemia | Cells with a sickle shape are resistant to the parasite that causes malaria |
| Thalassemia | Offers protection against malaria and is particularly prevalent in regions where malaria was historically common |
| Cystic fibrosis | Numerous hypotheses have been advanced including protection against cholera, typhoid, and tuberculosis |
| Dental crowding, crookedness | Eating processed, refined, and cooked food has made our jaws weaker and smaller, leaving less room for our teeth to grow in straight |
| Natural Aging | Genes that promote health or reproduction early in life may cause susceptibility to aging later in life (antagonistic pleiotropy) |
| Cancer | All cells are programmed to divide, aging causes DNA damage that disinhibits cell division resulting in the formation of tumors |
| Thrombosis | Tendency toward blood clotting (hypercoagulability) may increase the risk of thrombosis in modern life but may have preventing death from bleeding |
| Hypertension (high blood pressure) | May be linked to survival advantages conferred by salt retention in some populations |
| Hyperthyroidism | An overactive thyroid gland, and high metabolic rate, might have been beneficial by increasing energy and cold tolerance |
People come in many forms. We have different blood types. These are alternative frequency-dependent morphs. The same is true of immune types. Different people have different immune systems optimized for solving different immunological challenges. The two sexes, females and males, are alternative though mutually reliant morphs. There are many such morphs and likely some that science is currently unaware of. Even personality disorders can be conceptualized as alternative behavioral strategies dependent on the environment and the frequency of other such strategies in the population. In the next section, we will start to consider various alternative mental strategies and their natural histories.
Evolutionary Psychopathology and Evolutionary Psychiatry
Evolutionary medicine has attempted to explain the origins of certain mental disorders. This growing field has been called evolutionary psychopathology, evolutionary psychiatry and Darwinian psychiatry. It is concerned with the application of evolutionary rationale to the understanding of psychological conditions (Nesse, 1999; Baron-Cohen, 1997; Brune, 2008; Panksepp, 2006; Cosmides & Tooby, 1992). Researchers in this field have concluded that there were probably multiple, alternative, cognitive strategies to deal with the problems that recurred in our evolutionary past (Bjorklund & Pellegrini, 2000; Hood & Jenkins, 2008). Furthermore, they emphasize that psychological and psychiatric issues may not always represent disease, but in fact represent naturally selected responses to pressing environmental concerns (Trevathan, 2007). These responses are unconscious and involuntary. Reinforcing this perspective, several “behavioral syndromes” have been discovered in mammalian species and are thought to represent adaptive responses to particular scenarios, despite the fact that they appear maladaptive when taken out of their ecological context [25].
Many articles in the last three decades have espoused this view and reconceptualized various forms of psychopathology as cognitive syndromes that have ecological utility [19]. There have been thoughtful treatments of disorders such as: anxiety, hypothesized to represent a careful, cautious strategy (Marks, 1994); depression, a socially submissive strategy (Allen & Badcock, 2006); psychopathy, a socially selfish and opportunistic strategy (Brune & Brune-Cohrs, 2006) and post-traumatic stress disorder (PTSD), a threat avoidant strategy (Bracha & Maser, 2008). Significant comparative evidence supports these four theories as the symptoms of anxiety, depression, psychopathy, and PTSD have been shown to exist in other animals, serving adaptive purposes (Hood & Jenkins, 2008; Reser, 2006). This suggests that many neurodiverse conditions may be coping mechanisms, tens to hundreds of millions of years old, that are hitting obstacles in our modern society.
Imagine the plight of a guppy born without the capacity for predator anxiety. It is going to get eaten. Despite the psychological pain associated with anxiety today, genetic susceptibility to it would have conferred specific adaptive benefits in prehistoric times. Certainly, a person who meets the criteria for an anxiety disorder takes this “cautious strategy” to an extreme; however, researchers speculate that such people may have achieved high reproductive success during highly volatile, violent, or unpredictable times (Nesse, 2007). Similarly, threat avoidance, self-subordination, and selfishness all represent important behavioral proclivities that have strong neurological foundations in a large variety of species. Of course, these behaviors also vary widely within species, meaning that some guppies of a particular species are fearful, some are fearless, and others fall somewhere in between.
Another good example of evolutionary psychopathology is phobias. Phobias can be helpful or pathological, we share them with other animal species, and some of the most common were surely naturally selected. Consider achluophobia or fear of the dark. It was beneficial for our ancestors to be wary of things that might have been invisible to them at night. Consider that large nocturnal felines with night vision hunted our ancestors for millions of years. The list below defines one dozen phobias and details what purpose each may have served. Most of these are instinctual fears and have a strong genetic component. Many of the phobias on the list affect most people to a least a small degree. Some of them help protect us even today.
| Phobia | Fear of | Function |
| Achluophobia | The dark | Avoidance of predators that could attack at night |
| Acrophobia | Heights | Avoidance of places where a fall is possible |
| Algophobia | Pain | Avoidance of situations that could cause injury |
| Arachnophobia | Spiders | Avoidance of venomous spiders |
| Autophobia | Being alone | Avoidance of exclusion or abandonment |
| Entomophobia | Insects | Avoidance of invertebrates that sting, poison, or parasitize |
| Hemophobia | Blood | Avoidance of situations that could create a laceration |
| Hypochondria | Illness | Avoidance of contagion or exposure to sickness |
| Necrophobia | Dead things | Avoidance of decomposing corpses that could cause sickness |
| Ophidiophobia | Snakes | Avoidance of snake bites and venom |
| Pyrophobia | Fire | Avoidance of fire and getting burned |
| Sociophobia | Social evaluation | Avoidance of social competition and status demotions |
Aside from fear, all emotions generally serve adaptive functions. The table below divides many human emotions into two categories, those linked to opportunities and those linked to threats. As you peruse the table, you might want to think about how these emotions guided our ancestor’s behavior in adaptive ways.
| Emotional Arousal | ||
| Opportunity | Threat | |
| Orientation | Approach | Withdrawal |
| Physical | Pleasure | Pain |
| Gain / Loss | Happiness | Sadness |
| Interest | Desire, seeking, curiosity | Fear, disgust, boredom |
| Mate | Love, lust | Grief, jealousy |
| Kin | Care | Loneliness |
| Allies | Camaraderie | Anger, Guilt, Envy |
| Status | Pride, dominance | Shame, submission |
Table 1.X This table of emotions helps categorize most human emotions into two groups, those associated with opportunity and those with threat. The emotions associated with threat tend to increase stress.
The adaptive value of the emotions listed above is pretty clear. But the value of many mental disorders is muddled by much more complexity. Schizophrenia is a great example of such complexity. In Chapter X we will discuss the “schizophrenia paradox.” It has been considered paradoxical because even though it seems to be unhelpful, many people around the world have it. The schizophrenia paradox is now known more generally as the paradox of common, harmful, heritable mental disorders. It was once expected that rare and new (de novo) mutations would be responsible for most brain disorders, but many alleles predisposing us to them are common in the population, making adaptive hypotheses more plausible (Huxley et. al., 1964). For now, let’s dive into further detail on depression.
Depression
The social competition hypothesis of depression sees depression as an involuntary strategy that communicates yielding and submission to dominant members of the group. It creates a subjective sense of incapacity and powerlessness resulting in the inhibition of competitiveness and aggression toward higher-ranked members. Imagine a monkey born without any instinct to act subordinate to larger, dominant monkeys. It is going to get put in its place and it could get killed in the process.
Depressive feelings cause humans, primates, mammals, and animals in general to accept a loss in rank (Price et al., 2007). Depressive behavior is extremely common across the animal kingdom in response to hierarchy conflict. When animals from crickets to chickens lose a fight, they are much more reluctant to fight again. The neurological circuits responsible for depression cause animals to submit and withdraw from social engagement. This can be highly adaptive because it convinces animals higher in the hierarchy not to attack, wound, or kill them. I know that behaving in a depressed manner has shielded me from the scrutiny, criticism, and abuse of peers. It has also helped me withdraw from my friend groups to focus more on reading, researching, and writing.
There are other views on depression. The “psychic pain” hypothesis interprets depression as the body’s way of keeping an animal from being fixated on an unreachable goal. Pessimism, discouragement, and lack of motivation may help us reassess our plans, influence us to escape a bad situation, or force us to stop wasting resources and energy when acting is futile (Nesse, 2000). The “cry for help” hypothesis points out that depression may encourage others to provide resources or assistance to the depressed person (Hagen, 2003) and there is some evidence to support this as well.
Depression is also common in humans and other animals during infection, disease, starvation, after childbirth (postpartum), and during the winter months. In these cases, it may beneficially reduce energy expenditure and risk-taking behavior. Bouts of depression are generally observed after people experience long-term stress, loneliness, trauma, grief, or romantic rejection and it is apparent that it may be a reasonably adaptive response to those situations as well.
Applying Natural Selection to Human Traits and Features
I have been fascinated by evolutionary explanations for human attributes since I was a child. I would notice something curious about myself and wonder why I share that propensity, ability, or body part with other people or animals. Then I would start to think about how the trait could have been naturally selected due to its beneficial properties. Of course, devising adaptive explanations is fraught with difficulty because many will be untestable or wrong.
While working on a minor in neuroscience, I took a course in evolutionary biology. During lectures, the instructor introduced us to hundreds of examples of adaptations. I realized that by wrapping my head around each example, I was refining my own ability to conjecture about evolution. The process helped me begin to understand how evolution works and how it doesn’t work. Soon I found myself reading and taking notes on all the academic literature on adaptations I could get my hands on. Thi helped me to refine and formulate some of my pet theories.
Most important for understanding evolution are the steps of natural selection as identified by Charles Darwin. Darwin asserted that natural selection could be explained through a series of four postulates, each integral to the process. So, before we continue discussing examples of adaptive evolution, please review this process in the table below.
| The Steps of Natural Selection | |
| Variation | There is variation within a population that can be inherited genetically (i.e., butterflies come in different colors) |
| Competition | Not all offspring can survive, and this leads to competition for survival (i.e., many butterflies get eaten by birds) |
| Adaptation | Individuals with beneficial adaptations are more likely to reproduce (i.e., certain colored butterflies have better camouflage) |
| Selection | Over many generations there is a change in the genetics of the population (i.e., the species gradually changes its color and evolves) |
Change in the peppered moth is one of the most well-known examples of evolution taking place in modern times. These moths were naturally selected to change their color in response to pollution during the industrial revolution. Air pollution from factories covered trees in soot. This soot camouflaged the dark moths but caused the light-colored moths to be more easily seen and eaten by birds. The population of moths shifted to a darker color for years. Later when the pollution was reduced, being a dark moth became maladaptive. This time, the light moths were selected because they blended better against the background of light-colored trees and came to predominate again. This was later called “industrial melanism” and was an early test of Darwin’s theory of natural selection in action.
Over time, natural selection is a powerful force, but it has many limitations. Natural selection is slow, it must work with constraints and compromises, and it favors successful reproduction rather than health or well-being. Also, new adaptations cannot make leaps. They can only amount to small incremental modifications, that must be immediately beneficial and must integrate flawlessly with the existing body plan. For example, our eyes did not appear overnight. Rather they started as small patches of photosensitive cells like the eye pits seen in some flatworms today. Our “camera-type” eyes are the result of miniscule changes happening over hundreds of millions of years. For the moths, the change was a simple increase or decrease in the production of the pigment melanin.
Next to the peppered moths, another canonical example of adaptive evolution is Darwin’s finches. Darwin encountered many different species of finch on the Galapagos islands. Most of these birds looked similar but had very different beak structures. After some observation, Darwin concluded that these various species all descended from a single finch ancestor but had developed beaks of different shapes and sizes because they helped the birds exploit different food resources. Beak specialization helped the animals gain access to different types of food. Some were specialized for nuts, and others for grubs, fruit, seeds, or insects. Looking at the image below, you can see how procuring a specific type of food with the wrong beak could have been problematic.
Figure 1.1. During his voyage to the new world, Charles Darwin noted the great variety of finches on the Galapagos islands. He realized that all the finches he saw must have derived from a recent common ancestor that then split into different species, each highly adapted to a different food source. He was right. Today the birds are known to have evolved from a single finch species that arrived on the islands just a few million years ago.
Now that we have some background, let’s jump right in and consider some common human tendencies and how they may have been naturally selected over time. The following table lists eight common behaviors and describes how, even though they may be destructive for modern people, they would have been constructive for our prehistoric ancestors. As you are reading, please try to imagine how these behaviors, that are normally thought of as negative today, might have benefitted someone living in a wild, natural environment.
| Common Behaviors | Implications for Modern People | Implications for Hunter-Gatherers |
| Addictiveness | Tendency to become addicted to drugs, alcohol, gambling etc. | Tendency to repeat behaviors that have positive outcomes |
| Aggression | Negative interactions leading to social isolation | Healthy self-promotion and protection |
| Chronic Stress | Unhealthy bodily effects. Unnecessary psychological discomfort and woe | Cautious of danger. Motivation to struggle, fight, and survive |
| Defensiveness | Unnecessarily high concern or fear of others’ intentions | Self-protection and healthy suspicion of others |
| Dominating Behavior | Putting others down and making them resentful or angry | Drive to attain food, mates, and prestige |
| Fear | Exaggerated fear responses to nonfatal threats | Enhanced awareness of potentially fatal threats |
| Impulsivity | Diminished capacity for patience, discipline, and goal setting | Drive to quickly attain food and other resources |
| Submissive Behavior | Superfluous tension, passivity, servility, and humility | Pacifying those who might harm you and de-escalating conflict |
Table 1.2 Some behaviors that were adaptive in our ancestral past are now maladaptive.
While working on a psychology major, I had to take courses in abnormal and clinical psychology, the study of mental disorders. It wasn’t long before my fascination with this area converged with my interest in evolutionary biology. I began to wonder if some psychological disorders are functional specializations like the beaks on Darwin’s finches. Before moving forward, let’s point out one major difference. With the finches, each species had its own unique beak type. This variation between related species is called interspecific variation. But all the mental variation between humans is found within a single species, homo sapiens. This variation within a species is called intraspecific variation. With its focus on neurodiversity, this book is mostly concerned with variation within humans. As you read over Table 1.2 you may have recognized that humans have a lot of intraspecific variation in the behaviors listed.
Just as most animal species are visually distinguishable from one another, every animal species has a different brain structure. This leads animals from distinct species to respond with unique behavior even when placed in the same conditions. Each species’ brain is fine-tuned to deal with the environment their bodies are born expecting. But for most animals that environment can be highly variable and when it is, you can expect intraspecific variation in brain structure. This is when a single species demonstrates different brain types or behavioral variants. Usually when a species does this, it is because those brain types are adapted to different niches. Every chapter in this book will elaborate on this concept.
The next table lists common mental disorders and the purpose they might have served in prehistory. These are hypotheses that have been discussed for decades now in the academic literature. Although there is much controversy in this research, the table includes what I personally believe to be the most compelling and realistic explanations. For the most part, much of the table corresponds to the general consensus in the field, but there are many experts that would disagree.
| Psychological Disorders | Rate | Negative Implications for Modern People | Positive Implications for Hunter-Gatherers |
| ADD / ADHD | 4% | Difficulty focusing attention on and completing complex tasks | Active scanning of the environment, creativity, and divergent thinking |
| Anxiety | 4% | Exaggerated fear responses to nonfatal threats | Enhanced awareness of potentially fatal threats |
| Bipolar | 3% | Mood swings and erratic behavior | Ability to take advantage of good situations and act defensively in bad ones. |
| Borderline | 1% | Unstable, chaotic interpersonal relationships and fear of abandonment | Distrust of questionable people, sensitivity to other’s emotions, loyalty, empathy, and courage |
| Depression | 5% | Loss of confidence, drive, and ambition | Accepting and adapting to defeat to avoid further defeat |
| Binge Eating | 9% | Overeating and unhealthy lifestyle | A strong desire to obtain calories and store fat |
| Histrionic Personality Disorder | 1% | Excessive and inappropriate attention seeking and need for approval | Engaging personality that is lively, dramatic, extroverted, and exhibitionistic |
| Narcissism | .5% | Need for admiration, inability to empathize, abusive | Confidence, self-centeredness and exaggerated feelings of importance |
| Obsessive Compulsive Disorder | 1% | Excessive and unreasonable worry about minor details | Need for order, safety, cleanliness, and hygiene |
| Paranoid | 3% | Unhealthy obsession with non-life-threatening things | Healthy focus on serious threats to life |
| Paraphilia | 8% | Persistent sexual interest in atypical activities or situations | Willingness to adapt to and experiment with sexual options |
| Post-Traumatic Stress | 4% | Tendency to obsess over low probability negative events | Tendency to be prepared for and avoidant of traumatic situations |
| Psychopathy | 1% | Hurting others, making them angry and becoming ostracized | Self-promotion, protection, and elevation |
Table 1.3 Mental disorders that can be maladaptive today may have been adaptive in the ancestral past. “Rate” corresponds to a very rough estimate of prevalence which varies by age, nationality, sex and other factors and can be measured in many ways.
It is estimated that 30% of Americans have at least one clinically diagnosable mental disorder. This percentage would be a lot higher if the criteria that psychologists and psychiatrists use were a little more lax. The criteria are calibrated to establish disorder when it significantly affects someone’s quality of life. Thus, when a disorder begins to impair someone socially and occupationally, clinicians slap it with a diagnosis. In truth though, much about diagnosis is arbitrary. We all experience the symptoms of most psychological disorders at some point in our life. I can vividly relate to each mental disorder above. Can you? But the real question is, can we relate to the prehistoric hunter-gatherers who found these disorders useful?
When I speak to people about the hypotheses in the table above, they often relate these ideas to things they have seen on television. Sometimes they imagine the ancestral past as medieval Europe or the Biblical Levant. This can mislead their thought process. As we will discuss, almost all the mental disorders considered by this book have nearly the same prevalence rates regardless of race or ethnicity. That means that they existed before humans split up into the racial categories we recognize today. Thus, they took much the same form they do today over 100,000 years ago and probably much earlier, during a period of human history that most people have trouble relating to or even imagining.
Evolutionary Mismatches
By explaining the negative implications for modern people, and the positive implications for hunter-gatherers, the last two tables encompass the concept of evolutionary mismatch. An evolutionary mismatch occurs when a trait was naturally selected but then became maladaptive when environmental conditions changed. Mismatches are common in nature. Environments can change when animals migrate, or when their habitat changes gradually such as with climatic change, or when new species are introduced. Because evolution is slow acting, it can take hundreds or thousands of generations for a species to adapt and thus when the environment changes much faster than a species genome does, there can be glaring incongruity.
Humans evolved to subsist off the land in small tribal bands. But modern humans live in ways that are very different from the stone-age environment our bodies are expecting. This makes us vulnerable to many health problems that are now being called, diseases of civilization. The biggest environmental change in human history was the Neolithic Revolution. This was the transition from a hunter-gatherer lifestyle to an agricultural lifestyle involving planting, farming, and animal domestication. This happened approximately 10,000 to 12,000 years ago and it revolutionized the way humans interacted with their environment. Since then, the industrial and electronic revolutions have placed us even further from the environment that our bodies are expecting. This has caused many of our key adaptations to become outdated and defective.
For example, the advent of agriculture led humans to have very limited grain-based diets rather than the more varied, plant-based diets of the past. This caused deficiency syndromes such as scurvy (a lack of vitamin C) and rickets (lack of vitamin D and calcium). As we will discuss in Chapter 8, the move away from raw plants still affects us today. Contemporary humans also engage in limited physical exercise and prolonged periods of inactivity. Early humans would have been highly active on a daily basis unless they were injured or ill. Thus, our bodies are not prepared for sedentariness which makes us susceptible to obesity, metabolic diseases, inflammation, and stress.
Osteoporosis is another example. It is a disease, common in older people, that causes bones to become weak and brittle. It may not have greatly affected our ancestors though. Fossil evidence of elderly foraging women living tens of thousands of years ago show no evidence of bone loss. What causes this decrease in bone density today? It is the sedentary lifestyles of modern Western populations. We don’t move as much or distribute loads throughout our bodies by turning, twisting, bending, kneeling, squatting, and the like. So, our bones don’t activate the genes and build the proteins necessary to build our skeletons up robustly. With reduced manual labor, many people, especially older women, are remarkably susceptible to osteoporosis. Knowing this, would you consider osteoporosis a disease or a mismatch?
Next, consider the hygiene hypothesis. High levels of washing and sanitation may be coddling our immune systems and making us weaker. There has been a profound recent increase in allergies, autoimmune diseases, and some chronic inflammatory diseases. Many specialists now believe this has happened because sterilization and other hygienic practices have kept our immune systems from being exposed to microbes and their antigens. Our hyper clean urban environments deplete microbiotic diversity. If we are rarely exposed to bacteria and viruses, then our immune systems cannot learn to protect us from them, and are likely to overreact to them (i.e. with excessive inflammation).
Humans currently live in something called the “delayed reaction environment.” Our behaviors and decisions do not create immediate results as our bodies expect them to. Instead, because our society is so advanced, and the interactions are so abstract, our behaviors create delayed results that may not manifest for days to years. In our past, real-time reward and punishment helped us escape predators, find a meal, and locate shelter each night. Today, our daily decisions impact our health, wealth, and quality of life months in the future. If you don’t go to work today, you are not going to go hungry, but that absence may be a factor in a lack of promotion a year from now.
Because of our previous environment, we have a need for instant gratification. Our brains have trouble being far-sighted and appreciating the long-term consequences of our actions. This is one of the reasons why it is challenging and unnatural for so many of us to thrive in the professional sector which requires discipline, self-restraint, and preparation. Our tendencies to seek immediate pleasure also influence us to gamble, use drugs, and overeat.
In the times of foragers, there were few loud noises, no demands for professional socializing, no crowds, and meeting new people was rare. Children would’ve spent most of their time playing together and learning to forage. There were no standardized tests, PowerPoint presentations, or competition for high-paying professional jobs. There was also less pressure to form an isolated nuclear family. Moreover, we did not perform abstract, intellectual work to gain food. Rather, we went out and actively sought out the food that we put into our mouths.
Because of this incongruity, we experience significant mismatches every hour of the day. Today, many of us work at unrewarding jobs that subject us to long hours of busy work, sitting, and bracing against a keyboard and mouse. Then we go home to languish on the couch by ourselves or with only a few similarly disillusioned family members. However, our bodies are expecting cathartic communal bonding over a bonfire each night where we sing, dance, and share stories with our entire community, surrounded by playful children and wise elders. This is how many modern hunter-gatherers live. Nearly no one literate has anything akin to that today.
In many ways, modernity predisposes us to developing background anxiety and depression. We are bombarded by media that scares, worries, and enrages us. Modern urban living limits our exposure to the natural environments of our ancestors, which are known to have various positive effects on mental health. We all want to be famous and rich despite the fact that this is extremely improbable. Nowadays, the constant barrage of information from the internet, social media, and 24-hour news cycles can lead to information overload. Our society constantly prods us to seek romance despite our deep need for nonsexual love. All these issues, along with those related in the table below, increase stress. And as we will soon see, stress is the most consequential factor in mental health.
| Unhealthy Aspects of the Modern World | Explanation |
| Drugs and Alcohol | The lure of addictive substances can cause grave physical and mental health problems |
| Loneliness | Rather than living in a tight-knit group of one hundred fellow hunter-gatherers we live partially isolated |
| Detachment | Many don’t have the time or energy to have close physical bonds with children, friends, or spouses |
| Negative News | We are inundated with world news that agitates, worries, and angers us |
| Meaningless Work | Many of us perform repetitive tasks that are not intrinsically interesting to us or for our own direct benefit |
| Immobilized Work | Many of us sit in chairs with our hands glued to keyboards causing cardiovascular and musculoskeletal problems |
| Divisive Politics | Politics cause us to see our neighbors as enemies and outgroup members |
| Violent Media | Television, movies, and social media feed us a stream of accidents, fails, fights, arguments, and horrific violence |
| Attention Disrupting Media | A never-ending series of provocative but brief video clips fractures our attention |
Attention-Deficit Hyperactivity Disorder
Next, let’s take the popular reconceptualization of attention-deficit / hyperactivity disorder (ADHD) as an example of the applicability of evolutionary thinking. American radio personality, Thom Hartmann (Hartmann, 1997) coined the phrase “hunter in a farmer’s world” to explain the predicament of individuals with ADHD. He explained that in the ancestral environment, children were naturally selected to hunt and forage for food, not to focus on schoolwork for hours on end. They never benefitted from concentrating carefully on chalkboard lessons or assignments. There were no classrooms, grades, or homework during prehistory. Even so, there was lots of cognitive work to be done. Hartmann explained that the ADHD mindset may have offered advantages during hunting and gathering times especially under time-critical, novel, and resource-depleted conditions.
Others (Jensen et al., 1997) have elaborated on this hypothesis since. They explained that the predilection for individuals with ADHD to be impulsive, hyperactive, and scattered could have been highly advantageous in prehistoric times. High levels of activity, scanning, response-readiness, and exploration may have kept humans with ADHD in tune with a rapidly changing environment and permitted them to identify immediate threats and opportunities. Today, diffuse attention can cause some people with ADHD to become sidetracked from a task. However, in the ancestral environment tasks were different, and AHDH may have helped people switch rapidly between related aspects of the same task. Moreover, the relentless need for stimulation may have kept them actively engaged with their environment helping them exploit resources. Researchers (e.g. Panksepp, 1998) have long concluded that even some severe cases of ADHD may reflect normal variation in these gainful tendencies.
People with an attention deficit may have difficulty with convergent thinking which is the ability to accumulate and utilize relevant knowledge in a focused effort to arrive at a logical solution to a problem. However, a recent literature review (Hoogman et al., 2020) found that ADHD symptoms are associated with convergent thinking’s opposite, divergent thinking. Divergent thinking, an equally important skill, is the ability to produce creative solutions and consider a problem from multiple, highly unique, viewpoints. Divergent thinking happens in a spontaneous, free-flowing manner where many creative ideas are generated and evaluated. Relatedly, individuals with ADD and ADHD tend to have flexible associative memory. This allows them to recall and employ more distantly related ideas keeping them from becoming fixated on a single ineffective solution and allowing them to conceive of ideas that would not occur to other people.
Underscoring the importance of neurodiversity, it is generally thought that using convergent thinking in conjunction with divergent thinking is preferable to either alone. For example, it may be best to use divergent thinking at the outset to brainstorm and explore possible solutions and to then evaluate these individually using convergent thinking. I can almost see two hunters losing the footprints of the big game they have been tracking, but then employing these two cognitive strategies to help each other infer the direction their quarry is traveling, a solution that neither one could reach by themselves.
Studies have shown that there are certain games that people with ADHD perform better on. In an online foraging game people with ADHD moved on from digital berry bushes faster than their peers and thereby played more effectively. Similar findings have shown that people with ADHD spend more time exploring than exploiting. After reading studies where individuals with ADHD perform optimally on computer games with fixed rules, I was able to see that the “disorder” is really an alternate mathematical strategy for reallocating attention. The typical algorithm for sustaining focus and losing interest is tweaked leading to rapid shifts in focus and a broader scope of attention. I hope that after reading the first few chapters you start to see various neurodiverse conditions as normal but fine-tuned to excel at slightly different versions of the same game.
High energy, skill at multitasking and risk taking, and a low tolerance for the mundane can make ADHD seem like a superpower in certain contexts, but in others it can feel like kryptonite. Difficulty paying attention can be frustrating especially when it conflicts with educational or occupational demands or causes the person to be labeled a disruptive troublemaker. The question I would like you to keep asking yourself as you read on is, “how can people with psychiatric conditions harness their unique strengths?”
Evolutionary Anachronisms
There are many cases where plants or animals have a straightforward adaptation that makes no sense in their current environment. Often in these cases, the adaptation in question was adaptive in the past before something in the environment changed that made it out of place in time. Some specialists refer to these cases as ecological anachronisms. The simplest form of an anachronism is a vestige. Vestigial traits are common in the animal kingdom and the table below gives some examples.
| Animal | Vestigial or Anachronistic Trait |
| Pandas | Pandas primarily eat bamboo, but they have a digestive system suited for carnivores. This is an anachronism from when their ancestors were meat-eaters. |
| Whales | Some whales have tiny, vestigial hind limbs buried within their bodies, remnants from when their ancestors walked on land. |
| Dogs | Dewclaws are the remnants of what used to be functional toes. In some dogs, these claws are not connected to the foot and serve no real purpose. |
| Flightless Birds | Birds like ostriches and kiwis have wings but cannot fly. These wings are evolutionary remnants from their flying ancestors. |
| Boa Constrictors | These snakes have tiny remnants of hind legs, known as pelvic spurs, used by their legged ancestors. |
| Horses | Modern horses walk on a single toe on each foot, but they still have vestigial bones in their legs, remnants of their multi-toed ancestors. |
| Blind Mole Rats | Some mole rats are blind, yet they retain small, non-functioning eyes. |
| Penguins | Penguin’s wings are vestigial in terms of flight but have been repurposed for their aquatic lifestyle. |
| Kiwi Bird | The kiwi lays one of the largest eggs in relation to its body size. This is thought to be a vestigial trait from when their ancestors were much larger. |
Table 1.6 Examples of how evolution can leave traces of a species’ past, even when those traits are no longer beneficial in their current environment.
The next table offers examples of vestigial traits in humans.
| Human Vestige | Functional Trait |
| Tailbone | Similar to humans, many animals have a tailbone (coccyx) but no tail, a remnant of their tailed ancestors. |
| Male Nipples | Male mammals of many species have nipples, despite having no use, a byproduct of the mammalian developmental pathway. |
| Gill Slits | In the embryonic stages, many vertebrates, including humans, show gill slit-like structures, a vestige from our evolution from fish. |
| Palmar Grasp Reflex | Human infants exhibit a hand grasping reflex part of our primate heritage from when infants needed to cling to their mothers’ fur. |
| Pineal Gland | The pineal gland, or third eye, in some reptiles, is a light-sensitive organ on the forehead that helps regulate circadian rhythms. In mammals it exists only inside the brain and is not light-sensitive. |
| Vomeronasal Organ | This organ is used for detecting pheromones for many mammals like cats and dogs. In humans, it is largely non-functional. |
| Goosebumps | In animals with thick fur, like cats and dogs, piloerection increases insulation and during tense encounters can make the animal appear bigger. |
| Appendix | In humans, the appendix serves no significant purpose and is susceptible to appendicitis, but is thought to have been useful in our herbivorous ancestors for digesting cellulose in plant material. |
| Ear Muscles | The muscles that allow some animals to move their ears independently are largely useless in humans. |
| Third Eyelid | In many mammals the third eyelid can be drawn across the eye horizontally for protection and moisture. Humans, have the remnant of this in the corner of the eye. |
| Wisdom Teeth | Today wisdom teeth commonly become impacted, but chewing raw plant matter would have afforded our ancestors larger jaws that would have benefitted from the additional molars |
| Body Hair | Millions of years ago body hair conserved warmth and offered a degree of protection from injury |
Table 1.7 Examples of human traits that are retained despite no longer being beneficial.
For example, there are many cases of plants that bear large fruit that fall to the ground and rot. Producing fruit is energy expensive, so plants do not produce fruit for it to go to waste. Fruit is meant to be eaten whole by animals that travel far away from the parent plant and then defecate, depositing the fruit’s seeds to grow in a new place. The poop acts as a fertilizer, and the animal acts as a seed disperser. This is the purpose of all botanical fruits. Most berries that contain small seeds are meant to be consumed and spread by birds and small mammals. However, when large fruit with large seeds go uneaten, it is often because the large animal that used to eat the fruit has disappeared.
Merely a few thousand years ago, giant mammals could be found foraging throughout the North American continent. These include camels, mastodons, mammoths, rhinoceroses, horses, gomphotheres, and giant ground sloths. Some of these animals were tremendous; some giant ground sloths weighted up to three tons. It seems that human hunters killed many of these large animals all around the world as they exited Africa and spread (megafauna overkill hypothesis). These megafauna have been gone for thousands of years but the trees are slow to catch on because of the slow pace of evolution. Most mammoth populations died out over 10,000 years ago, but for a tree that lives 200 years, this only represents about 50 generations. This is not nearly enough time for the right mutations to accumulate to change various characteristics of the fruit to make it more suitable for smaller animals.
The avocado may be an example of a fruit from a different time. Avocados have giant seeds that no existing animals can consume and pass through their digestive tract. Would you ever swallow an avocado seed? I don’t think so. In the wild, the fruit usually falls straight to the ground, where the young avocado sapling must compete with its larger more established parent tree for sunlight and room to grow. The genes that produce the large avocado seeds are expecting a mutualistic relationship with the enormous animals that used to consume the seeds whole.
Some authors state poetically that those giants still haunt the avocado as evolutionary ghosts. Nevertheless, avocados did not die out. This is because the same animals responsible for the extinction of the American megafauna, humans, started cultivating them avocados. In the process, we artificially selected avocados to have more flesh and smaller seeds. The avocado isn’t alone in being an anachronistic fruit. Others include the papaya, prickly pear, cherimoya, sapote and many more. Plants are not alone in demonstrating anachronisms.
The California Condor is the largest bird in North America. It is a scavenger, often very successful creatures. However, these birds are critically endangered today partly because the giant animal carcasses they are prepared to scavenge are gone. These birds can no longer locate and then descend on a dead mastodon. Instead, they must settle for much smaller carcasses that are often too small to sustain their large bodies. These condors are too big for the environment they find themselves in today. Something similar may have happened to the largest ape of all time.
Evolutionary ghosts refer to instances where a species’ adaptations are shaped by interactions with another species that has since gone extinct. For example, the feeding behaviors and adaptations seen in modern Asian elephants may be partially a result of coexisting with now-extinct woolly mammoths, as they had to adapt to different or less optimal food sources to avoid direct competition. As another example, it is hypothesized that the dodo bird played a crucial role in the lifecycle of the Calvaria tree by eating and then dispersing its seeds. Since the extinction of the dodo, these trees have become increasingly rare. The thorns and large seed pods of the honey locust tree suggest an adaptation to protect its seeds from, and be dispersed by, Pleistocene megafauna such as mammoths, which are now extinct. These examples illustrate how the “ghosts” of extinct species can continue to influence living ones.
Gigantopithecus was a giant ape living in south Asian tropical forests from 2 million to 300,000 years ago. Only large teeth and huge jaw bones have been found, but scientists scale from these remains (using allometric techniques) to suggest that Gigantopithecus could have weighed from 500 to 1,000 pounds and been taller than nine feet. Proteins taken from fossil teeth enamel tell us that today its closest living relative is the orangutan. Detailed examinations of its teeth and their isotopic composition have revealed that these animals were herbivorous, eating fruits, leaves, and other forest plants. It would have had a brain bigger than a chimp’s or a gorilla’s. It is possible that this tremendous primate was bipedal and it could have used tools. Ancient humans, namely homo erectus, lived in the same areas during the same time periods, and they likely interacted. If we had a specimen today, its behavior and brain structure would constitute a fascinating addition to our understanding of primate neurodiversity. It is not clear why it went extinct, but it is likely that, as conditions shifted, its large body became too energy intensive to maintain.
As the next section will discuss, it is not just size that can be anachronistic. Behaviors and the neurological circuitry responsible for them can be as well.
Relict Behavior
Similar to the concept of evolutionary anachronisms, the term relict behavior describes actions an animal will take that are no longer helpful. These functionless or vestigial behaviors were beneficial in the animal’s past. Let us start with some examples.
California ground squirrels living in areas that have not had rattlesnakes for 50,000 years still instinctively know how to defend themselves from snakes using complex, instinctual action patterns. Such relict anti-predator behaviors are common and occur in many species, from fish to mammals.
The North American pronghorn antelope can run nearly 60 miles per hour. It can keep up this incredible pace for a half-mile. This fast running may have evolved in response to now-extinct predators, including jaguars, lions, long-legged hyenas, short-faced bears, and American cheetahs, all of which no longer exist in North America and were probably mostly wiped out by humans in the last 15,000 years. It is expensive to build and maintain a body that can run so fast, but a fetal pronghorn’s genes have no way of knowing that their natural predators are gone.
The ring-tailed lemur from Madagascar exhibits clear instincts and reflexes to take flight from birds of prey. However, none of the present-day birds on the island are big enough to attack or eat a lemur. Scientists know that the Malagasy crowned eagle, which went extinct around 1500 AD, would have been big enough to prey on lemurs. Specialists believe the lemur’s behavior reflects their past with this long-gone predator.
Humans may also show relict behaviors. Careful studies have revealed that human males report being most fearful about being attacked from the side, and females report being most fearful of being attacked from below. Some scientists think this may be a holdover from earlier hominin evolution when females spent more time in trees than males and, like many arboreal monkeys, were more likely to be attacked by large cats climbing trees from below. Of course, we no longer try to avoid predation by hiding in trees, but this gender discrepancy in innate fear circuitry may reflect an ancient past.
| Vantage Points | Our visual systems are tuned to derive pleasure from expansive views from high places which would have allowed us to spot prey and predators |
| Allure of Water | The preference to be near oceans, lakes, or rivers, and to find the sound of running water relaxing, would have ensured proximity to a vital resource |
| Allure of Fire | The hypnotic allure of fire, crucial for warmth and cooking in ancestral times, persists today in the form of campfires and fireplaces |
| Music and Dance | The universal human enjoyment of rhythmic music and dance could be a relict of ancient communal bonding practices. |
| Storytelling | The universal human love of narratives, movies, and novels may stem from storytelling, the way our ancestors passed down knowledge and built bonds. |
| Attraction to Colors | Our attraction to bright colors in art, clothing, and decoration likely stems from our attraction to and need to identify ripe fruits. |
| Dietary Variety | We desire great variety in our cuisine because a varied diet was crucial for obtaining all necessary nutrients. |
| Neophilia | Our love of novelty and new experiences would have helped our ancestors explore new resources and environments. |
| Hoarding | The tendency to collect and store items was beneficial in times of scarcity whereas in the modern world this can manifest as a psychological disorder. |
| Xenophobia | Strong in-group loyalty and tribalism was crucial for survival but today can manifest as prejudice, racism, or xenophobia. |
| Sexual Fetishes | Normal sexual arousal and anchoring instincts can imprint on unusual objects or situations |
| Food aversions | Tendencies to dislike certain foods may be related to adaptations intended to identify and avoid toxic foods or undigestible compounds |
| Pica | Pica, the craving for clay or dirt, may be a response to mineral deficiencies and provided nutrients lacking in the diet. |
| Seasonal affective disorder | Too much time indoors without exposure to sunlight can cause depression may also be an evolved mechanisms to conserve energy during darker months |
| Insomnia or Night Owl Tendencies | Night watch for predators and threats seen in some hunter gatherers today |
| Sweet Tooth | In ancestral environments sweet tastes indicated ripe, energy rich fruit |
Psychopathy and Antisocial Personality Disorder
Psychopathy, sociopathy, and antisocial personality disorder (ASPD), are related mental disorders. Each is characterized by a lack of social fear that can result in deviance and criminal behavior. Because these three similar disorders have subtle technical differences, here we will focus on psychopathy. Psychopaths can generally empathize with others but often do not feel compelled to act on empathy because of their lack of fear. For example, they may realize that insulting you will make you mad, but this will not keep them from doing it. So, they are less afraid to cross social boundaries. This can result in amoral conduct, and unfortunately, they can tend to hurt or abuse others often landing in jail. Because about 1% of people have psychopathy, evolutionary psychologists have come to believe that psychopathy may represent an ecological strategy that helps an individual to selfishly gain resources by taking advantage of others. Our hunting and gathering ancestors have relied on cooperation for millions of years and this is why most people are not psychopathic. Perhaps though, in small numbers, genes that influenced individuals to be less helpful and more selfish benefited those that bore them.
Clearly, it would be helpful if a baby could know in advance if their reality was going to be friendly, and conducive to teamwork, or hostile rewarding those that defect from the team. Well, it is possible that psychopaths can be made by bad experiences. Perhaps even ones that start as early as the womb. Studies of the Dutch Hunger Winter of 1944-1945 have shown that individuals born just after this famine, whose mothers were exposed to food shortages, are more likely to demonstrate psychopathic behavior. Many experts believe this is due to pathological effects of malnutrition on neurodevelopment. However, it could alternatively represent an adaptive response to scarcity.
This relationship between famine and psychopathy suggests that humans can be programmed by early environmental stress to be manipulative, guiltless, and antisocial. It would make sense for natural selection to program us to be more deceitful and cunning in a hostile environment that is marked by resource shortage and thus high levels of competition. This may be when cheaters prosper. The brain changes of psychopathy would force the person or animal to look out for number one. Some evolutionary scholars see psychopathy’s high self-interest, risk taking, and cheating and conclude that it could function as a short-term mating strategy that results in a large numbers of sexual partners. Peruse the following list from the “Psychopathy Checklist Revised” and ask yourself: “what kind of environment would these traits be suitable for?”
| Psychopathic Traits: | |
| Factor 1: Aggressive narcissism | Factor 2: Socially deviant lifestyle |
| Glibness / superficial charm | Need for stimulation / prone to boredom |
| Grandiose sense of self-worth | Parasitic lifestyle |
| Pathological lying | Poor behavioral control |
| Cunning / manipulative | Promiscuous sexual behavior |
| Lack of remorse or guilt | Lack of realistic, long-term goals |
| Emotionally shallow | Impulsiveness |
| Callous / lack of empathy | Irresponsibility |
| Failure to accept responsibility | Juvenile delinquency |
I feel strongly that psychopaths have redeeming characteristics. One of my best friends is a psychopath and in some ways, he is a definite asset in my life and our relationship has been one of my most rewarding. In other ways, it has also been detrimental to my emotional stability. I think, as with other neurotypes, that the right therapeutic interventions could really help people with psychopathy to become aware of their tendencies for social fearlessness so they can stop themselves from hurting others. Moreover, their lack of fear is very powerful and if harnessed properly could be put to productive use.
It is possible that psychopathy is not an adaptation but a spandrel. A spandrel is a trait that is an unintentional byproduct of an adaptive characteristic. As an example, some experts believe the human adoration of music is a spandrel in that it was never beneficial but was a nonevolutionary side effect of bigger brains and the capacity for rhythmic movement. Can you think of why our love of music might have been positively selected or does it seem like a spandrel to you? Robin Dunbar thinks it might have been more than a spandrel and played a role in social bonding, bringing people together around a campfire. He also thinks people with musical abilities may have been favored during mating (Dunbar, 2012).
Well, some scientists see psychopathy as a spandrel. They don’t think that acting in a psychopathic way would have been helpful. Instead, they think selection for psychopathy happened indirectly because it is tied to sexuality (mating effort) and dominance, and that those two things were what was being selected, not antisociality per se (Leedom & Almas, 2012). As you continue to read about evolutionary hypotheses don’t forget to ask yourself if the conditions I purport to be adaptive are actually spandrels.
The Adaptationist Agenda
Many evolutionary explanations are criticized for being “adaptationist.” They take a trait and use vivid imagination to dream up some purported function for it. The worst of these fanciful explanations are deemed “just so stories” because they attempt to explain the origin of something using fanciful tall tales that are difficult to validate or gather evidence to support. In their widely cited critique of the “adaptationist paradigm,” Gould and Lewontin provided many reasons why directionless evolutionary theorizing can be useless or even counterproductive.
Consider the idea that our ancestors started walking on two feet to see farther. Ok, now consider that they did it to free up the hands. It is very difficult to find strong evidence for either of these hypotheses. They may be unprovable. We cannot go back in time 5 million years and poll ancient hominins on the reasons for why walking bipedally was helpful for them. However, they both seem very compelling to common sense and they probably each contributed to bipedalism in some way. Many hypotheses, even adaptationist ones, in the field are similar. They resonate strongly with some people but may not with others. However, many of these hypotheses are probably at least somewhat true. Does this mean they are worth our consideration though?
I believe strongly in the 10 evolutionary hypotheses, found in Table 1.4 below, that I present in this book. At first glance, they may seem strange or farfetched. But these chapters aim to provide you specific knowledge and an altered perspective that may be capable of convincing you of their validity. Also, unlike the explanations for human bipedality, I believe that the hypotheses in this book are falsifiable. I see them less as a matter of opinion and more as a detective story. I believe this detective work will be verified in the future as brain and genetic analytical techniques improve.
Conclusion
This book dedicates a chapter to each of 10 different diagnoses. These are mental disorders we have not discussed thus far and that have less obvious adaptive properties. I will introduce novel hypotheses as to how each of these represent natural human variation and optimization for a different ecological niche. These disorders, possibly better understood as forms of diversity, are listed in the table below.
| Brain Disorders | Rate | Negative Implications for Modern People | Positive Implications for Hunter-Gatherers |
| Stress Cascade (Chapter 4) | 100% | Problems with memory and high-order brain function | Enhanced caution, awareness, and disinhibition |
| Schizophrenia (Chapter 5) | .5% | Psychosis, social withdrawal, and disorganized thinking | Instinctual, impulsive, emotional, and defensive behavior |
| Metabolic Syndrome (Chapter 6) | 22% | Obesity, diabetes, and cardiovascular disease | Reduced metabolic rate conserves energy, reducing risk of starvation |
| Intellectual Disability (Chapter 7) | 2% | Social and occupational difficulties and challenges | Reduced metabolic rate in the body and brain |
| Down’s Syndrome (Chapter 8) | .14% | Social and occupational difficulties and challenges | Reduced metabolic rate in the body and brain |
| Autism (Chapter 9) | 2% | Difficulties in social interaction and communication | Tendency to forage solitarily and ability to survive in small groups |
| Tourette’s Syndrome (Chapter 10) | 1% | Unwanted yet compulsive tics and outbursts | Quick-acting, spontaneous movements and actions |
| Arthritis (Chapter 11) | 20% | Joint pain and diminished mobility | Energy conservation due to minimization of joint use |
| Asthma (Chapter 12) | 8% | Wheezing, coughing, and shortness of breath | Energy conservation due to reduced respiratory output |
| Alzheimer’s Disease (Chapter 13) | 33% (over 85) | Forgetfulness, disorientation, and dementia | As expertise increases brain energy expenditure can be reduced |
Table 1.4 Some of the diseases and disorders that will be discussed in depth in this book and what some of their adaptive properties may have been. “Rate” corresponds to a rough estimate of prevalence which varies by age, nationality, sex and other factors and can be measured in many ways.
The next table lists the original articles that I wrote spelling out these hypotheses formally using scientific terminology. They contain information not included in this book for the sake of clarity and brevity. However, this book also contains information not available in these articles because of new findings and further research.
The Original Theoretical Articles on Which This Book is Based
| Reser, J. 2016. Tourette syndrome in the context of evolution and behavioral ecology. Medical Hypotheses. 99(35-39). Creative Commons CC-BY-NC-ND |
| Reser, J. 2016. Incremental change in the set of coactive cortical assemblies enables mental continuity. Physiology and Behavior. 167: 222-237. Creative Commons CC-BY Unrestricted use |
| Reser, J. 2016. Chronic stress, cortical plasticity and neuroecology. Behavioural Processes. 129:105-115. Creative Commons CC-BY Unrestricted use |
| Reser, J. 2014. Solitary mammals provide an animal model for autism spectrum disorders. Journal of Comparative Psychology. 128(1): 99-113. © 2023 American Psychological Association |
| Reser, J. 2011. Schizophrenia and the Metabolic Syndrome. In Victory Preedy (ed.) Handbook of Behavior, Food and Nutrition. New York, NY: Springer. © Springer Science |
| Reser, J. 2011. Nutrition, Behavior, and the Developmental Origins of the Metabolic Syndrome. In Victory Preedy (ed.) Handbook of Behavior, Food and Nutrition. New York, NY: Springer. Springer Science |
| Reser, J. 2011. Conceptualizing the autism spectrum in terms of natural selection & natural history: The solitary forager theory. Evolutionary Psychology. 9(2): 207- 238. © Sage Creative Commons Attribution-NonCommercial 3.0 License |
| Reser, J. 2009. Does rheumatoid arthritis represent an adaptive, thrifty condition? Medical Hypotheses. 74(1): 189-194. © 2009 Elsevier Ltd |
| Reser, J. 2009. Alzheimer’s disease and natural cognitive aging may represent adaptive metabolism reduction programs. Behavioral and Brain Functions. 5 (13). Creative Commons CC-BY Unrestricted use |
| Reser, J. 2007. Schizophrenia and phenotypic plasticity: Schizophrenia may represent a predicitive, adaptive response to severe environmental adversity that allows both bioenergetic thrift and a defensive behavioral strategy. Medical Hypotheses, Volume 69, Issue 2, 383-394. © 2007 Elsevier Ltd |
| Reser, J. 2006. Evolutionary Neuropathology and Down syndrome: An analysis of the etiological and phenotypical characteristics of Down syndrome suggests that it may represent an adaptive response to severe maternal deprivation. Medical Hypotheses, Volume 67, Issue 3, 474-481. © 2007 Elsevier Ltd |
| Reser, J. 2006. Evolutionary Neuropathology and Congenital Mental Retardation: Environmental cues predictive of maternal deprivation influence the fetus to minimize cerebral metabolism in order to express bioenergetic thrift. Medical Hypotheses, Volume 67, Issue 3, 529-544. © 2007 Elsevier Ltd |
As we further analyze the evolutionary roots of mental disorders, I also want to encourage you to keep Niko Tinbergen’s (1963) “four questions” in mind. These will help you see each condition from four individual yet intersecting vantage points. Applying these questions to autism for instance would get one to consider how the brain is different in autism, how these differences arise during an individual during development, what purpose it would have served, and why it evolved in the first place. The fifth vantage point I would like you to take with you is this idea of the original, ancestral environment. Prehistoric individuals with autism were very different from individuals with autism today because they grew up hungry and focused on acquiring food.
| Tinbergen’s 4 Questions | ||
| Contemporary: Current Form | Historical: Chronological sequence | |
| Proximate: Present day explanation (How?) | Mechanism: How does it work biologically? (Causation) | Development: How did it develop during lifespan? (Ontogeny) |
| Ultimate: Evolutionary explanation (Why?) | Adaptive Value: Why does the organism do it? (Function) | Evolution: Why did it evolve? (Phylogeny) |
Table 1.X Tinbergen’s four questions are a framework for understanding animal behavior, developed by biologist Nikolaas Tinbergen. The four questions are used to comprehensively understand animal behaviors by examining them through different lenses of biological inquiry.
Anthropologists who have worked for years with hunter-gatherers have a profound respect for the survival skills of their subjects. They have seen how many years it takes to learn to fashion tools, start fires, locate vegetables, and predict the behavior of wild animals. For someone like you and me, who were not raised in the wild, it would be extremely difficult to become a proficient forager. As with domestic animals, even a young human adult may be too old to internalize all the knowledge needed to thrive in most wild habitats. Knowledge of this should change our perspective on the mental disorders in the tables above.
The diagnoses discussed in this book are usually seen as barriers to success in the modern world. The common assumption is that they would have been even more detrimental in the wild. However, I hope this book helps you understand that people with these diagnoses were reared and immersed in nature during prehistory. They were focused by hunger, thirst, pain, instinct, curiosity, and various emotions to hone their foraging techniques and thrive in their respective environments. People with ADHD, autism, intellectual disability, and schizophrenia grew up intensely focused on staying alive and this would have made them more effective survival experts than you or I would be capable of becoming today.
References:
Robin Dunbar, “On the Evolutionary Function of Song and Dance”, in N. Bannan (ed.) Music, Language, and Human Evolution. Oxford University Press, 2012, p. 201-202.
Leedom LJ, Almas LH. Is psychopathy a disorder or an adaptation? Front Psychol. 2012 Dec 18;3:549.
Huxley J, Mayr E, Osmond H, Hoffer A (October 1964). “Schizophrenia as a Genetic Morphism”. Nature. 204 (4955): 220–1.
Price JS, Gardner Jr R, Wilson DR, Sloman L, Rohde P, Erickson M (July 2007). “Territory, Rank and Mental Health: The History of an Idea”. Evolutionary Psychology. 5 (3): 147470490700500305.
Nesse RM (January 2000). “Is depression an adaptation?”. Archives of General Psychiatry. 57 (1): 14–20.
Hagen, E.H. “The Bargaining Model of Depression”, Genetic and Cultural Evolution of Cooperation, The MIT Press, 2003.
Hoogman M, Stolte M, Baas M, Kroesbergen E (December 2020). “Creativity and ADHD: A review of behavioral studies, the effect of psychostimulants and neural underpinnings”. Neuroscience and Biobehavioral Reviews. 119: 66–85.
Tinbergen, N. (1963). On the aims and methods of ethology. Zeitschrift fur Tierpsychologie, 20:410-433.
Adaptive Neurodiveristy 