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Evolutionary approaches to mental illness are theoretical and methodological approaches, which are predicated on the theory of evolution, used by evolutionary psychologists to understand the prevalence and incidence of various mental illnesses in the human species. Research in this field is focused on understanding the ultimate, as opposed to proximate, underlying causal mechanisms that lead to various mental illnesses. Evolution by natural selection dictates that deleterious traits that negatively affect reproductive fitness should be put under strong negative selection, thereby eliminating the trait from the population. The overwhelming persistence of mental illness across human populations therefore poses a quandary for evolutionary theory, leading to a variety of theoretical perspectives.

According to evolutionary theory, mental disorders are anomalous for a variety of reasons, including:
 * They are vastly common, and affect about half of the world's population
 * Their onset tends to be at reproductive maturity
 * High comorbidity
 * High heritability

Evolutionary Psychology
Evolutionary Psychology holds that evolutionary processes are just as key to understanding animal and human psychology as they are to understanding animal and human physiology. Researchers in this field seek to illuminate the trajectory of the human mind, and view psychological mechanisms as adaptation to problems that confronted human ancestors during the hunter-gatherer stage of human history. Evolutionary psychology investigates a variety of issues including: emotion, aggression, attachment, mothering, development, personality, psychological defenses, placebo, stigmatization, sleep, theory of mind, decision-making, and the brain.

Academic controversy in sociobiology sparked criticisms of evolutionary discussions of human cognition, some of which still circulate today. An influential objection has been that evolutionary psychology conflates adaptations with exaptations. However, key developments in evolutionary theory including understanding altruism and cooperation, emphasizing individual selection, and incorporating game theory, were particularly influential in establishing a more sophisticated analysis of human behavior.

Evolutionary Psychiatry
Evolutionary psychiatry is a field of research that attempts to apply evolutionary theory to psychopathology. As a discipline, it parallels evolutionary psychology and evolutionary medicine. Proponents of evolutionary psychiatry support disease models of mental illness, and view mental disorders as equivalent to any other physical illness like diabetes. This field of research is premised on the argument that an understanding of the evolution of the human mind and brain is essential in explaining the origins of particular psychiatric disorders. Although to some extent it remains outside mainstream psychiatry, due to the some difficulty in identifying physical abnormalities in brain structures, it continues to exert an increasing impact on the field. Researchers argue, for instance, that it is important to know whether a psychological symptom arises from a defect (e.g. seizures), from a defense (e.g. pain), or from a dysregulation of a defense (e.g. dehydration from diarrhea).

There are three broad categories of mental problems:
 * Disorders that arise primarily from brain abnormalities with high heritability such as: schizophrenia, autism, obsessive compulsive disorder (OCD), bipolar disorder
 * Disorders of emotional or behavioral dysregulation such as: addiction, anxiety, and depression
 * Affective states that are aversive or socially unacceptable but are nonetheless normal (or are at least useful for genes)

Evolutionary Psychopharmacology
Evolutionary psychopharmacology is a relatively new field of research that seeks to apply evolutionary perspectives to psychopharmacological studies. Psychopharmacology is the scientific study of the effects drugs have on mood, sensation, thinking, and behavior. Researchers in this field contend that substances need to be understood within an evolutionary perspective, arguing that since reward mechanisms have evolved over time and in ancestral environments, substances available in the modern environment act on brain circuitry to falsely indicate a fitness benefit. Recent work in evolutionary psychiatry is promising in understanding the relationship between proximal mechanisms (involving genes and proteins) and distal mechanisms (involving evolutionary processes) in its application to psychopharmacology. (find one more reference?)

Adaptation and Darwinian Medicine
Adaptationist approaches, generally, aim to understand the particular forces of natural selection that have generated and shaped particular traits. This involves identifying one or more functions that a trait is designed to accomplish. Ultimately, this helps researchers understand the ways in which a particular trait or set of traits may malfunction, or even how to suppress or eliminate a function that is no longer desirable. Darwinian medicine, furthermore, applies the principles of evolutionary biology to the problems of medicine, which some researchers argue is useful to psychiatry. According to this view, there are six possible answers to the problem of disease:


 * 1) Mismatch between our bodies and novel environments
 * 2) Co-evolution of hosts and fast-evolving pathogens
 * 3) Constraints on what natural selection can do
 * 4) Design trade-offs
 * 5) Traits that increase reproduction at the expense of health and
 * 6) Defenses that are useful even though they are costly and aversive

It is argued that these six evolutionarily informed possibilities offer psychiatry a diagnostic framework that distinguishes defenses from defects in the same way the rest of medicine does.

Depression

 * See also: Evolutionary approaches to depression

Major depressive disorder (MDD) has been associated with a variety of brain abnormalities, but there is still no consensus with regard to any particular underlying neuropathology. It is typically characterized as a malfunction of neurotransmission or brain circuitry that normally regulates mood and executive function. According to evolutionary perspectives, although brain malfunctions must occur sometimes in a population, they should be relatively uncommon, since properly functioning brains are directly related to Darwinian fitness. And although an evolutionary approach might not say that depression is useful, it could posit that low mood is useful in some way. Supporters of adaptationist approaches also argue that the utility of intense states of low mood cannot be understood until their is a better understanding of the functions and normal regulations of low mood.

Postpartum depression (PPD) has also been theorized as an adaptation in relation to maternal age and number of children. One study found that for each additional year of maternal age, there was a 4% reduction in the chance of developing PPD, which was not modified by the number of children already present in the home. It is suggested that adaptive mechanisms shaped through human generations persist and contribute to Postpartum depression in modern societies.

The social risk hypothesis argues that depression is an adaptive response to the perceived threat of social exclusion from important social relationships that are critical to the maintenance of the individual's fitness prospects. Depression may therefore reduce social exclusion by: (1) cognitive hypersensitivity to social threat indicators; (2): signaling behaviors designed to decrease social threat and elicit social support; and (3) a general reduction in an individual's propensity to engage in risky behaviors.

Schizophrenia
There is a popular notion in evolutionary psychology that schizophrenia is an adaptation of some sort. However, the combination of increasing evidence for brain damage and the very low Darwinian fitness (about half the average) makes this and implausible notion. Some have suggested that someone with schizophrenic tendencies might assist the group, but this is inconsistent with recent understandings of the limits of selection at the group level. Others suggest that the genes that cause schizophrenia are advantageous except in combination with other specific genes or particular environmental situations. Another possibility is the cliff-edge effect, originally proposed by David Lack and colleagues, which explains the number of eggs birds lay. According to this theory, some individual traits such as height, egg number, speed or intelligence, may produce increasing fitness benefits, but extremes of a trait may lead to failure for some individuals. If high intelligence or verbal ability has been strongly selected for in recent millennia, then these traits could have been pushed to extreme levels where some individuals "fall of the cliff" because of a failure of some crucial system.

Difference Amplification Model
Difference amplification is the process whereby the initial differences between competing individuals is amplified by the outcome of competition, whereby cycles of adaptation are generated in the winner and maladaptive cycles are generated in the looser. This model proposes that in ancestral populations, those who were most successful in winning hierarchical competitions showed more self-confidence and a relative absence of depression and anxiety, thus raising their inclusive fitness.

Attachment Model
The attachment system is predicated on caretaker-dependent proximity in times of potential threat, which serves as a protective function. The protest reaction produced by the infant during separation from the caretaker involves both the seeking system, which is designed to achieve reunification, and the opiate system, which is activated in the absence of the caregiver and upon return. During separation, feelings of anxiety are prominent, which would also cause immobility and possibly depression. Both anxiety and depression have been shown to be related to immobility.

Social Rank Model
Social rank theory argues that depression and anxiety serve the adaptive function of ending conflict and maintaining the stability of hierarchical relationships. The involuntary Defeat Strategy (IDS) is triggered when an individuals perceives inevitable defeat in hierarchical competitions, which is characterized by feelings of helplessness, hopelessness, and inferiority. Thus, the IDS is adaptive in that it motivates individuals into submission when defeat is inevitable. It can become maladaptive when the IDS is continuously triggered.

Creativity and Short-Term Mating Success
Some researchers have argued that creativity such as painting, poetry, music, and even humor have evolved as a mate choice for cultural displays. The idea is that the more ornamental and creative the display, the more indicative it is of the individual's developmental stability. Furthermore, if creativity is the ancestral product of displays of fitness to multiple potential mates, creativity would then be an indicator, to some extent, of an individual's biological fitness. Recent research suggests that artistic accomplishment in visual art is related to sexual success for men.

The nature and extent of the relationship between creativity and mental illness is still not fully understood, but there is a growing body of research that indicates there is some type of link. There is a correlations between creative occupations and mental illness, where individuals and first-degree relatives with certain forms of mental illness are more likely to be in creative jobs. This assumed relationship suggests that creative ability may have possible advantages for the genes associated with psychopathology, where negative fitness effects are counterbalanced.

Harmful Dysfunction
Harmful Dysfunction (HD) is a theory developed by Jerome C. Wakefield for the conceptualization and analysis of mental disorders that aims to address conceptual problems in abnormal psychology. Wakefield argues that disorder is a hybrid concept comprising a factual component and a value component. Dysfunction is a scientific and factual term based in evolutionary biology, and refers to the failure of an internal mechanism to perform a designed function. Harmful is a value term referring to consequences that occur as a result of the dysfunction, which are deemed negative by sociocultural standards. Here, the attribution of a disorder requires both a scientific judgment that there exists a failure of designed function, and a value judgment that this failure harms the afflicted individual. Disorder, then, lies on the boundary between the natural world and the socially constructed world. However, a harmful condition is not considered a disorder unless it involves a failure of a psychobiological mechanism to perform its natural function. Wakefield argues that the establishment of an analysis of disorder is important, as the dispute over the concept has far-reaching implications for diagnosis, treatment, policy, and research.

In his analysis, Wakefield does not distinguish between disorder, disease, and illness; he instead uses the term internal mechanism in a general sense to refer to both physical structures and organs, as well as mental structures and dispositions. These include motivational, cognitive, affective, and perceptual mechanisms. Furthermore, in order to accurately understand dysfunction, natural functions of psychical and mental mechanisms must be determined, which requires that one identify the evolutionary purpose that explains its existence. This concept of adaptation is therefore an instance of ultimate (as opposed to proximate) biological explanation for the existence of mental disorders.

Natural functions are therefore effects that explain the existence and structure of naturally occurring physical and mental structures. Wakefield further argues that evolutionary theory provides a better explanation of how a mechanism's effects can explain the mechanism's presence and structure; natural function refers to a casual explanation in terms of natural selection. Since natural selection is the only known means by which an effect can explain a naturally occurring mechanism, evolutionary explanations must underlie all correct ascriptions of natural functions. Evolutionary approaches are therefore the most useful in studies of personality and mental functioning, and they are central to an understanding of psychopathology. The concept of disorder, in Wakefield's view, places two constraints on any theory of mental disorder: the value criterion implies that any successful theory of disorder must link up with the concept of harm; the explanatory criterion implies that any successful theory must offer an account of dysfunctions.

In one example, Wakefield uses Sigmund Freud's repression account of neurotic disorder. According to Freud, repression is designed to provide the benefit of keeping extremely painful ideas and affects from reaching consciousness and impairing the functioning of the organism. However, sometimes repression fails to function properly, and the indirect expressions of repressed material might reach consciousness in the form of neurotic symptoms. Here, repression itself is not the disorder, it is the malfunction of repressive mechanisms and the consequences, such as anxiety, that constitute the neurotic disorder.

Dual Origin Hypothesis
The dual origin hypothesis refers to the development of the cerebral cortex of higher mammals. It is theorized that the cerebral cortex develops in evolution by outgrowth from two primordial structures: the amygdala and the hippocampus, which both generate distinctive ventral and dorsal protogradations, or lines of cortical development. Consequently, the dual origin of the cerebral cortex has significant implications for cortical connectivity. The ventral system, which arises from the amygdala, specializes in the identification of stimuli, assigning meaning, and elaborating the motivational aspects of behavior. The dorsal system, on the other hand, which stems from the hippocampal-induseal cortex, specializes in the representation of spatial environments and for organizing temporal and spatial action. These distinctive yet interactive cortical networks have immense implications for emotion, cognition, and behavior. It is theorized that particular psychiatric symptoms may stem from abnormal processing within the dorsal (time, space) or ventral (meaning-motivation) systems, or from a dysfunction in the functional and interactional balance of the two systems. For example, functional imaging studies have shown repeatedly hyperactivity within ventral prefrontal sectors in obsessive-compulsive disorder (OCD). Conversely, in schizophrenia, research has shown structural pathology localized in specific areas of the dorsal system. These pathologies readily account for schizophrenic symptoms such as disorganization and executive dysfunction.

Genetics
Research into the genetic causes of mental illness has followed the direction of medical research. Various methods have been used to estimate the heritability of mental illness including: clustering of illness in families, higher concordance rates between monozygotic than dizygotic twins, and similarity of adoptee to their biological rather than adoptive relatives. High heritability estimates resulting from these methods point to the importance of genetic substrate passed from generation to generation. However, twin and adoption studies have been relatively uninformative in understanding how many genetic loci constitute such substrate. One of the major questions facing psychiatric genetics pertains to the difficulties in finding a molecular substrate for the presumably large genetic contribution to mental illness.

The high heritability of mental illness is puzzling as it appears to defy natural selection. Mental illness usually has an onset early in reproductive age and is associated with reproductive disadvantage, and according to the theory of natural selection, genetic variants associated with vulnerability to mental illness should be under strong negative selection pressure and therefore be eliminated from the genetic pool. And yet, mental disorders remain common. Moreover, one of the second major questions facing psychiatric genetics is related to the problem of how heritable yet harmful mental illnesses survive the fitness-maximization process of evolution.

The combination of high prevalence and early age onset distinguishes mental illnesses from most medical conditions; the life expectancy and average age of reproduction, which can explain diseases like cardiovascular illness and type II diabetes, is not a plausible explanation for most types of mental illness. There are also important differences in the prevalence of particular mental illnesses across time and space. For instance, bipolar affective disorder is relatively consistent across countries, but the prevalence of unipolar depression varies substantially. Evolutionary theory supposes that the persistence of any genetic trait in a population depends on its potential to increase fitness for individual survival and reproduction, and yet, all types of mental illness are associated with increased mortality (from both natural and unnatural causes) and reduced fertility.

There are two evolutionary plausible mechanisms that together may account for the persistence of harmful and heritable mental illness. The first involves interactions between common genetic variants and environmental factors that have changed throughout evolutionary history. This mechanism is consistent with the general properties of complex biological systems and with individually variable effects of environmental factors on the risk of common mental illnesses. The second mechanism involves a cumulative effect of multiple mildly deleterious pleiotropic mutations that are under negative selection, but are still present in the gene pool due to recent origin.