Schizophrenia is classified by the American Psychiatric Association in the DSM-5 (2013) as a severe and chronic mental disorder characterised mainly by hallucinations, bizarre delusions, disorganised thoughts, and abnormal motor behaviour that impairs social and occupational functioning (Tandon, Gaebel, Barch, Bustillo, Gur, Heckers, Malaspina, Owen, Schultz, Tsuang, Van Os & Carpenter, 2013). The prevalence rate for schizophrenia is about 1%, which is considered high when taking into account the prevalence of deleterious mutations (Jablensky, 1995; Polimeni & Reiss, 2003). Many researchers seek external factors responsible for the occurrence of schizophrenia. Complications during pregnancy, for instance bleeding, diabetes and infections, as well as birth complications, emergency caesarean section, asphyxia, low birth weight or reduced head circumference have been classified as having a significant association with schizophrenia (Cannon et al., 2002).

Schizophrenia is confirmed as a heritable disorder. Many studies have shown a genetic basis for schizophrenia, as its occurrence in families is high. The concordance rate for monozygotic twins is 44.30% and for dizygotic twins 12.08%, according to data gathered by Gottesman et al. (1987). According to Gottesman (1991) and Jablensky et al. (1992), the appearance of schizophrenia appears stable across cultures and over time, regardless of geographical location. Consequently, it has been concluded that schizophrenia was present before the last racial isolation of Australian Aboriginals around 60,000 years ago. Polimeni and Reiss (2003) characterise schizophrenia as an ancient condition.

Once schizophrenia was concluded to have an evolutionary aetiology, the question that emerged was: why, despite many negative characteristics, has the disease not yet become extinct?

Generally, schizophrenia is seen as a burden that families of sufferers must bear, often keeping them at a distance or isolated from society. Social stigma surrounding schizophrenia remains high, with the general population convinced of potential danger from schizophrenic patients (Nichols, 2009).

Research has led to the conclusion that schizophrenia is a disadvantageous by-product of normal brain evolution in the human species. However, many scientists have argued that schizophrenia can also be considered advantageous for those affected, for their kin, and for the whole of humankind in terms of societal evolution (Polimeni & Reiss, 2003).

This paper aims to outline some disadvantageous and advantageous characteristics of schizophrenia from an evolutionary viewpoint and its implications for further research. It is worth mentioning that due to the linkage of Darwinian Theory of Evolution and Mendelian genetics, the work of Huxley, Mayr, Osmond, and Hoffer (1964) popularised contemporary research seeking reasons why schizophrenia has survived for hundreds of thousands of years despite its negative characteristics (Crow, 2008).

First, it must be underlined that genetic research has shown the human DNA code differs from other species, making humankind unique in many aspects. Researchers have identified traits specific to what makes humans susceptible to schizophrenia. One specific trait that differentiates humankind from other species is language.

Human language is the main aspect far more developed than any other species’ communication system. Human language ability relies on cerebral asymmetry and connections between hemispheres. It has been suggested that human brain evolution brought, alongside the appearance of language, a by-product in the form of mental disease – schizophrenia. Crow (2008) argued that the nuclear symptoms of schizophrenia, i.e., hearing voices, passivity, thought insertion, thought withdrawal or broadcast, and delusions (Wing et al., 1974), are in fact language abnormalities. These symptoms classify as anomalies in the pathway linking language production and perception. Consequently, Crow (2008) concluded that the nuclear symptoms of schizophrenia represent a dissolution of the core elements of language. The bi-hemispheric theory (Cook, 1986; Crow, 1998) posits that the evolution of language occurred due to the development of brain asymmetry and the separation of hemispheric functions. Abnormalities in these brain regions can be assumed to play a crucial role in language production and comprehension, thus explaining such nuclear symptoms. In fact, people who suffer from schizophrenia often show reduced or absent brain asymmetry. Studies have linked schizophrenia, a lack of lateralisation, and a tendency towards ambidexterity (Sommer, Ramsey, Kahn, Aleman & Bouma, 2001).

Horrobin (1998) argued that schizophrenia is the illness that made us unique in nature, that made us human. The scientist suggested that human brain size and function are due to lipid metabolism changes and genetic reactions to challenges during the lifespan. Geographical location and ethnicity do not play a role in schizophrenia onset, leading to the conclusion that schizophrenia is a result of normal human brain evolution. The researcher concluded that, in terms of heritability, relatives of a schizophrenic individual have a high probability of onset of mental disorders like schizophrenia, manic-depression, dyslexia, sociopathy and psychopathy, and it is also highly probable that characteristics like creativity, leadership qualities, achievements in many fields, high musical skills and an intense interest in religion are highly heritable (Horrobin, 1998).

In support of the above theory, it is worth mentioning Randall’s (1983) conclusion that schizophrenia symptoms may rely on abnormally distributed connections between specialised brain regions, so behavioural variations can be seen as results of these deviations (Polimeni & Reiss, 2003). The concept of the triune brain, which assumes the human brain is built from three components: the reptilian, the paleomammalian, and the neomammalian (brain stem, limbic system, and cortex respectively), led Millar and Maclean to assume that schizophrenia pathology could be caused by a failure of integration between the limbic system and the cortex (Millar, 1987).

To further address the question of why a disease like schizophrenia persists, advantageous characteristics should be outlined. Schizophrenia has existed for a minimum of sixty thousand years, long before the last racial splitting. It has been argued that schizophrenia has many advantageous characteristics that play a substantial role in human group splitting. According to Silber (2014), a key principle is that group size in every species is optimal. Price and Stevens (1996) argued that schizophrenia plays a crucial role in human group splitting. They concluded that in human group splitting, there are two common modes of dispersal. The first assumes that when a group splits, for instance during the colonisation of America and Australia, the ‘parent group’ influenced newly initiated daughter groups, and existing assumptions were copied. The second kind of dispersal is characterised by adopting rules newly formulated by a new leader. Historically, this can be seen as following a new prophet who established different, favourable views. However, if a person’s assumptions are not favoured by potential followers, they are classified as psychotic (Price & Stevens, 1998). Delusions and hallucinations are the most common symptoms of schizophrenia. According to many studies, sufferers often see themselves as prophets who can lead millions along a new, better path.

Kellett (1973) suggested that schizophrenic characteristics helped early humans during territorial location and in building hierarchy (Polimeni & Reiss, 2003). Schizophrenia genes may have a heterozygous advantage related to territoriality (Kellett, 1973). Schizophrenic characteristics like paranoia, delusions and excessive religiousness have been used by leaders to seed new cultures. History shows individuals characterised by schizophrenic traits; among others, Adolf Hitler, Napoleon Bonaparte or Charles Manson serve as examples of psychotic behaviour that led to the formation of new groups based on new, revolutionary worldviews. The power of their personalities led many followers.

Polimeni and Reiss (2003) proposed shamanism as a form of schizophrenic persons’ ability to conduct and lead religious rituals. This behaviour has been compared to religious practices worldwide. Among the most significant religious leaders are Jesus of Nazareth and Buddha. Rituals practised by shamans are observed globally in other religions. Polimeni and Reiss (2003) suggest the schizophrenic trait of psychosis could be seen as advantageous in the context of shamanism.

Polimeni and Reiss (2003) illustrated balanced polymorphism using the sickle cell anaemia model, comparing it to advantageous characteristics of schizophrenia. The main assumption is that genes associated with deleterious phenotypes can survive if the heterozygote demonstrates some advantage over its associated homozygotes. If some schizophrenic characteristics can be seen as plausible social behaviour, schizophrenia could be seen as a balanced polymorphism.

According to Allen and Sarich (1988), the schizophrenia genotype runs as a balanced polymorphism in the human population. They propose that the decreased fitness of overt schizophrenics must be balanced by a corresponding increase in the fitness, relative to the population as a whole, of individuals who carry the gene, or some part of the polygenic complex, but who do not manifest the pathological condition. However, they did not assume that sickle cell anaemia and schizophrenia are directly comparable.

Creativity and psychosis have been assumed to accompany the evolution of intelligence (Brüne, 2004). Data from the relatively stable genetic population of Iceland have shown that exceptional creativity is connected to relatives of psychotic individuals. Great creativity has been shown in patients with bipolar disorder, and research reveals that schizophrenia and bipolar disorder share some genetic basis.

Anecdotal evidence of genius suffering from mental illness has been noted in recent evolutionary research. The relationship between psychotic episodes and genius is presented in examples like Isaac Newton, who suffered from psychosis in late adulthood, and the prominent Nobel laureate mathematician John Nash, diagnosed with schizophrenia in his 30s.

Research has shown that schizophrenic individuals suffer from reduced fecundity. However, Huxley, Mayr, Osmond, and Hoffer (1964) stated that advantages include higher resistance to allergies, infections, shock, insulin, visceral perforation, and high doses of histamine. Polimeni and Reiss (2003) noted that the schizophrenia paradox is associated with reduced fecundity and high prevalence. Research shows schizophrenic males have a reproductive deficit of around 70%, women around 30% (Silber, 2014). According to Polimeni and Reiss, assumptions of the schizophrenia paradox are considered in most evolutionary hypotheses about the origin of schizophrenia. Nichols (2009) suggested that female schizophrenics’ advantages overweigh male disadvantages, and that affected women do marry, with a possible higher than average fertility rate.

Darwin’s theory of evolution (1859), which underlines the survival of the fittest, became a main attachment point for the aetiology of many diseases. Since then, researchers have conducted studies based on assumptions from On the Origin of Species. Schizophrenia, a serious mental illness with a lifetime prevalence of about 1% (Horrobin, 1998), has come to be seen as originating during the earliest evolutionary modifications of the human brain (Horrobin, 1998; Crow, 2008). Many studies conclude schizophrenia is associated with the human capacity for language, even if language developed gradually. Schizophrenia has a genetic basis, as revealed by twin, family and adoption studies. Evolutionary scientists (Polimeni & Reiss, 2003; Crow, 2008; Horrobin, 1998) conclude schizophrenia is a by-product of normal human brain evolution, involving disruptions in brain asymmetry development and lateralisation. Randall (1983) also suggests that abnormalities of functional connection between specialised areas in the human brain may underlie the symptoms which constitute the schizophrenia syndrome. Polimeni and Reiss (2003) highlighted Randall’s conclusions about establishing new neural pathways through a process of biological trial and error, suggesting neural connections could be supernormal or classified as nonadaptive misconnection. As research demonstrates, schizophrenia cannot be seen only as a brain disorder (Horrobin, 1998; Nichols, 2009). It can be assumed that some schizophrenic individuals have contributed schizophrenic traits to human society, including interest in agriculture, art and music.

Commonly assumed social and cognitive disturbances in schizophrenic individuals lead to a lower ability to distinguish others’ emotions and actions, usually resulting in misunderstanding, psychosis, delusions and hallucinations. It has been proposed that abnormalities in the mirror neuron system are associated with schizophrenia (McCormick, Brumm, Beadle, Paradiso, Yamada & Andersen, 2012). Researchers conducted an EEG study on schizophrenic patients with active psychosis, revealing about 20% greater mirror neuron activity than in a control group (McCormick et al., 2012). Moreover, higher mirror neuron activity correlated with psychotic severity. A study by Abu-Akel (2003) revealed a high association of greater empathy and Theory of Mind abilities in schizophrenic patients with active psychosis. These findings could lead to the conclusion that while psychosis may be a disadvantageous characteristic of human brain evolution, the accompanying advantage could be that schizophrenic individuals tend to have heightened connectivity in brain areas responsible for reading others’ intentions and actions. However, further research should consider these findings and the observation that schizophrenic individuals often overestimate the world around them.

According to Nichols (2009), schizophrenia has come to be seen as a price we have to pay for the essentially human cognitive qualities we possess. Further research into variants of schizotypal traits in relation to possible advantages should be conducted.

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