Evolutionary approaches to schizophrenia

Schizophrenia is a mental illness characterized by persistent hallucination, delusions, paranoia, and thought disorder. These experiences are evident in multiple sensory modalities and include deviation in all facets of thought, cognition, and emotion. Compared to other psychological disorders like major depressive disorder (MDD) and general anxiety disorder (GAD), schizophrenia has significantly higher heritability. It has been found present cross-culturally, and it almost always has 0.1% prevalence rate in a given population, though some studies have cast doubts on this result. Schizophrenia has also been hypothesized to be unique to human beings and has been existing for a long time.[1]

Schizophrenia has raised a perplexing question for evolutionary psychologists and anthropologists. Why has such a debilitating disorder persisted for so long, with such frequency? This paradox has been referred to as the schizophrenia enigma, and many evolutionary models have been created to explain it.

Evolutionary approaches reflect how human genes can change over time. Under evolutionary pressures, certain phenotypes are favored, and this selection influences the frequency of genes that control these phenotypes. Evolutionary approaches have been used to try to explain the schizophrenia enigma. Major views include schizophrenia as a disadvantageous byproduct of human evolution and schizophrenia as evolutionarily beneficial.[2]

Main explanations

There are mainly two views explaining the persistence of schizophrenia under selective pressures. One argues that schizophrenia is a byproduct of evolution that occurs unavoidably, and the other argues there are evolutionary benefits countering the effects of the costs of this disorder.

The disadvantageous byproduct view sees schizophrenia as functionally disabling. This view argues that schizophrenia has occurred when the human brain evolves to possess advanced cognitive power. Models under this view commonly posit that the occurrence of schizophrenia is closely linked to human cognition.

Another major view argues that schizophrenia’s existence and prevalence indicate certain evolutionary benefits. This could be advantageous to the individuals suffering from this disorder, their kin, or the social groups that the afflicted belong to. The benefits of schizophrenia are essentially counter-balancing the negative selective effects of the costs. Thus, schizophrenia has been preserved under evolutionary selections with consistent prevalence.

Schizophrenia as by-product of human evolution

The disadvantageous byproduct view hypothesizes that schizophrenia started to occur when human diverged from close primates. This view believes that schizophrenia symptoms are the extreme versions of normal social behaviors.[3] Examples like delusions indicate that there are similar cognitive processes, which on a less severe level can be greatly beneficial. Symptoms of schizophrenia that are at the undesirable extreme of normality, however, result in more harm than benefits.

Some also believe that schizophrenia is unavoidable in the evolution of human brain and it results from a trail process in human biology.[4][5] Although this view is favored by some, it is also criticized for neglecting the negative selective pressure as well as the inability to explain the maintenance of this disorder over time.[2]

Some researchers also hypothesize that schizophrenia has close relations with human language development.[6] This is mainly supported by the significant language dysfunction that shows up in schizophreniacs.

Another related model argues that schizophrenia results from certain inabilities to buffer the deleterious effects of mutations.[7] This model of developmental instability is compatible with the stress diathesis model.

For those who view that schizophrenia and other frontal lobe disorders are connected with delayed cerebral maturation, this pathophysiological model explains schizophrenia as one unwanted end of normal variation.[8]

Schizophrenia as advantageous to individual, kin, and group

Contrary to the previous models, there are also models rooted in theories which view schizophrenia as possessing beneficial powers to individuals, kins, or groups of the patients. This is used to mainly explain the prevalence and persistence of schizophrenia over time and across culture. Researchers using these models argue that the existence of schizophrenia in human is not random. This view sees some benefits in promoting the fitness of individuals, kins, or groups and thus enhancing the survival chances. The benefits of schizophrenia are likely to have been buffering the deficits of the disorder and thus balancing the selective pressures. This is primarily based on the 1% prevalence rate, which is higher than common mutation rates.[2]

Benefits to individuals

For researchers who see individual advantages in schizophrenia, they believe that the disorder itself can make individuals more tolerant towards harmful factors like shock and infections.[9] This view has not been substantiated.[2] Another related model suggests that certain traits in schizophrenia may have helped with developing territorial instincts, and thus becoming advantageous to humans.[10] However, this has been criticized by the idea that hierarchal structures are of more importance in human societies compared to territorial ones.[11]

Benefits to kin

More researchers support the idea that schizophrenia brings evolutionary benefits to the kins by enhancing the adaption of relatives.[2] Researchers hypothesize schizophrenia to be a homozygotic condition, and that the asymptomatic heterzygotes are favored over average people.[12] However, these advantages have not been substantiated.[2]

One study in favor of this beneficial kins model argues that female children of patients with schizophrenia have increased survival.[13] Similarly, there is a report indicating less incidence of virus infections among relatives of schizophrenia patients.[14] However, criticism remains because of the failure to replicate such results.[2]

This advantageous view of schizophrenia partially corresponds to the public fantasy of mental illness patients being these tormented geniuses. In support of this, superior academic success among relatives of schizophrenia patients has been found.[15] This suggests possible adaptive benefits. Criticism over this model mainly addresses the lack of evidence to suggest creativity-mental illness linkage.[16]

Benefits to groups

Group advantages have also been considered in this general beneficial model. Many hypotheses draw a connection between schizophrenia symptoms to the shamanic characteristics seen in religious or spiritual leaders. Such view observes a universally present shamanism or shaman-like behaviors and argues that there is a need to consider the genetic root of such behaviors.[2] One related hypothesis argues that schizophrenia helps maintain charismatic leaders using certain symptoms like paranoia and delusions. Due to the altruistic nature of leadership, schizophrenia is selected to be maintained and bring benefits to the social group.[17]

Schizophrenia as sexual selection indicative factors

Unlike viewing schizophrenia as a by-product of brain evolution, one model presents schizophrenia to be one extreme of a sexually selected fitness indicator.[18] This model hypothesizes that schizophrenia to be a side effect in sexual selection for certain traits.

Sexual selection concerns the mating choices of human and other animals. Traits that function as fitness indicators are those revealing potential benefits rooted in genetic qualities. While choosing potential mates, animals go for those with better fitness indicators to ensure better benefits for them and their offsprings. These indicators can be morphological traits as well as behavioral traits.[18] Peacock’s tail[19] and nightingale’s courtship songs[20] are examples of the two kinds. Sexual selection studies have shown that male height, muscularity, and facial structure, female breasts and buttocks are important indicators. Previously introduced Crow[6] and Randall[5] partially integrated the idea of sexual selection in their models to explain schizophrenia.

Fitness indicatory theory, a branch in sexual selection theory, has been used to explain the evolutionary origins of schizophrenia.[18] This model hypothesizes schizophrenia to be at the undesirable and abnormal end of mental and behavioral traits which functions as fitness indicators. In this model, the fitness indicator related to schizophrenia is courtship ability, which involves verbal and behavioral aspects. Failure of developing this ability means impaired communication skills, which can be seen in patients with schizophrenia.

Compared with other models, this one better explains the post-puberty onset of schizophrenia and frequent cases beginning at ages correlated with peak mating efforts. Evidence has been found that patients with schizophrenia have reduced marriage and reproduction rates.[21]

Other studies using evolutionary explanations

There are other studies using evolutionary approaches to explain schizophrenia, but their main focus is not explaining the schizophrenia enigma. The idea of a schizophrenia enigma is based on the acknowledgement of a 1% consistent prevalence rate across culture, and not all scholars agree to it.

Some scholars focus on explaining the variance of prevalence rates among certain societies. The biocultural approach is used to try to explain is variance.[22] This model primarily looks at the interaction between genetic heritage and cultural influences and how it impacts the onset of schizophrenia. Studies like this still take into account evolutionary factors but do not accept the schizophrenia enigma.

See also

References

  1. ↑ Picchioni, Marco M.; Murray, Robin M. (2007-07-12). "Schizophrenia". BMJ. 335 (7610): 91–95. doi:10.1136/bmj.39227.616447.BE. PMC 1914490. PMID 17626963.
  2. 1 2 3 4 5 6 7 8 Polimeni, J; Reiss, J (2013). "Evolutionary Perspectives on Schizophrenia". The Canadian Journal of Psychiatry. 48 (1): 34–39. doi:10.1177/070674370304800107. PMID 12635562.
  3. ↑ Farley, J. D. (March 1976). "PHYLOGENETIC ADAPTATIONS AND THE GENETICS OF PSYCHOSIS". Acta Psychiatrica Scandinavica. 53 (3): 173–192. doi:10.1111/j.1600-0447.1976.tb00073.x. ISSN 0001-690X.
  4. ↑ Randall, P.L. (March 1983). "Schizophrenia, abnormal connection, and brain evolution". Medical Hypotheses. 10 (3): 247–280. doi:10.1016/0306-9877(83)90114-7. ISSN 0306-9877.
  5. 1 2 Randall, P.L. (March 1998). "Schizophrenia as a consequence of brain evolution". Schizophrenia Research. 30 (2): 143–148. doi:10.1016/s0920-9964(97)00143-6. ISSN 0920-9964.
  6. 1 2 Crow, T (March 2000). "Schizophrenia as the price that Homo sapiens pays for language: a resolution of the central paradox in the origin of the species". Brain Research Reviews. 31 (2–3): 118–129. doi:10.1016/s0165-0173(99)00029-6. ISSN 0165-0173.
  7. ↑ Yeo, Ronald A.; Gangestad, Steven W.; Edgar, Chris; Thoma, Robert (October 1999). "The evolutionary genetic underpinnings of schizophrenia: the developmental instability model". Schizophrenia Research. 39 (3): 197–206. doi:10.1016/s0920-9964(99)00074-2. ISSN 0920-9964. PMID 10507512.
  8. ↑ Saugstad, Letten F. (October 1999). "A lack of cerebral lateralization in schizophrenia is within the normal variation in brain maturation but indicates late, slow maturation". Schizophrenia Research. 39 (3): 183–196. doi:10.1016/s0920-9964(99)00073-0. ISSN 0920-9964.
  9. ↑ Huxley, J; Mayr, E; Osmond, H; Hoffer, A (1964). "SCHIZOPHRENIA AS A GENETIC MORPHISM". Nature. 204: 220–1. PMID 14212411.
  10. ↑ Kellett, J. M. (1973-04-21). "Evolutionary theory for the dichotomy of the functional psychoses". Lancet. 1 (7808): 860–863. ISSN 0140-6736. PMID 4123405.
  11. ↑ De Waal, F (1996). Good Natured. Cambridge: Harvard University Press.
  12. ↑ Kuttner, R; Lorincz, A; Swan, D (1967). "The Schizophrenia Gene and Social Evolution". Psychological Reports. 20 (2): 407–412. doi:10.2466/pr0.1967.20.2.407. PMID 6042998.
  13. ↑ ERLENMEYER-KIMLING, L. (November 1968). "Mortality Rates in the Offspring of Schizophrenic Parents and a Physiological Advantage Hypothesis". Nature. 220 (5169): 798–800. doi:10.1038/220798a0. ISSN 0028-0836. PMID 5698756.
  14. ↑ Carter, M.; Watts, C. A. (April 1971). "Possible biological advantages among schizophrenics' relatives". The British Journal of Psychiatry. 118 (545): 453–460. ISSN 0007-1250. PMID 5576646.
  15. ↑ Karlsson, JL (1974). "Inheritance of Schizophrenia". Acta Psychiatr Scand. 247 Suppl: 77–88. PMID 4533231.
  16. ↑ Waddell, C (1998). "Creativity and Mental Illness: Is There a Link?". Can J Psychiatry. 43 (2): 166–72. doi:10.1177/070674379804300206. PMID 9533970.
  17. ↑ Stevens, A; Price, J (2000). Evolutionary Psychiatry. London and Philadelphia: Routledge.
  18. 1 2 3 Shaner, Andrew; Miller, Geoffrey; Mintz, Jim (2003). "Schizophrenia as one extreme of a sexually selected fitness indicator". Schizophrenia Research. 70: 101–109.
  19. ↑ Petrie, M (1994). "Improved growth and survival of offspring of peacocks with more elaborative trains". Nature. 371 (6498): 598–599.
  20. ↑ Andersson, M (1994). Monographs in Behavior and Ecology. New Jersey: Princeton.
  21. ↑ Hilger, Tamara; Propping, Peter; Haverkamp, Fritz (1983-07-01). "Is there an increase of reproductive rates in schizophrenics?". Archiv für Psychiatrie und Nervenkrankheiten. 233 (3): 177–186. doi:10.1007/BF00343595. ISSN 0003-9373.
  22. ↑ Sullivan, Roger J.; Allen, John S.; Nero, Karen L. (April 2007). "Schizophrenia in Palau". Current Anthropology. 48 (2): 189–213. doi:10.1086/510474. ISSN 0011-3204.
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