Schizophrenia: An Evolutionary Enigma
Written by Brett Weiss
Schizophrenia affects about 1% of the world’s population and has presented throughout human cultures and ethnicities throughout recorded history (Srinivasan et al., 2016). Furthermore, the disease seems specific to humans. Hallucinations and delusions, thought disorders, and impairments of higher order cognitive functions characterize patients with schizophrenia. Although mechanisms of schizophrenia remain not well understood, heritability of schizophrenia is high, between 60% and 80%, suggesting a strong genetic component (Srinivasan et al., 2016). Many people with schizophrenia do not reproduce (reduced fecundity). Nevertheless, the prevalence of this disease remains stable across generations. This has led to those who study the disease to regard schizophrenia as an “evolutionary enigma (Srinivasan et al., 2016).” Perhaps complex traits exist in families of those affected with schizophrenia that undergo ‘polygenic’ or multi-gene selection. In that case, traits associated with schizophrenia could confer some sort of advantage. The following will explore the hypothesis that higher degrees of ‘creativity’ constitute an evolutionary advantage in families of patients with schizophrenia. If this possibility held some truth, it might help to solve the ‘evolutionary enigma” of schizophrenia. Such a familial evolutionary advantage could explain the stable prevalence of schizophrenia across world cultures and ethnicities.
The theoretical model of schizotypy in psychology posits a continuum of personality characteristics and experiences ranging from imaginative states to psychosis and schizophrenia. Schizotypal personality encompasses a set of personality traits similar to schizophrenia symptoms but to a lesser degree (Wang et al., 2018). People with schizotypal traits often display magical ideation with non-conventional beliefs and accepted causality which is not culturally valid (Wang et al., 2018). Those with schizotypal traits also have distorted perceptions of body and objects along with anhedonia (lack of pleasure), social withdrawal, odd speech, eccentric behavior, and impulsive nonconformity (unstable mood and behavior with regard to social convention) (Wang et al., 2018). Among schizotypy symptoms, magical ideation and perceptual aberration are considered positive schizotypy, while social withdrawal and anhedonia are considered negative schizotypy (Wang et al., 2018). It turns out that many studies have associated high schizotypy with enhanced creativity. Many researchers have agreed upon a model of creativity where mild psychopathology such as schizotypy may facilitate creativity but full psychopathology symptoms associated with psychosis and schizophrenia may damper creativity (Wang et al., 2018).
Wang et al. (2018) performed a study comparing high schizotypal individuals with low schizotypal individuals in creativity tasks. The purpose of the study was to find out what cognitive processes contribute to enhanced creativity in those with high schizotypy. The results of the study indicated that the high schizotypal group outperformed the low schizotypal group in creativity tasks, as expected. Furthermore, the study found that overinclusive thinking mediates higher creativity in the high schizotypal group. Overinclusive thinking relates to broader conceptual boundaries. For instance, if asked whether ‘feet’ would be included in the category of ‘vehicles,’ someone with overinclusive thought might say, “Yes, because feet can take a person from one place to another.” Another factor that the study found to mediate creativity in the high schizotypal group was ‘cognitive inhibition,’ which refers to the mind’s ability to ignore stimuli that are irrelevant. Higher cognitive inhibition found in schizotypal individuals may include a reduction in the effects of past regularities in experience on current perception (Wang et al., 2018). How does schizotypy relate to schizophrenia?
Linscott et al. (2018) performed a study associating increased inheritance of schizotypy in siblings of patients with schizophrenia. According to their study, in the general population, prevalence of positive and negative schizotypy is 7.9% and 11.1%, respectively. The group found that prevalence of positive and negative schizotypy in siblings of patients with schizophrenia was 14.1% and 21.8%, respectively. The authors went on to say that familial loads for bipolar disorder or drug use disorders did not predict siblings’ membership in schizotypy classes; however, familial load for schizophrenia did (Linscott et al., 2018). Hence, according to this study, siblings of individuals with schizophrenia have much higher prevalence of schizotypy than the general population, suggesting a genetic link between schizotypy and schizophrenia and that perhaps a schizotypy continuum exists in families of patients with schizophnria. The proposed predisposition to schizotypy in family of patients with schizophrenia may confer abilities related to creativity which might give an evolutionary advantage. This could explain schizophrenia’s stable prevalence in human populations.
Srinivasan et al. (2016) performed an analysis of genetic information from genome-wide association studies of 80,000 participants with schizophrenia. The group compared genes implicated in schizophrenia with genes that have undergone positive selection (new, advantageous genetic variants sweeping a population) since human divergence from Neanderthals. The group found that the majority of genes implicated in schizophrenia have undergone positive selection evolutionarily since human divergence with Neanderthals. Several of the genes found are involved in cognitive function and associate with language and creativity (Srinivasan et al., 2016). Hence, according the the authors, “…persistence of schizophrenia is related to the evolutionary process of becoming human (Srinivasan et al., 2016).”
Schizophrenia is a complex disorder and entails complex genetic explanations. For instance, many researchers now believe that a large number of genetic risk loci have small effects in the illness (Srinivasan et al., 2016). Could a complex, polygenic trait such as propensity toward creativity explain the stability of schizophrenia prevalence in the human population? No one seems to know the answer for sure. Only further research in the field can provide clues.
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