Schizophrenia is a psychiatric disorder that manifests in a variety of ways, including disorganized thoughts, hallucinations, delusions, and social withdrawal. The schizophrenia paradox is described by the fact that there is a lower reproductive rate of those with schizophrenia (about 50% lower compared to a healthy population) than prevalent in the population. In every culture, schizophrenia is prevalent in about 1% of the population, a greater percentage than can be accounted for by mutation.
Studies have indicated that schizophrenia occurs more through genetic inheritance than environmental influence. Adopted children with biological relatives with schizophrenia have an increased risk of expressing the disorder. In 40% of identical twins, if one has schizophrenia, so does the other, compared to only 15% in fraternal twins. If individuals that express this mental disorder do not often have children, then how is it that nature has not selected for the extinction of schizophrenia, especially when it has had thousands of years to fade from the gene pool?
Cases of schizophrenia have been recorded in the population for at least 60,000 years. The articles reviewed addressed several possible theories as to why schizophrenia still exists. Some researchers have attempted to explain schizophrenia in terms of irregularities in brain development. Because schizophrenia is most commonly expressed in the late teens or twenties, and more prevalent in men than in women, researchers have examined how the rate of maturation of the brain might contribute to schizophrenia.
Throughout youth, neurons in the human brain are pruned to get rid of unused neural pathways and strengthen pathways that are used more often. Genes that prolong the maturation period or change the rate of maturation could result in over-pruning, which would decrease neural connectivity (Brune 2003, Polimeni 2003). This could account for why schizophrenia is more prevalent in men than in women since men have a more delayed maturation period, allowing for a longer time for pruning.
If his is the case, one would predict that hallucinations that occur earlier in the onset of schizophrenia would decrease in time as more pruning occurs. This theory is compatible with cases of chronic schizophrenia in which hallucinations do in fact decrease over time. The neoteny-based theory also focuses on brain structure and development to explain the evolutionary basis for schizophrenia. The lack of playfulness and curiosity (thought to be neotenous traits) in schizophrenic patients may be due to a revert back to the earlier brain of human ancestors. Read about role of extinction in evolution
Millar (1987, as cited in Brune 2003) even proposed that schizophrenia is related to insufficient suppression of the reptilian brain (the limbic lobe). This would mean there’s some kind of malfunction in the neocortex. Sexual selection for intelligence led to cerebral asymmetry. Increased cerebral flexibility and variation in brain function came with the “disadvantageous byproduct” of personality disorders and psychosis. One theory proposes that new neural pathways established in the brain have been advantageous, but are randomized so that nonadaptive “misconnections” are also formed.
Such trial and error would produce various results, potentially producing characteristics that could be classified as schizophrenia. However, the theory that schizophrenia is a random phenomenon fails to address the fact that schizophrenia and other mental disorders have a specific, organized array of symptoms. Diet may have been a factor in the evolution of schizophrenia. In the last two million years, some essential fatty acids have been more available which may have led to an increase in neural microconnectivity (Polemeni 2003).
Such an increase could potentially result in over-excitability in some areas of the brain, which may be associated with paranoia or hallucination. Conversely, the fact that those with schizophrenia have lower rates of reproduction may be offset by possible benefits associated with schizophrenia genes such as resistance to shock, allergies, and infection. Other traits associated with schizophrenia, such as creativity and remaining alert with low levels of stimulation, may have been selected for as territorial instincts in early man (Polimeni 2003).
The argument against this theory is that ancestry of Homo sapiens has been more hierarchical than territorial. Schizophrenia may also have favored early man due to the benefit of high energy observed in mania and the awareness and vigilance of paranoia. One theory of schizophrenia proposes that it is a homozygous trait, but heterozygous carriers have some sort of benefit. An example of this would be sickle-cell anemia as a homozygous trait, but heterozygous carriers of the gene benefit from a resistance to malaria.
Genes that provide an advantage when heterozygous can be selected for and remain in the gene pool even though they are detrimental in homozygous cases. A sample from state hospitals in New York observed that female children of parents with schizophrenia had an increased survival rate. However, three previous studies of the same kind found no such correlation. Along the same lines, Carter and Watts (1971, as cited in Brune 2003) found that relatives of patients with schizophrenia had significantly lower rates of viral infection.
Another study found that relatives of patients with schizophrenia had superior academic success. Genius and mental illness have always had a close relationship. Examples of this include Albert Einstein, whose son had schizophrenia, and Sir Isaac Newton, who was diagnosed with schizophrenia when he was 51. Schizophrenia is also linked to increased creative potential, which would be another benefit of the gene. It’s possible that the schizophrenia genes themselves provide the advantage or it may be a case of pleiotropy where schizophrenia genes are closely linked to the genes that provide the advantage.
Another theory is that, historically, characteristics of schizophrenia have led to benefits in group settings due to nonconformity. Not sharing biases and misconceptions of the rest of group, but having a unique point of view could provide advantage. It can benefit the society as a whole when some individuals offer a different perspective. Prevalence of religious-based delusions in schizophrenia supports the notion that schizophrenia is linked to shamanism. Religious rituals and shamanism have occurred in all cultures since the beginning of hunter-gatherer societies.
In this sense, the tendency of shamanism to remain in a single family is supported by the heritability of schizophrenia. There are many theories of the evolution of schizophrenia, but most of them either have gaps in the argument; support the development of some characteristics of schizophrenia, but conflict with others; or do not have enough empirical evidence to support their claims. The current paper only scratches the surface of the proposed explanations of schizophrenia, summarizing a few of the more widely speculated theories.