The evolution of schizophrenia refers to the theory of natural selection working in favor of selecting traits that are characteristic of the disorder. Positive symptoms are features that are not present in healthy individuals but appear as a result of the disease process. These include visual and/or auditory hallucinations, delusions, paranoia, and major thought disorders. Negative symptoms refer to features that are normally present but are reduced or absent as a result of the disease process, including social withdrawal, apathy, anhedonia, alogia, and behavioral perseveration. Cognitive symptoms of schizophrenia involve disturbances in executive functions, working memory impairment, and inability to sustain attention.
Given the high numbers of individuals diagnosed with schizophrenia (nearly 1% of modern-day populations), it is unlikely that the disorder has arisen solely from random mutations. Instead it is believed that, despite its maladaptive nature, schizophrenia has been either selected for throughout the years or exists as a selective by-product.
Hypotheses
Balancing Selection and Positive Selection Hypothesis
The balancing selection hypothesis suggests that balancing selection, an evolutionary mechanism, has allowed for the persistence of certain schizophrenia genes. This mechanism is defined as maintaining multiple alleles of a gene in the gene pool of a population despite having selective pressures. Heterozygote advantage, a mechanism of balancing selection, is when the presence of both the dominant and recessive allele for a particular gene allow for greater fitness in an individual as compared to if the individual only expressed one type of allele. This mechanism can be seen in the carriers for the schizophrenia gene who express both the dominant and recessive allele. These carriers may express certain advantageous traits that would allow the schizophrenia gene to be selected for. Evidence has suggested a carrier of the schizophrenia gene could experience selective advantage due to their expression of advantageous traits as compared to those who do not express the schizophrenia gene. Studies have shown that some of the carriers for the schizophrenia gene may express adaptive benefits such as a decreased frequency of viral infections. Additional beneficial traits may include a higher IQ, increased creativity, and mathematical reasoning. Due to the presence of these beneficial traits, the schizophrenia gene has not been selected against and has remained prevalent in human development over numerous generations. While the idea of balancing selection hypothesis sounds plausible, there is no substantial evidence in support of this hypothesis. Within the studies that found a positive correlation between specific favorable characteristics and the schizophrenia gene, only a few carriers were tested, meaning that there is no sufficient evidence to assume a direct correlation between these advantageous traits and the carriers of schizophrenia. Although this hypothesis has not yet been substantiated, the advantageous traits that these carriers express could provide a reasonable explanation for why the genes for schizophrenia have not been eliminated.