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Schizophrenia, the psychotic disorder marked by hallucinations, delusions and cognitive disorganization, affects roughly 1 percent of the U.S. population. Many of those afflicted, however, also have reduced reproductive fitness, which means they are less likely to pass a genetic profile associated with the condition onto their offspring.
Therefore it’s genetic persistence must indicate a positive selection based on some adaptive advantages (Erlenmeyer-Kimling). These advantages may be (Polimeni and Reiss)
- advantages to human brain development although the condition itself when expressed, perhaps epigenically, is a disadvantage,
- an evolutionary advantage from the condition itself.
The first explanation, a result of brain development, may be related to the “kluge” or Rube Goldberg nature of brain evolution. Over a rather short evolutionary time span, cognitive cortical expansion resulted in some genetic mismatches that persisted, although not always expressed.
The second explanation involves many theories including that which states there is a relation between schizophrenic genes and exceptional abilities. Steve Dorus, an evolutionary geneticist at the University of Bath in England analyzed human DNA from several populations around the world and primate genomes dating back to the shared ancestor of both humans and chimpanzees. He and his colleagues reached the striking conclusion that several gene variants linked to schizophrenia were actually positively selected and remained largely unchanged over time, suggesting that there was some advantage to having them.
Schizophrenia can be explained by a lot of individual alleles (variations of genes). There are many different loci that impact the actual manifestation of the disease so an entire range of neurodevelopmental processes may be effected by these genes in addition to the expression of schizophrenia.
Erlenmeyer-Kimling, L.; William Paradowski (Nov–Dec 1966). “Selection and Schizophrenia”. The American Naturalist 100 (916): 651–665.
Polimeni J, Reiss J. Evolutionary perspectives on schizophrenia. Canadian Journal Of Psychiatry. Revue Canadienne De Psychiatrie [serial online]. February 2003;48(1):34-39.
Crespi, B., Summers, K. & Dorus, S., et al. Proc. R. Soc. B doi:10.1098/rspb.2007.0876 (2007)