Schizophrenia—Evolution of Humanness

Is schizophrenia bound to human evolution? Schizophrenia is a neuro-developmental disorder characterised by delusions, hallucinations, and bizarre behaviours. No other animal displays these symptoms. Depression, addiction, anxiety are all found in other animal species, but not schizophrenia. Schizophrenia is not even found in chimpanzees our most recent evolutionary ancestors. It is inheritable, and highly disadvantageous to survival of the affected person. Given this, schizophrenia should be almost non-existent. Yet it continues to affect a massive 1% of the global population. Something is pushing for the persistence of this disorder and its spontaneous manifestation in humans.

Human evolution separated from the chimpanzees 5.5 million years ago when we walked upright and then acquired language abilities. Language ability developed after 'lateralisation', the separation of brain functions into the left (sequential) and right (parallel processing) hemispheres. The peculiar delusions and hallucinations of schizophrenia can be understood as failure of the complex brain mechanism that enables the speaker to distinguish his thoughts from his speech or that of others. This brain mechanism evolved with lateralisation of brain functions. Loss of brain laterality in schizophrenia has been demonstrated.

Comparison of the gene sequences of early humans and their close evolutionary relatives, the Neanderthals have shown that regions of the human genome that underwent positive selection are enriched by association with schizophrenia. This suggests that schizophrenia susceptibility factors may be a "side effect" of human achievements like language and creative thinking. 

Recent evolutionary modifications in brain wiring and connections may have played a role in the development of schizophrenia in humans. Compared to our closest living relative the chimpanzee, brain connections present only in humans show a higher involvement in schizophrenia. Evolutionary changes in the human brain related to supporting more complex brain functions are paralleled with a higher risk for brain dysfunctions that can manifest as schizophrenia.

However, this genetic susceptibility is actually reducing. A study comparing modern-human-specific gene sites with archaic ones has shown that schizophrenia-risk related genes in modern humans are much less than those in Neanderthals and Denisovans (archaic humans). So negative selection of schizophrenia risk-related genes are probably being gradually removed from the modern human genome.

References

1. https://en.wikipedia.org/wiki/Human_evolution
2. Crow TJ. Is schizophrenia the price that Homo sapiens pays for language? Schizophr Res. 1997;28(2-3):127–141. doi:10.1016/s0920-9964(97)00110-2
3. Crow TJ. Schizophrenia as the price that homo sapiens pays for language: a resolution of the central paradox in the origin of the species. Brain Res Brain Res Rev. 2000;31(2-3):118–129. doi:10.1016/s0165-0173(99)00029-6
4. Srinivasan S, Bettella F, Mattingsdal M, et al. Genetic Markers of Human Evolution Are Enriched in Schizophrenia. Biol Psychiatry. 2016;80(4):284–292. doi:10.1016/j.biopsych.2015.10.009
5. van den Heuvel MP, Scholtens LH, de Lange SC, et al. Evolutionary modifications in human brain connectivity associated with schizophrenia. Brain. 2019;142(12):3991–4002. doi:10.1093/brain/awz330
6. Liu C, Everall I, Pantelis C, Bousman C. Interrogating the Evolutionary Paradox of Schizophrenia: A Novel Framework and Evidence Supporting Recent Negative Selection of Schizophrenia Risk Alleles. Front Genet. 2019;10:389. Published 2019 Apr 30. doi:10.3389/fgene.2019.00389

How do I know if my partner is cheating on me - evolution, sexuality and relationships

Last week I gave a talk on Personality and Sexuality. One question that cropped up was - "How do I know if my partner is cheating on me?" By chance the next morning there was a newspaper article attributing uncommitted sex, one-night stands, and acts of infidelity to genetics.

Individuals who have frequent sex with multiple partners are psychologically different from those who have frequent sex with single partner (Simpson & Gangestad, 1991). This sociosexual difference places individuals in two groups

1. Unrestricted sociosexual types, have a higher number of sexual partners, and one night stands. They have a permissive attitude to casual and uncommitted sex.
2. Restricted sociosexual types, require greater closeness and commitment before sex with their romantic partner.

 Genetics plays an important role in sociosexuality

Genes evolve to enhance survival of the species. From an evolutionary viewpoint females reproduce with a mate who will invest in their offspring and produce offspring with an advantaged genetic makeup. Female socio-sexual orientation exists because males vary in the quality of their genes.

• Unrestricted type females benefit from passing the genes of men with greater reproductive success to their own offspring. To induce males to invest in their offspring they frequently engage in pretense and deceit, traits that are strongly associated with unrestricted sociosexuality.
• Restricted type females benefit from caring males who limit reproductive efforts to the females own offspring.
• Male sociosexuality evolved through competition. Males who do not succeed with an unrestricted socio-sexual orientation become Restricted.

How do you know your partner's sociosexuality?

Lynda Boothroyd showed that observers were able to identify restricted vs. unrestricted individuals from cues in thier faces. My audience successfully differentiated the Restricted and Unrestricted pairs from the same picture. Test yourself.

What does this mean for a relationship?

• Restricted socio-sexual personalities are more willing to remain in an unsatisfactory marriage. They are less likely to be drawn out of such a relationship by attractive alternate partners.
• For Unrestricted individuals long term stability in the relationship depends on the extent to which the partner is highly attractive and possesses high social visibility. Decisions to continue or terminate a relationship depend more on changes in the partners physical attractiveness and social status.

However, personality is not all in the genes. Genes contribute to the biological aspect of personality - temperament. Personality also has an acquired aspect - character. When fully developed, character defines the mature personality.

How do I know if my partner is cheating on me?

To return to the question. Asking the question indicates there are already boundaries being overstepped. It indicates an erosion of trust - a core component in any long-term relationship. That is what needs to be addressed.

Jeffry A Simpson, Steven W Gangestad. Personality and sexuality: empirical relations and an integrative theoretical model. In: Sexuality in close relationships. Kathleen McKinney, Susan Sprecher Editors. Lawrence Erlbaum Associates. New Jersey. 1991:71-92.