The social brain hypothesis and social dysfunction: A Darwinian paradox

The social brain hypothesis (SBH) states that the evolution of the primate neocortex to levels beyond what is expected based on total brain size is the result of selection pressure for high levels of social complexity. In situations of selection pressure such as the one suggested by the SBH, genetic variants related to the phenotype in question are assumed to eventually either become lost or reach fixation in the population, reducing the genetic variation related to the phenotype. However, variation in social functioning has been found to be related to genetic variation (Bralten et al., 2021) and reduced social functioning is connected to a wide variety of psychiatric disorders (Porcelli et al., 2019) which are assumed to result in reduced reproductive fitness. Although this genetic variation may appear to be an argument against the SBH, this paradox is a commonly known paradox in evolutionary psychiatry, where genetic variation related to severe psychiatric disorders maintained despite the apparent evolutionary disadvantage.
Hypotheses attempting to explain this paradox include ancestral neutrality, balancing selection and mutation-selection balance, which are compatible with the SBH to variable extents. Recent methodological advances have also created many opportunities to use genetic data to examine evolutionary markers (Song et al., 2021). Using methodologies such as selection pressure timelines and cross-species conservation of genetic variants, researchers can now examine the evolutionary processes involved in the continued existence of genetic variation in social functioning. Findings from such analyses may contribute to our understanding of the evolution of the brain and to our understanding of social dysfunction such as that observed in psychiatric disorders like schizophrenia and autism spectrum disorder