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Max Marty's avatar

I would look back to works like Robert Axelrod's The Evolution of Cooperation for an answer to this question. If you think of psychological traits as strategies being played by competing sets of genes - then it pays to try explore the space of possibilities to look for new strategies to play to one's advantage. The bigger and faster the environmental (including social) changes are happening, the more it pays to look for new angles to maximize private gains within that environment. Since we're living in particularly tumultuous times relative to the span of human evolution, it pays to look far and wide for social/psychological niches to exploit within the social environment.

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Mark's avatar

For some traits, possibly including schizophrenia, it's likely just a matter of zygosity. Having one copy of a particular variant may be advantageous (e.g., make you smart) while two copies make you schizophrenic. The homozygous disadvantage may - from an evolutionary standpoint - be a price worth paying, since, if we think of it as a dosage effect, a variant that is 'safe and effective' when there are two copies may do nothing much when there's only one copy, and as population geneticists will tell you, a mutation with no fitness advantage when heterozygous is highly unlikely to succeed, since it requires two people with it to mate and produce a homozygous offspring.

With complex traits resulting affected by many different loci, one inevitably ends up with a continuous distribution, and the 'optimal distribution' in terms of producing the highest avg. fitness population will still consistently produce a good many individuals who are suboptimally extreme, and the population doesn't necessarily converge to everyone having the same genotype because the homozygote disadvantage discourages the allele frequency of the beneficial variants from getting too high.

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