Schizophrenia affects about 1% of people and reduces reproductive fitness, posing an evolutionary puzzle.
Cliff-edged fitness functions propose that certain traits boost fitness up to a threshold, beyond which fitness drops sharply.
Evolution may favor cognitive and social traits linked to schizophrenia risk alleles below the threshold, offering advantages.
Mathematical models estimate a very weak positive selection gradient (0.0135) is enough to maintain schizophrenia risk genes.
Genome studies find signals of both positive and negative selection on schizophrenia-related genetic variants over time.
Higher polygenic risk scores for schizophrenia correlate with slightly more children, supporting weak fitness benefits below the clinical threshold.
Schizophrenia likely emerges as an extreme byproduct of evolution selecting for subthreshold cognitive and social advantages.
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