Schizophrenia may persist because the same traits that enhance fitness up to a point can cause a sharp fitness drop when pushed too far.
Cliff-edged fitness functions describe traits that increase reproductive success until a critical threshold, beyond which fitness plummets.
Natural selection stabilizes traits just below the threshold, but genetic variation causes a small fraction of individuals to exceed it and develop schizophrenia.
A mathematical cliff-edge model estimates a very weak positive selection gradient (0.0135) is sufficient to sustain schizophrenia prevalence.
Schizophrenia genetics are highly polygenic, involving hundreds to thousands of small-effect alleles under balancing selection.
Risk alleles show mixed signals of positive and negative selection depending on their effect sizes and the population’s trait distribution.
Higher polygenic risk scores for schizophrenia are slightly associated with more children among unaffected individuals, consistent with weak positive selection.
Schizophrenia is viewed as an extreme outcome of cognitive and social traits that confer evolutionary advantages in most people.
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