Natural Selection should remove mental illness / Article

Behav Brain Sci. 2006 Aug;29(4):385-404; discussion 405-52.

Resolving the paradox of common, harmful, heritable mental disorders: which evolutionary genetic models work best?

Author information
  • 1Virginia Institute for Psychiatric and Behavioral Genetics, Virginia Commonwealth University, Richmond, VA 23219. Matthew. C. Keller


Given that natural selection is so powerful at optimizing complex adaptations; why does it seem unable to eliminate genes (susceptibility alleles) that predispose to common, harmful, heritable mental disorders, such as schizophrenia or bipolar disorder?

Comment: Natural selection is not a “thinking being”. It is an evolutionary process without intent. Nature does not say, “It’s time to eliminate those harmful, heritable mental disorders”. The disorders are NOT inherited: “susceptibility alleles” may be inherited, but the simple presence of these alleles does not constitute bipolar disorder nor schizophrenia. Bipolar and schizophrenia are the manifestation, or expression, of these alleles becoming active at some time in the carrier’s life.

We assess three leading explanations for this apparent paradox from evolutionary genetic theory: (1) ancestral neutrality (susceptibility alleles were not harmful among ancestors), (2) balancing selection (susceptibility alleles sometimes increased fitness), and (3) polygenic mutation-selection balance (mental disorders reflect the inevitable mutational load on the thousands of genes underlying human behavior). The first two explanations are commonly assumed in psychiatric genetics and Darwinian psychiatry, while mutation-selection has often been discounted. All three models can explain persistent genetic variance in some traits under some conditions, but the first two have serious problems in explaining human mental disorders.

Ancestral neutrality fails to explain low mental disorder frequencies and requires implausibly small selection coefficients against mental disorders given the data on the reproductive costs and impairment of mental disorders. Balancing selection (including spatio-temporal variation in selection, heterozygote advantage, antagonistic pleiotropy, and frequency-dependent selection) tends to favor environmentally contingent adaptations (which would show no heritability) or high-frequency alleles (which psychiatric genetics would have already found). Only polygenic mutation-selection balance seems consistent with the data on mental disorder prevalence rates, fitness costs, the likely rarity of susceptibility alleles, and the increased risks of mental disorders with brain trauma, inbreeding, and paternal age. This evolutionary genetic framework for mental disorders has wide-ranging implications for psychology, psychiatry, behavior genetics, molecular genetics, and evolutionary approaches to studying human behavior.






2 thoughts on “Natural Selection should remove mental illness / Article

  1. I see the problem with natural selection, at this time, as moot. Mental problems seem to have existed for as long as humans have recorded ourselves – at least as far as the Egyptians (no wonder, with the lead in their make-up).
    Today we save every possible human, whether they are capable of any kind of self-care or not. We might have more aberrant humans than ever. We also use plenty of intervention (for those who can afford it) just so we can use our own eggs and sperm to get off-spring. This god-gene that I sometimes read about has to be the case with humans. We are dumber than brick.
    Then we pollute with stuff that causes mental and physical problems. Just look at all of the lead piping still present in Michigan, not to mention the huge spill in Flynn. Oil-spills into drinking water, plastic, DDT.
    Evolution won’t help any problems resulting from that. Unless they create some kind of mutant human capable of surviving such abuse.


    • I think the interesting question is, Why hasn’t natural selection removed “mental illness” if it’s genetically determined and a reproductive disadvantage? Nature doesn’t “judge” certain behaviors to be “mentally defective” and even if certain behaviors jeopardize survival for the individual, that doesn’t mean they won’t reproduce and have unaffected and successful children. The entire “mental illness construct is a bit daffy!

      Liked by 1 person

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