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Homosexuality:. A Darwinian. Paradox?. Matt Schechter 4/26/05. What Paradox?. Homosexuals, by definition, don’t reproduce Natural Selection selects for reproductive success How, if natural selection operates on humans, does homosexuality persist?. ?. Assumptions.

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A Darwinian


Matt Schechter



  • Homosexuals, by definition, don’t reproduce

  • Natural Selection selects for reproductive success

  • How, if natural selection operates on humans, does homosexuality persist?



  • Homosexuality is heritable

  • Prevalence and history of homosexuality is constant

  • Homosexuals have decreased reproductive output (fitness)


  • Male Bias

    • Frequency

    • Exclusivity

  • Two Categories of Explanation

    • Direct benefit hypothesis

    • Indirect benefit hypothesis


  • Homosexuality is a byproduct of a trait that has fitness benefits

    • Does this ever happen?

  • Selection acts on biology, so what have we discovered about the causes of homosexuality?


  • The Difficulties

    • Political/Social

    • Scientific

  • Nature vs Nurture?

    • Genes vs Environment

    • The two in consort


  • A blend of male and female

    • Female characteristics

      • Gender-atypical childhood behavior

    • Male characteristics

      • Promiscuity in male homosexuals

      • Male gender identification

  • Is there a biological basis for these observations?


  • Female brain characteristics present in male homosexuals

    • LH Response

    • SCN Nucleus

  • Cause of these characteristics

    • Brain development occurs in utero

    • Gender is defined in utero during sexual differentiation

      • Eve principle


Either XX or XY Fetus

Female Gender “Seeds”

Male Gender “Seeds”




XY Fetus – Heterosexual Development

Female Gender “Seeds”

Male Gender “Seeds”






XY Fetus – Homosexual Development

Female Gender “Seeds”

Male Gender “Seeds”






  • Homosexuality and birth order

    • Homosexuals tend have older male siblings

      • More older brothers means higher chances of homosexuality in younger siblings

      • Females are unaffected

    • The Mechanism

      • Maternal immune response provoked by H-Y antigens

      • One of the H-Y antigens is MIH


  • Maternal immune response to MIH is greater with each male child born

    • More femaleness left in brain) (defeminization is decreased) with each male child

  • Variations in sexual personality among male siblings are advantageous for survival

    • Don’t kill each other during childhood

    • Don’t compete for similar females

  • Homosexuality is this advantageous process gone too far


  • But why does this process go too far?

  • And, if this physiological cause of homosexuality increases with the amount of sons born, why aren’t all males who have a given number of older brothers gay?

  • By the same token, why are some only-children gay?

    • An individual’s genes may determine the degree of susceptibility to these effects


  • A “gay gene”

    • Maternal heritability

    • Xq28

      • 33 of 40 gay brothers shared the same gene in the same region on the X-chromosome

  • The X chromosome

    • Males get it only from their mothers

    • All X-linked traits are found twice as often in females, so the the selection of these traits depends more on their effect on females than on males


  • What might Xq28 or similar x-linked genes do to females?

    • The maternally related females of gay men have larger families than paternally related females

      • Ex. My mom’s sister has more kids than my dad’s sister

      • Ex. The daughters of my mom’s sister have bigger families than my do the daughters of my dad’s sister

    • This is clearly advantageous


  • Homosexual male brains are more feminine than heterosexual male brains

  • The defeminization of developing male brains varies by birth order and is advantageous

  • This variation has a genetic component that may be due to a group of genes

  • AND…

    • There is at least one gene, located on the x-chromosome that influences homosexuality

    • An X-linked gene that is related to homosexuality also has a beneficial effect on the fertility of females related to homosexuals

  • Homosexuality

    • In homosexual males, the same trait that increases fecundity in females causes too much susceptibility to the normally-advantageous defeminization-limiting immune response

    • The increase in the fertility of females with this trait is more than enough to keep the x-linked trait in the gene pool

    • It is likely that a group of genes control this trait, and different combinations produces more/less severe effects


    • What about lesbians?

    • What about bisexuality?

    • What about first-born gay sons with straight younger brothers?

    • There’s no evidence for genes that increase female fecundity

    • There’s no evidence for genes other than Xq28 that affect homosexuality


    • Ethics of understanding the mechansim

      • Genetic/hormonal surgery

      • Prenatal screening

    • Implications of understand the evolutionary basis

      • Homosexuality as a product of success?

      • Choice in behavior?

    • Political implications

      • Don’t expect revolution