2. Matt Schechter 4/26/05

3. What Paradox? Homosexuals, by definition, don’t reproduce Natural Selection selects for reproductive success How, if natural selection operates on humans, does homosexuality persist?

4. Assumptions Homosexuality is heritable Prevalence and history of homosexuality is constant Homosexuals have decreased reproductive output (fitness)

5. The Nature of the Answers Male Bias Frequency Exclusivity Two Categories of Explanation Direct benefit hypothesis Indirect benefit hypothesis

6. 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?

7. The Science of Homosexuality The Difficulties Political/Social Scientific Nature vs Nurture? Genes vs Environment The two in consort

8. Behavior 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?

9. Biology 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

10. Eve Principle Either XX or XY Fetus Brain Body Male Gender “Seeds” Female Gender “Seeds”

11. Male Differentiation XY Fetus – Heterosexual Development Brain Body Male Gender “Seeds” Female Gender “Seeds” TESTOSTERONE MIH

12. Male Differentiation XY Fetus – Homosexual Development Brain Body Male Gender “Seeds” Female Gender “Seeds” TESTOSTERONE MIH

13. Biology 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

14. Why does this happen? 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

15. Wait just a minute… 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

16. Genetics 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

17. The Effect on Females 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

18. Recall… 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

19. A Possible Answer to the Paradox 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

20. Problems With the Answer 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

21. Beyond The Science 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