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Homosexuality

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  • 1. Genetic Factors Predisposing to Homosexuality May Increase Mating Success in Heterosexuals Written by Zietsch et. al By Michael Berman and Lindsay Tooley Evolutionary Psychology 459 – Winter 2010
  • 2. Introduction
    • How do genes predisposing homosexuality affect heterosexuals?
      • Creates a mating advantage
      • Evolution and the maintenance of homosexuality
    • Research suggests sexual orientation is genetically influenced
    • Recent evidence also suggests that homosexual men tend to come from larger families
      • Which has been interpreted as greater fecundity in relatives of homosexual men
  • 3. Background
    • Kin Selection and kin altruism model
      • Classically, this model has been used as an explanation for why heterosexuals might be at an advantage if related to a homosexual
      • It proposes that homosexuals could balance their fitness loss by caring for their relatives thus increasing their inclusive fitness
    • However, there has been a lack of sufficient empirical evidence to support this theory
  • 4. Introduction of Hypotheses
    • A less discussed hypothesis concerns feminine-masculine traits and their presence in homosexuals and heterosexuals
    • Zietsch et. al examined that hypothesis and hypothesized that a number of pleiotropic genes predispose to homosexuality and also contribute to reproductive fitness in heterosexuals
      • It was postulated that the genes that predispose for homosexuality confer a mating advantage in heterosexuals who carry some of the same genes
      • Females are more attracted to males with certain behavioral traits
      • In males, alleles that promote femininity
      • In females, alleles that promote masculinity
      • In order to empirically test the hypothesis they tested for correlations between homosexual traits and mating success in heterosexuals
  • 5. Hypotheses
    • Hypothesis 1: Sex-atypical gender identity is associated both with homosexuality and, in heterosexuals, with an elevated number of opposite-sex partners
    • Hypothesis 2: These associations are due, in part, to the same genetic factors influencing each trait
    • Hypothesis 3: Heterosexuals who have a homosexual twin will have an elevated number of sex partners
  • 6. Methods
    • Large community based twin sample
      • N = 4904
      • Australian twins reared together
      • Aged 19-52
    • Data collected through a mailed questionnaire regarding sexual attitudes and behavior, demographics and personality types
  • 7. Definitions of Measures
    • Sexual orientation:
      • Operationally defined those with any degree of sexual attraction to the same sex as homosexuals, and the associated trait as homosexuality
    • Gender identity:
      • Treated as a continuum of self-reported masculine and feminine traits in women and men respectively
    • Mating Success
      • Defined not as the number of children but rather the number of sexual partners
  • 8. Methods
    • To test for sexual orientation, Kinsey type questionnaire with a 0-7 scale was used
      • “ Which of the following best describes your sexual feelings at present?” with responses ranging from 0-7 (0 = “I am attracted to women only, never men” and 7 = vice versa)
    • To test for gender identity, six dichotomous items were used to assess the degree to which participants’ self-concept was masculine or feminine
      • “ In many ways, I feel more similar to women [men] than to men [women],” “I don’t feel very masculine [feminine]” etc.
    • To test for mating success, the number of sexual interactions with others was reported
      • None, 1 only, 2, 3-5,6-10, 11-20, 21-50, and over 50
  • 9. Results
    • Hypothesis I : Sex-atypical gender identity is associated with homosexual behavior as well as a higher level of sexual partners in heterosexuals
    • 2 key correlations were found to support this hypothesis: (Fig. 1)
    • 1) Among the entire sample, a positive correlation between sex-atypical factors and homosexual identification was observed
    • 2) When examining only heterosexual data, a positive correlation exists between sex-atypical behavior and the number of sexual partners reported
    • Summary: This hypothesis is supported by the data!
  • 10. Figure 1
  • 11. Results, contd.
    • Hypothesis II: The correlations that were observed are due, in part, to the same genetic factors influencing each trait
    • Here, a key observation can be made: Genetic modeling (Fig. 2) reveals that the alleles that code for the previously mentioned correlations are significantly related with one another
    • Thus, a single allele pairing could be the reason for the relationship between gender-atypical behavior and sexual orientation, as well as the number of sexual partners one typically attains
    • Summary: This hypothesis is at least partially supported by the data!
  • 12. Figure 2-Genetic Model
  • 13. Results
    • Hypothesis III: Heterosexuals who have a homosexual twin will have a greater number of sexual partners
    • The results of the survey of the participants revealed significant data in support of heterosexual females having a larger number of sexual partners if they have a homosexual twin. (Figure 3A). The effect was not significant for males
    • When comparing exclusive homosexuals with heterosexuals, a more powerful effect was observed for both males and females, but due to the small sample size for the former individuals, the statistical power is weak (Figure 3B)
    • Summary: The hypothesis is somewhat supported here
  • 14. Figure 3
  • 15. Discussion
    • To summarize, this experiment has sought to derive homosexuality to its genetic factors in order to relate it to alleles that affect mating success
    • In regards to the experiment, the results of the first two hypotheses were significant, however the comparison of twin data was only a tentative support for the previous hypotheses
    • The core question to be answered by this experiment: How has the gene that codes for homosexual traits maintained itself over evolutionary history?
  • 16. Antagonistic Pleiotropy
    • This concept is viable because it explains why a trait that so obviously contradicts Darwinian fitness could have persisted over time
    • The results of this study indicated a positive correlation of occurrence for alleles that code for varying degrees of gender identity and the number of sexual partners
    • Antagonistic pleiotropy is a theory that genes antagonistic to fitness will persist if they occur simultaneously with genes that are beneficial to fitness. Thus, the progression towards genetic fixation is slowed
  • 17. Explanations
    • Alternative explanations for observed heterosexual mating success:
      • Heterosexuals with a homosexual twin may be socially pressured to seek out more sexual partners
      • Also, such individuals could have a greater insight towards the sexuality of the opposite sex which could increase their mating abilities
  • 18. Limitations
    • Due to the relatively low number of reported homosexual behavior (13% for men, 11% for women) and an even lower percentage of reported exclusive homosexuality (2.2 % men, .6% women), the statistical power of the experiment was rather low
    • The method of data collection, the questionnaire, allows for lying or responses that are socially acceptable
    • Lack of evidence describing the link between number of sexual partners and true reproductive success on a present or evolutionary scale
  • 19. Concluding Thoughts
    • This experiment leaves many avenues of future research open, such as:
    • Relation between homosexuality and fecundity of the family
      • Search for evidence in support of Kin selection in this issue
      • Correlation between birth order and prevalence of homosexual traits

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