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

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

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

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

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

Methods Large community based twin sample Australian twins reared together Aged 19-52 Data collected through a mailed questionnaire regarding sexual attitudes and behavior, demographics and personality types

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

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

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!

Figure 1

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!

Figure 2-Genetic Model

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

Figure 3

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?

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

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

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

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