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
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