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In this research proposal it contains the biological and environmental factors in gay and lesbian twin, both identical and fraternal. The objective is to find what factors are stronger and can prove homosexuality in both male and female twins. I am using 100 participants and will use a questionnaire format. I will use a preliminary questionnaire and then a post questionnaire. In this research I will use several differen


Monoygotic, same-sex dizygotic, and opposite-sex twins are going to respond to a sexual orientation survey. I will use frequencies of heterosexuals, bisexuals, and homosexuals for the participants to respond to. Sexuality orientation is a fundamental part of human sexuality, men mate with women. Sexual orientation is linked to gender roles, also showing that some origins of sexual orientation could also shed some light on the development of other important sexual differences. This paper is the beginning of my research. I will be looking at the biological and environmental factors in homosexual twins. This will also include gender, which is a big part of this study. Lesbians are not particularly studied much. Also I will be including different types of measures in my methods. This will include the zygosity, sexual orientation, and childhood gender nonconformity. This study is being replicated to help further the results.

Literature Review

The causes of sexual orientation have provoked intense scientific interest. "This interest stems, in part, from the mostly mistaken belief that different etiological accounts of sexual orientation have different social and ethical implications" (Bailey, J., Dunne, M., & Martin, N. 2000, pp 524-536). Research of the studies done has been generally organized in the nature-nurture category. Most of the research today has focused on gay men in particular. Many studies have been done, but the ones that I have chosen for my research involve gays and lesbians in monozygotic (identical) and dizygotic (fraternal) twins. I have reviewed many sources and different types of studies and chose to do my research on the biological and environmental factors of homosexual twins.

Biological or Environmental

In past studies it was not decided if there was a factor called the "gay gene." Michael J. Bailey and Richard Pillard did research on familial studies of sexual orientation in twins. Bailey and Pillard found that the concordance rate for homosexuality was higher in monozygotic twins than dizygotic twins. Furthermore, the concordance rate was higher in dizygotic twins than in non-twin siblings. This suggested that there was a stronger hereditary contribution to homosexual orientation (Bailey, Dunn, & Martin 2000). Many studies show that sexual orientation relies a great amount on genetics. Because both men and women have an X chromosome, scientists believe that "male sexual orientation is influenced by a gene on the X chromosome" (Bailey, J. Dunne, M., & Martin, N. 2000, pp 524-536). Also apart from the twin studies, neurobiologists have just recently identified discrepancies between homosexual and heterosexual men in the hypothalamus and other brain structures that could help determine sexual behavior (Angier, N. 1993).

"Genes don't account for all individual differences in sexual orientation" (Bower, B. 1992, p 117). Because of environmental factors, scientists cannot directly say that someone's sexual orientation is strictly determined by their genes (Eysenck H.J. 1990, p 58). "Hormonal fluctuations in the womb or parental treatment may affect twins differently in ways that have yet to be gauged" (Angier, N. 1993). Genes act in part with biological, psychological, and social environment (Bryne, W. 1993) (Hershberger, S. 1997). L. L. Heston stated that no environmental factors were detected which differentiated the homosexual from heterosexual twins (1968). Although Heston states this, Neville Parkers states otherwise. "Factors other than genetical may play an important part in the etiology of homosexuality" (Parker, 1964). Pillard did a study of two male identical twins that were separated at birth, raised in different families, and were reunited as adults. "These men displayed striking similarities in virtually all aspects of their lives. They even discovered a shared proclivity for wearing leather garments during sex and employing various devices to heighten stimulation. Yet one man was gay, the other heterosexual" (Bower, B. 1992, p 117).


Gender plays an immense role in scientists' experiments. Scientists recall that men have a "gay gene." Although Hamer's scientific articles made it clear that he had not found a gay gene, he did find an association between genetic markers. He found an associations between the "genetic markers on 'X q28' (a segment of the X chromosome with several hundred genes) and the trait on homosexuality in forty pairs of gay brothers" (Rosario 2003, p 40). Hamer tried to show the linkage between genetic markers on the X chromosome and male homosexuality. Although he was able to find some linkage, another paper was done years after and failed to find any linkage between the X chromosome and sexual orientation.

Some scientists feel that genetics plays a role in gay women, while others feel that it is environmental factors, because no actual linkage with genes has been found. In a study done researchers recruited homosexual women with an identical, fraternal, or adoptive sister and interviewed them. Most of the gay women stated that they had masculine traits as children. "Using estimates of female homosexuality in the general population and the assumption that several genes influence sexual orientation, Pillard and Bailey calculate that genes account for half of the individual differences in women's sexual orientation. Scientists refer to this as a heritability estimate" (Bower, B. 1992, p 117).

Most of the research on this topic is done on the nature-nurture aspect. This topic has not widely been studied; this is why I have chosen to research it. While much research has been done on gay men, I have also chosen to do it on homosexual women. Unfortunately, most twin research that has been done involves twins with psychological disorders. This is why I have chosen to do my research on the biological and environmental factors of homosexual twins, and will continue to further my research.



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