西蒙•列维（Simon LeVay）关于同性恋男尸下丘脑的研究和Marc Breedloves关于生者的出生顺序以及手指长度比例研究，都显示出出生前荷尔蒙对性取向决定问题上所产生的影响，前者指出男性同性恋者的女性化趋势，后者则指出同性恋者，不论男性还是女性，都有男性化的趋势。
对非异性恋的研究和试验为另外一个观点开辟了道路：性取向是在孩童时期或更早的时候被固定的。对同性恋双胞胎的研究表明，如果其中一个人是同性恋，那么另一个人有40-60%的机会成为同性恋者；异卵双生的比率为15-30%。对于不是双胞胎的同性兄弟姐妹来说，这个比率是5-10%（参看：http://www.worldpolicy.org/americas/sexorient/twins.html 和 http://researchmag.asu.edu/stories/supporting.html ）。
摘自wikepedia的Biology and sexual orientation条目：
Chromosome linkage studies
Earlier chromosome studies of homosexuality in males have not been replicated, or have had doubt cast on these early suggestions. For example, in 1993, Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families. Hamer et al. found that the gay men had more gay male uncles and cousins on the maternal side of the family than on the paternal side. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles. In another finding, thirty-three of the forty sibling pairs tested were found to have similar alleles in the distal region of Xq28, which was significantly higher than the expected rates of 50% for fraternal brothers. This was popularly (but inaccurately) dubbed as the ‘gay gene’ in the media, causing significant controversy.
However, a later analysis by Hu et al. revealed that 67% of gay brothers in a new saturated sample shared a marker on the X chromosome at Xq28. Sanders et al. (1998) replicated the study, finding 66% Xq28 marker sharing in 54 pairs of gay brothers. On the other hand, two other studies (Bailey et al., 1999; McKnight and Malcolm, 2000) failed to find a preponderance of gay relatives in the maternal line of homosexual men. Also, a study by Rice et al. in 1999 failed to replicate the Xq28 linkage results.
Additionally, Mustanski et al. (2005) performed a full-genome scan (instead of just an X chromosome scan) on individuals and families previously reported on in Hamer et al. (1993) and Hu et al. (1995), as well as additional new subjects. With the larger sample set and complete genome scan, the study found much weaker link for Xq28 than reported by Hamer et al. However, they did find other markers with significant likelihood scores at 8p12, 7q36 and 10q26, the latter two having approximately equivalent maternal and paternal contributions.
^ Rutter, M. (2006). Genes and Behavior. Oxford, UK: Blackwell Publishing.
^ Hamer DH, Hu S, Magnuson VL, Hu N, Pattatucci AM (July 1993). “A linkage between DNA markers on the X chromosome and male sexual orientation”. Science (journal) 261 (5119): 321–7. doi:10.1126/science.8332896. PMID 8332896.
^ Hu S, Pattatucci AM, Patterson C, et al (November 1995). “Linkage between sexual orientation and chromosome Xq28 in males but not in females”. Nat. Genet. 11 (3): 248–56. doi:10.1038/ng1195-248. PMID 7581447.
^ a b Wilson, G.D., & Rahman, Q. (2005). Born Gay: The Biology of Sex Orientation. London: Peter Owen Publishers.
^ Vilain E (2000). “Genetics of sexual development”. Annu Rev Sex Res 11: 1–25. PMID 11351829.