Homosexuality in Humans: Nurture or Nature?
Human sexuality has been discussed over millennia, and many theories have been explored regarding the causes of sexual orientation. Leading evidence suggests a mixture of both biological and psychosocial factors.
Hormonal Exposure in Utero
In some case studies of women with Congenital Adrenal Hyperplasia (CAH), where they were exposed to high levels of testosterone in utero, there a significantly increased rate of non-heterosexual attraction, compared to women without CAH ((http://www.ncbi.nlm.nih.gov/pubmed/25172350#).
It is particularly interesting that there is evidence that this exposure to masculinity hormones in early development seems to increase the prevalence of bisexuality specifically, rather than exclusive homosexuality (http://www.ncbi.nlm.nih.gov/pubmed/7560936).
The Gay Gene
The Northshore Research Institute in the United States has recently conducted a study on the genetics of 409 pairs of homosexual brothers. They have identified two specific genetic regions (Xq28 identified in 1993, and 8q12 identified in 2005) that are linked to homosexuality. Although other single nucleotide polymorphisms were frequently identified as being present in the 818, these two were the most commonly found.
Fraternal Birth Order Effect
Some studies have been published that show men with older biological brothers are more likely to identify as gay, rather than men with older female siblings, or men with no siblings. These papers have shown that the likelihood of being gay increases by approximately 33% with each additional older brother. The same is not seen in cases where older brothers have been adopted into the family. (http://classes.biology.ucsd.edu/bisp194-1.FA09/Blanchard_2001.pdf).
It is likely that homosexuality in humans is as a result of a complex mix of hormonal, genetic and social factors, but as of yet there remains no definite answer. It has been established, however, that homosexuality has not been identified as a disease by the World Health Organisation since 1990, and few people have expressed the belief that their sexual orientation is a choice.