To say that this topic is controversial would be an immense understatement. Gay and lesbian persons have been the subject of much research and debate. At the core of the issue is whether people are born homosexual with homosexual tendencies, or if it is more about the milieu they are raised in or even a choice as many conservative religions frame the issue in terms of sin, based on their interpretation of scriptural verses.
I. Homosexuality is biological and/or inherited
1. Swedish study finds gay men respond differently from straight men when exposed to suspected sexual stimulus found in male sweat. Researchers found a testosterone compound activated the hypothalamus in homosexual men and heterosexual women, but not heterosexual men. An estrogen compound activated the hypothalmaus only in heterosexual men. PNAS journal by National Geographic News. Link. Link
2. Scientists at the Monell Center in Pennsylvania found that gay men perferred odors from other gay men, while odors from gay men were least preferred by straight men and women. Link.
3. Study shows gay men and heterosexual women have similar size brain halves, whereas in heterosexual men and gay women the right side was larger than the left. Also, heterosexual women and gay men had more connections in the left amygdala than heterosexual men and lesbian women. "As far as I'm concerned, there is no argument anymore - if you are gay, you are born gay", Dr. Rahman, Univ of London. Link. Link
4. Studies at Oregon State University using a population of male-orientated sheep suggests sexual orientation is largely hard-wired into the sheep's brain before birth.
Link. Ranchers have long observed that about 8% of the rams never father offspring becuase they are attracted only to other males. In Australia they are called "shy breeders". OSU researchers have also found a brain region linked to sexual behavior was twice as big in heterosexual as compared to homosexual rams, and the difference was noted before birth.
5. Austrian scientists report switching a single gene was enough to make female fruit flies reject males and attempt to mate with other females. Link. Link
6. Italian at the Univ of Padua found that mothers and aunts of gay men had more offspring than female relatives of homsexuals, suggesting genes that influence homosexuality in men may increase fertility in females. Link. Explained through "sexually antagonistic selection" Link
7. A high pecentage of girls exposed to male hormones before birth grow up lesbian. There are also about 40 cases of boys who were surgically altered to be girls and raised as girls due to genital deformities still were overwhelmingly attracted to females once they reached puberty. "The evidence is starting to look pretty good that hormones early in life influence the probablity of who you will be attracted to 10 years later, when people start to get their first crushes"; neuroscientist Marc Breedlove at Michigan State University. Link
8. Lesbian finger lengths more like men's than other women (in heterosexual women, the index and ring fingers are usually the same length). This trend also held for eye-blink patterns and inner-ear function. Link
9. Homosexuality runs in families. The number of older brothers a man has can increase his chance of being gay. Being homosexual is 3 - 5 X higher among people who have a gay brother or sister. Link A man with four older brothers is 3 X more likely to be gay than a man with no brothers.
10. For fraternal male twins, the gay-gay concordance rate is about 22%; for idential twins it is 52%. Link Based on these studies, scientists estimate homosexuality is due 40% to genes. "There have been psychological and social explanations for homosexuality for 100 years, and they haven't come up with anything concrete", Ray Blanchard, Toronto.
11. Fruit fly gene GB mutation turns the flies bisexual. Study indicates gender attraction is biologial but not hard-wired in fruit flies. Link
12. Female mice turned lesbian after altering one gene. Link Link
13. Homosexualy behavior has been observed in over 500 animal species. Link Link
14. Biology and sexual orientation. Discussion of evidences. Wikipedia Link
15. Homosexuality among human cultures is common and found around the world. Wikipedia Link
16. Discussion of various religious attitudes towards homosexuality. Table on contrasting views. Success rate of reversion therapies extremely low; easy to change sexual behavior, difficult to change sexual orientation. Link
17. Rugby player awakens after stroke gay (2011) Link
18. Men in study who were homophobic had arousals to homosexual visual presentations that did not occur with non-homophobic men. The homophobic men were not aware of their arousal tendencies. Link
II. Homosexuality is not biological
1. Summary article critical of the "gay gene" hypothesis. 2003. No gay gene has been found. Discussed: brain is plastic, critical of twin studies including study design and potential sampling errors, LeVay's study and its problems, criticisms regarding Hamer's study. There is some evidence that sexual orientation change is possible through reparative therapy. True.origin.org Link (note: this seems to be mostly a straw man argument. Few, if any researchers, are looking for a single gene to explain homosexuality and none would say that homosexuality is all genetic - Biomed).
2. Conservapedia summary article on homosexuality. Homosexuality is condemned in the Bible. Acceptance of homosexuality is tied to belief in evolution and being a liberal. In 29 of 79 cultures surveyed in 1952, homosexuality was rare or absent. Childhood molestation can be a contributing factor in homosexual behavior. Studies have failed to show genetic determinism for homosexuality (only one mentioned). Evidence shows homosexuality is a choice. Ex-homosexuals and programs to change homosexual orientation show that orientation can be changed and conversion to Christianity has the best outcomes. Much of the site discusses that homosexality leads to promiscuity, murder, homicides, AIDS, MRSA, STDS, mental health issues, cancer, obesity in women, drug use. Homosexuals prey on vulnerable youth. The media is biased towards promoting homosexuality. Homosexuality does not exist in animals. Link
(Seems to list most of the arguments against biological basis for homosexuality, although only a section deals with the question of genetic contributions to homosexuality - Biomed)
3. Even if homosexuality is not a choice, homosexual behavior is. If we are governed by feelings only, we are reduced to just being animals. Thus, homosexuality if practiced is immoral and a choice. A 2007 article in response to Rev. R. Albert Mohler, Jr. president of Southern Baptist Theological Seminary Link . In Mohler's article (Link), he suggested that if homosexuality is found by science to be largely genetic, it is still immoral and he would hope medical treatments would be developed to change an unborn child's sexual orientation. Link
4. Gay gene theory is not supported again. Link
5. There are many sources that are against homosexuality, but do not specifically address the issue of inborn and genetic factors, but rather approach the issue from a religious viewpoint only and thus are not included in this review.
It seems from my review that there is indeed a strong genetic component to homosexuality, but it is a continuum with multiple contributions. In general, I found the antihomosexual reviews weaker in their arguments. In addition, at least in lesbians, there seems to be a much higher incidence of childhood sexual abuse contributing to sexual orientation than with gay males. Studies of sexuality in women indicate strong sexual bonding towards relationships ahead of physical attributes which could explain a component of lesbianism in addition to possible childhood abuse. Thus, homosexuality in women is probably much more complex than in males and the two genders should be studied and discussed separately.
Lastly, as a physician I have worked with gay providers and have many gay and lesbian patients. I find they struggle with all the same relationship concerns as the heterosexual community, many knew they were gay at a very young age, many are kind and intelligent, and it is doubtful from my experience that in most cases of homosexuality it is a choice and thus sinful ~ Biomed
Related topic: A 19 yr. old Zach Wahls gives a passionate speech in Iowa about being raised by lesbian parents. Link (3 min)
What's Going On?
Apple Employees - It Gets Better. ( 6 min.) Pixar - It Gets Better (8 min.)
With You: The Mark Bingham Story
"I 'Came Out' For Marriage Equality": An Anti-Gay Activist Changes His Mind. Link