By Bill Sullivan
Across cultures, 2% to 10% of people report having same-sex relations. In the U.S., 1% to 2.2% of women and men, respectively, identify as gay. Despite these numbers, many people still consider homosexual behavior to be an anomalous choice. However, biologists have documented homosexual behavior in more than 450 species, arguing that same-sex behavior is not an unnatural choice, and may in fact play a vital role within populations.
In a 2019 issue of Science magazine, geneticist Andrea Ganna at the Broad Institute of MIT and Harvard, and colleagues, described the largest survey to date for genes associated with same-sex behavior. By analyzing the DNA of nearly half a million people from the U.S. and the U.K., they concluded that genes account for between 8% and 25% of same-sex behavior.
Numerous studies have established that sex is not just male or female. Rather, it is a continuum that emerges from a person’s genetic makeup. Nonetheless, misconceptions persist that same-sex attraction is a choice that warrants condemnation or conversion, and leads to discrimination and persecution.
I am a molecular biologist and am interested in this new study as it further illuminates the genetic contribution to human behavior. As the author of the book, “Pleased to Meet Me: Genes, Germs, and the Curious Forces That Make Us Who We Are,” I have done extensive research into the biological forces that conspire to shape human personality and behavior, including the factors influencing sexual attraction.
The hunt for ‘gay genes’
The new finding is consistent with multiple earlier studies of twins that indicated same-sex attraction is a heritable trait.
The 2019 study is the latest in a hunt for “gay genes” that began in 1993, when Dean Hamer linked male homosexuality to a section of the X chromosome. As the ease and affordability of genome sequencing increased, additional gene candidates have emerged with potential links to homosexual behavior. So-called genome-wide association studies identified a gene called SLITRK6, which is active in a brain region called the diencephalon that differs in size between people who are homosexual or heterosexual.
Genetic studies in mice have uncovered additional gene candidates that could influence sexual preference. A 2010 study linked sexual preference to a gene called fucose mutarotase. When the gene was deleted in female mice, they were attracted to female odors and preferred to mount females rather than males.
Other studies have shown that disruption of a gene called TRPC2 can cause female mice to act like males. Male mice lacking TRPC2 no longer display male-male aggression, and they initiate sexual behaviors toward both males and females. Expressed in the brain, TRPC2 functions in the recognition of pheromones, chemicals that are released by one member of a species to elicit a response in another.
With multiple gene candidates being linked to homosexuality, it seemed highly unlikely that a single “gay” gene exists. This idea is further supported by the new study, which identified five new genetic loci (fixed positions on chromosomes) correlating with same-sex activity: two that appeared in men and women, two only in men, and one only in women.
How might these genes influence same-sex behavior?
I find it intriguing that some of the genes from men identified in Ganna’s study are associated with olfactory systems, a finding that has parallels to the work in mice. Ganna’s group found other gene variants that may be linked with sex hormone regulation, which other scientists have previously suggested plays a large role in shaping the brain in ways that influence sexual behavior.
Males with a genetic condition called androgen insensitivity syndrome can develop female genitalia and are usually brought up as girls, despite being genetically male – with an X and Y chromosome – and they are attracted to men. This suggests that testosterone is needed to “masculinize” a prenatal brain; if that doesn’t happen, the child will grow up to desire men.
Similarly, girls who have a genetic condition called congenital adrenal hyperplasia are exposed to unusually high levels of male hormones like testosterone while in the womb, which may masculinize their brain and increase the odds of lesbianism.
It’s also possible that hormonal shifts during pregnancy could affect how a fetus’ brain is configured. In rats, manipulation of hormones during pregnancy produces offspring that exhibit homosexual behavior.
Why does homosexual behavior exist?
Several hypotheses have been advanced to explain how homosexuality can be beneficial in perpetuating familial genes. One idea involves the concept of kin selection, whereby people work to ensure the passage of their family’s genes into subsequent generations. Gay uncles and aunts, for example, are “helpers in the nest” that help raise other family members’ children to nurture the family tree.
Another idea suggests that homosexuality is a “trade-off trait.” For example, certain genes in women help increase their fertility, but if these genes are expressed in a male, they predispose him toward homosexuality.
Sexual behavior is widely diverse and governed by sophisticated mechanisms throughout the animal kingdom. As with other complex behaviors, it is not possible to predict sexuality by gazing into a DNA sequence as if it were a crystal ball. Such behaviors emerge from constellations of hundreds, perhaps thousands, of genes, and how they are regulated by the environment.
While there is no single “gay gene,” there is overwhelming evidence of a biological basis for sexual orientation that is programmed into the brain before birth based on a mix of genetics and prenatal conditions, none of which the fetus chooses.
Bill Sullivan is Professor of Pharmacology and Toxicology at Indiana University School of Medicine. This piece was originally written in 2019.
Bo Peep says
Well it is a choice or it is a mental illness so I prefer to call it a decision and a desire.
Skibum says
Little Bo Peep, you’re such a lost sheep. You talk as if you are some semblance of a mental health expert, declaring homosexuality a choice or a mental illness. Laughable, but absolutely FALSE!
I suppose YOU thought about your sexuality, how you were attracted to both those of the opposite sex as well as those of your own sex, and you thought, gee, I like ’em both and I have a choice, huh? Is that how it works?
Please enlighten all of us and do tell, Bo. Or, if there never was a choice for you, well then, obviously the only other remaining scientific quantification according to YOU is that you too, must be mentally ill to have a pre-existing sexual attraction.
Now, doesn’t all of this make your head spin, dear Bo, with dreamt up tidbits tossed haphazardly into the air and spouted as factoids?
I know myself as well as you know YOU. I can tell you, absolutely truthfully, that I have always been gay, I don’t know why – it just happened, and it was NEVER a choice! And oh, by the way, I have no history of mental illness either… just mental exhaustion and extreme frustration from a lifetime of listening to heterosexual bozos spout nonsense and trying to diagnose something that needs no more diagnosing than if all of the gay people in the world (and there are a LOT of us, dear) tried to examine just what kind of mental illness causes a person have a sexual attraction for others of the opposite sex???
Give me a friggin break! But before I go, there is one tiny little end cap I would love to bring to your attention, Bo Peep. And that is how apropos the name Bo Peep is, especially in light of the topic at hand. I would refer you to the link below, and I hope you take the time to read the article. Ewe might just learn something. Good Baaaaaa.
https://en.wikipedia.org/wiki/Homosexual_behavior_in_sheep
Laurel says
Bo Peep Land, where men are men, and sheep are nervous.
Sherry says
@Skibum. . . I’m actually convinced that bo peep is nothing but a “TrollBot”. Notice that entity posts nothing but Fox BS “inflammatory” one liners. Never presenting “ANY” credible facts, reasoning or evidence. A complete waste of our time in trying to debate “It”.
Skibum says
Sherry, I think I was a wee bit more ornery and mischievous than usual late last night when I wrote that response to Bo Peep, who only illuminated themself as a know nothing quackaboodle when it comes to both the reality of, as well as scientific factual statistics regarding the innumerable complexities of human sexuality.
Sherry says
Dear wonderful Skibum! I love it that “you do you”!!! Your writings are most certainly not wasted on me, I can assure you.
I’m just suggesting that ole bo peep is just a “Bot” built to inflame and divide. Certainly, there are “human” Maga trolls that comment here all the time. Since I don’t have time to waste on “complete FOOLS”, I choose not to engage with ANY of them. . . usually don’t bother to read one word of their complete “garbage”.
You, Jim, Laurel, Pogo, Dude, and others have much more to offer. Happy Holidays from this educated, safe, sane, kind, peaceful California bubble! HUGS!
Skibum says
Much love and respect returning to you! Happy Holidays to you out there where I grew up, learned to ski, and also learned a lot of life’s lessons… in the Golden State.
Been There says
Then that means heterosexuality is a choice as well. When did you make your choice?
Sherry says
@ Been There. . . Love your “snarky” remark! HO! HO! HO!
Dusty says
Well everyone deserves to love and be happy but until such time that the behavior falls within the second standard deviation it will be considered abnormal. However it will likely eventually reach the point where it falls within that bell curve anr be considered normal as supported by mathematics and science.
JC says
I was at Walmart at Cypress Point today and I saw a guy wearing a dress, yes I am not kidding. The guy was wearing a dress and he had a beard/body hair and have a carrying bag with the LGBT bag. Homosexuality not a choice, but I have a choice of calling a guy in a dress having some form of mental illness. It isn’t even Halloween.
Skibum says
JC, besides the fact that there are many men who are cross dressers and love to go out in public dressed up in women’s clothing who are not gay, don’t you realize that Walmart is the cheapest form of entertainment in town?
Walmart fashion faux pas: https://thunderdungeon.com/2025/07/09/people-of-walmart-29-fashion-violations-in-the-wild/
Sherry says
OK @ jc. . . that man in a dress “hurts” you how exactly?
Meanwhile, I haven’t seen you post your objections and disgust regarding trump being found GUILTY of “Sexual Abuse”. What about “School Shootings”. . . murdered children don’t bother you? Hummmm. . . .
Katrina Friel says
Agreed. We should stop punishing people whose behavior doesn’t fall within that second standard deviation just because it makes somebody else (who is completely unaffected by the behavior) feel uncomfortable. If somebody is doing something that is not your cup of tea, learn to say, “I’m happy they’re happy.” and move on or mind your own business.
Been There says
Your limited knowledge of world fashion is an indication that you should try traveling to someplace besides Walmart. Men wear “dresses” in many cultures. Men have also been teasing their hair and wearing make-up for centuries.
JC says
So many supporters of a grown man wearing a dress that I saw yesterday at Walmart with a beard, a crapton of body hair, was balding on his head, a beer belly and wearing a LGBTA flag Carrying Bag. I even saw some adults who were about to laugh at the guy’s face but was nice enough to not do it. This was not someone who truly went from Male to Female and can pass off as Female now, this was a dude in a real dress that was for women! Furthermore, I don’t care about other cultures than the culture we are living in Florida. I don’t care about world cultures when clearly this guy might have a mental illness problem. The supporters of this guy from some of the posters here are clearly why some people stop voting for Democrats because when you have supporters of a dude in a dress that isn’t on Halloween, that’s one level too fucked up.
Pierre Tristam says
Yet JC has no problem with women wearing pants, while a dude wearing a dress appears to obsess him considerably more than, say, a dude serially murdering people in boats.
feddy says
So although this is not something that is seen everyday in palm coast and it does not bother me one way or the other but did anyone consider that this individual was dressed like that because he might of lost a bet?
Samuel L. Bronkowitz says
It’s truly sad how the people that cry the loudest about it being a mental illness or some sort of aberration are deflecting attention away from their own homosexual desires. Trump himself went down on Bill Clinton, he’s normalized it for conservatives so you should be ok with your true gay.
Pogo says
@It’s also interesting
… how the quick response team for LGBT rights have been in the vanguard of political spoilers, eccentric perfectionists, Epicurean warrior poets (saved us from old Joe, didn’t ya: anything, or anyone — else?) one, and all.
FREE — magical mystery tour will take you away today
https://www.google.com/search?q=pleasure+as+a+philosohpy
We are all in the gutter, but some of us are looking at the stars.
— Oscar Wilde, from his play Lady Windermere’s Fan
Laurel says
Well of course people are going to deny everything they just read in the article (if indeed they did read) and snuggy themselves in their cocoon like comfort zone. They, themselves, are comfortably “normal.” Those of you, who are “different” are not. Therefore, the cocoon people are superior.
The deniers are the uninteresting people to me. They tend to sport the same tribal mind, right down the line, on all subjects. Tribalism is a powerful thing, though it tends to be tedious. It prevents a person from thinking something out on their own. Acceptance is all that matters to them. Safety in numbers.
DaleL says
Life is not just one thing (black) or the other (white). The myriad of genes which determine each living creature’s basic structure are not in perfect synchrony with each other. Natural selection favors reproductive success of the species genome as a whole, not individuals. The activation of many genes depends on outside influences. For example, the sword tail fish, which is kept in aquariums, can change from female to male. This will happen in a tank in which there are no male fish. The most masculine individual is typically the one to transform.
Similarly, genes for maleness and femaleness will be expressed differently depending on various outside influences from the embryo throughout life. Behavior can influence this, so there can be a degree of choice. How much, I don’t know.
In the end though, why do so many people choose to judge others? Whether sexual orientation is a choice really does not matter to me. I don’t care if the driver of the car ahead of me is gay or straight. I just want them to go when the light turns green.
Happy Holidays everyone!