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Biological Factors

I don’t know of published scientists who come out and say that there is no evidence for any biological influence on sexual orientation. I have not met any. So, I think we’re in the realm here of there is a lot of evidence for biological influence. And still a lot to do in understanding how it works.Dr. Eric Vilain, UCLA, Professor of Human Genetics

When I speak at universities, students often ask if people are “born gay” and whether same-sex attraction is a “choice.” I often turn the question around and ask, “When did you choose to be straight?”

The vast majority of these students look bewildered because they understand their sexuality to be a natural, deeply ingrained part of who they are – and certainly not a conscious choice. Well, it’s exactly the same phenomenon for gay, lesbian, bisexual, and transgender people. We simply are who we are, and love the person we fall in love with — just like everyone else.

The latest scientific research shows that homosexuality and bisexuality are normal variations of biological diversity. Indeed, Dr. Bruce Bagemihl, author of Biological Exuberance: Animal Homosexuality and Natural Diversity, has scientifically documented homosexuality in more than 1,500 species of mammals, birds, reptiles, insects, and other animals worldwide. While humans are certainly different than other animal species and are not easily compared, the incontrovertible existence of homosexuality in the wild does show that this behavior is a natural phenomenon.

In one of the most widely-cited animal studies, Dr. Charles Roselli, a scientist at Oregon Health & Science University, found that homosexually oriented rams had brains that resembled female sheep:

All the evidence that we have point to there being a biological influence on the development of the brain area. And by inference on development of the behavior. There has been some research on rearing. Rams are reared in all male groups, so you might think it’s sort of a boarding school effect. But there has been some research on that. And actually it doesn’t matter if they are reared with other males, reared with females, or reared alone. There is still a proportion of animals that shows same-sex behaviors.

Anti-gay activists routinely claim that abuse, neglect, or bad parenting causes homosexuality. The scientific literature, however, does not support these obsolete views. According to Dr. Eric Vilain, Professor of Human Genetics, Pediatrics and Urology at UCLA, and the Director of the Institute for Society and Genetics:

For the purposes of sexual orientation, every time researchers have looked at the broad external environment — the macro environment — so for instance family structure. Is it true that single mothers are more likely to raise gay kids? That would be an influence from the environment. Well, you can actually test this hypothesis, and it’s been done by teams at the Kinsey Institute. And there is absolutely no evidence that this kind of environment, the family structure, influences sexual orientation.

The environment can be past sexual experience whether pleasant or unpleasant, and again, this has been studied and there no evidence that that is the case. Child abuse. You can ask gay men or lesbian women if they have been abused sexually. And if you do that and do that in a control group of straight men and women, you actually find that there is no evidence that these kind of environments change you, or is associated with different sexual orientations.

Contemporary research points to a combination of genetic and biological factors as powerful influences on sexual orientation. While genes get the most attention, that is only part of the story:

It’s not [sexual orientation] as high as a lot of conditions that are studied genetically,” said Dr. Alan Sanders, a Clinical Associate Professor of Psychiatry at Northwestern University. “It is low to moderate heritability. There are different estimates. Maybe somewhere around thirty to forty percent of the variation.”

Scientists say that the interplay between genes and the intrauterine environment are critical in the formation of one’s sexual orientation. Dr. Simon LeVay, a neuroscientist who has served on the faculties of Harvard Medical School and the Salk Institute for Biological Studies, sums up the influence of biological factors:

Generally these [biological studies] point to the idea that what’s going on differently between gay and straight people is something in their brain when first descending itself before birth. In other words, there are processes when the hypothalamus and parts of the brain are being created when the nerve cells are being born and start to develop and form connections before birth. It goes forward differently in fetuses that ultimately become gay adults and those that become straight adults. And the difference seems to be brought about, at least, in-part, by sex hormones that are circulating in the blood of the fetus. These hormones come from the gonads, from the testes or ovaries of the fetus. They enter the brain and they influence how the brain develops in a sexual sense, whether it develops in a more masculine or more feminine direction. And there is more evidence now that this process really goes forward differently in, if you like, in gay and straight fetuses.

Subtle anatomical differences or physiological differences that have been described recently, like finger length ratios, the physiological properties of the inner ear, finger print patterns, and so forth, they all seem to fit into the same basic picture. Which is they seem to be caused or result from differences in the way that sex hormones are interacting with the brain and the body during early life. So, high levels of testosterone seem to drive the brain and the body in a male typical direction. Low levels allow the brain and the body to develop in a more female typical direction. And its that sort of general process, if you like, that causes there to be some kind of link between these anatomical features like finger length ratios and a person’s sexual orientation. It’s as if it’s part of a big package of traits, all of which have some common developmental process behind them.

Another biological phenomenon that has been studied is the “fraternal birth order effect.” Studies have shown that the more older brothers a boy has, the more likely he will be gay. Although Dr LeVay calls it a “pretty weak effect,” he says it is noteworthy:

You’d actually have to have about ten older brothers before you’d have even a fifty-fifty chance of being gay by the birth order effect alone. But, it does seem to be real and it is an intriguing finding that really needs to be explored.

Dr. Ray Blanchard, a scientist at the University of Toronto’s School of Psychiatry, pioneered research on the “fraternal birth order effect,” and explains his theory on why it occurs:

The full-blown theory goes like this. We know that there are certain proteins in male cells that either have counterparts in females that are different or have no counterparts in females. We also know that fetal material gets from the fetus into the maternal circulation. Especially at childbirth where there is a lot of tearing of maternal tissue and a lot of fetal material gets into the woman’s circulation.

So, I hypothesize that tissue from cells or cell fragments from a male fetus get into the mother’s circulation. The mother perceives these male substances as foreign and develops an immune response to them in the form of anti-bodies. These anti-bodies now cross the placenta barrier when she has another male fetus. And affect the male fetus in such a way as to partially affect differentiation of the brain in a male typical fashion. So, that the part of the fetuses brain that controls sexual orientation is left in the default position of sexually preferring males rather than females.

Additionally, a study released in May 2006 by Swedish scientists demonstrates that biology plays a key role in determining a person’ sexuality. The research shows that the portion of the brain that helps regulate sexuality — the hypothalamus – reacted the exact same way in straight women and gay men when exposed to male pheromones, which are chemicals designed to provoke a behavior, such as sexual arousal. The same area of the brain only became stimulated in heterosexual men when introduced to female pheromones.

We don’t have all the answers, but the science is marching forward and continuously expanding our knowledge of sexual orientation. According to Dr. Marc Breedlove, Rosenberg Professor of Neuroscience at Michigan State University:

Will we understand everything about sexual orientation soon? I don’t know, that’s a big question.  We don’t understand everything about very many biological processes. We certainly don’t understand everything about childbirth or cancer or almost any process that any of us really find interesting. So, I think the great thing about being a scientist is there is always plenty of stuff we still don’t know. The second great thing about being a scientist is that with the right training, you have at least a fighting chance of finding out things you didn’t know before. That’s what makes it fun.