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Gays and genetic predisposition
Gays and genetic predisposition
Twin studies point to a genetic contribution to gay orientation. Not all identical twins share the same sexual orientation but it appears to run in families! How complex human behaior can be!
Twin studies provide evidence that homosexuality runs in the family
When Vince Healy finally came out as gay, his disapproving Catholic family was familiar with the story. His older brother had been living with a man for several years. It didn't make things any easier, the 45-year-old Ballard man recalled.
"I was very unhappy at the prospect of being gay," he said. "I kept thinking: I must be a late bloomer."
As the youngest of three brothers, one of whom is straight, Healy illustrates the two most robust findings in the science of homosexuality: It runs in families, and the number of older brothers a man has can increase his chances of being gay.
About 3 percent of American men and 1.5 percent of women describe themselves as gay or bisexual, according to the National Institutes of Health. Those percentages are three to five times higher among people who have a gay brother or sister.
Of course, family dynamics might be the reason, not biology.
What scientists call slam-dunk proof that genes are part of the equation comes from twin studies.
Genetically influenced traits are more likely to be shared among the closest relatives, and that pattern holds for homosexuality.
For fraternal male twins, the gay-gay concordance rate is about 22 percent. For identical twins, it's 52 percent.
Based on those results, scientists conservatively estimate homosexuality is about 40 percent due to genes, said Alan Sanders, director of behavior genetics at Northwestern Healthcare Research Institute in Illinois.
But genes clearly are not the only factor, or identical twins would always share the same sexual orientation.
"That means there's a significant environmental contribution," said Sanders, who is leading a five-year, $2.5 million project for the National Institutes of Health to try to identify the genes involved.
Earlier research has pointed to several possible gene regions, but those studies were small and not definitive. With DNA from 1,000 pairs of gay brothers, Sanders' project will be much more powerful.
It's very unlikely to uncover a single "gay" gene, he said. As in most complex traits, multiple genes and environmental factors probably work together.
So far, scientists can only speculate how genes linked with sexual orientation might work. Perhaps they dictate the size of brain structures, which in turn regulate hormones before birth. Perhaps genes directly adjust prenatal hormone levels, or merely predispose people to a gay orientation.
Environmental factors could be exclusively biological, like chemical exposure or infection. One theory, backed by some evidence in rats, is that the chemical and hormonal milieu of the developing fetus can be disrupted when pregnant mothers are stressed.
Social factors may ultimately prove to play a role as well, Sanders said.
None of the psychosocial theories for homosexuality have panned out so far, including Freud's distant-father/domineering-mother dynamic.
"There have been psychological and social explanations for homosexuality for 100 years, and they haven't come up with anything concrete," said Ray Blanchard, head of Clinical Sexology Services at the Centre for Addiction and Mental Health in Toronto.
A few studies suggest a higher rate of childhood sexual abuse in gays and lesbians, though there's no evidence such experiences trigger homosexuality, said Mustanski, the University of Illinois geneticist.
3/13/2006 10:22 am
Having "chosen" the lesbian lifestyle for about a decade of my life in order to avoid men and heterosexual dynamics, I am always interested to read about anything that explains sexuality. I can honestly say that I have probably been firmly bi-sexual most of my life as I do not recall anytime that I found woman as anything but attractive. I still find both sexes attractive, I just prefer right now not to be lesbian although sexually I am still attracted to women.|
I see it as more of a style of living choice for me, not a sexual preferance.