With gay marriage now supported by Massachusetts' highest court and gay rights likely to be an issue in the presidential campaign, the question of whether sexual orientation is an innate or acquired trait is an increasingly urgent one.
Since at least 1991, some scientific research has suggested a biological basis to homosexuality -- meaning sexual orientation is probably at least partly natural destiny, not simply choice. But that point is open to political and scientific debate, and our understanding of how biology may drive sexual orientation is still fuzzy.
Understanding homosexuality or heterosexuality involves, among other things, figuring out how the brain, the seat of all complex behavior, becomes male or female in the first place.
Until recently, researchers thought that a surge in the male hormone testosterone sets the brain on a male track. Without testosterone, the brain continues developing on a female track.
But in October, California researchers studying fetal development identified 54 genes that play a role in the expression of sex -- before hormones are ever released.
"This refutes the idea that hormones are the only story in sexual differentiation of the brain," said Dr. Eric Vilain, an assistant professor of human genetics and urology at the David Geffen School of Medicine at UCLA, who led the research.
The findings' implications are many. An estimated one in 4,000 babies worldwide is born with "ambiguous genitalia," making it difficult to tell whether the baby is a boy or a girl. By analyzing chromosomes and looking for internal sexual organs such as ovaries or a prostate gland, doctors make their best guess as to the true sex of the child and sometimes perform surgery to make the anatomy conform to that. DNA analysis of the variations in these 54 genes and other genes that interact with them may help doctors figure out to which gender the child belongs, Vilain says.
The 54 genes may also help explain transgenderism, in which a person feels he or she was born the "wrong" sex. The situation is found in about one in every 50,000 people.
The UCLA study does not address homosexuality directly. But other data suggest that 75% of boys who were confused about their gender identity as children grow up to be gay, said Vilain. The new study, he said, may help "pave the way to find out about gender identity" in such children.
Other studies on the genetic roots of homosexuality are mixed.
Dr. Richard C. Pillard, a professor of psychiatry at Boston University School of Medicine, has studied male and female homosexuals, believed to make up at least 3% to 4% of the population. Pillard holds that men's sexual orientation is often inherited while in women, "sexuality is not as rigidly set."
In identical male twins, his research shows, if one is gay, there's a 50% chance that the other one is too. Granted, if homosexuality were totally genetically determined, that figure should be 100%. On the other hand, in male fraternal twins, his studies show there's a 20% chance that if one is homosexual, the other will be.
In 1991, an autopsy study by Simon LeVay at the Salk Institute for Biological Studies in San Diego found that part of the brain called the anterior hypothalamus was twice as large in heterosexual men as in homosexual men, suggesting a biological basis for homosexuality. Because the gay men all had AIDS, it is possible that the disease, rather than their homosexuality, transformed their brains.
Other studies that have tried to draw a biological link to homosexuality have faced problems as well.
In 1993, Dean Hamer, a molecular biologist at the National Cancer Institute, studied 40 pairs of gay brothers and published his results in Science. Hamer identified a region called Xq28 on the X chromosome (inherited from the mother) that was statistically correlated to homosexuality. In 1995, a second study by Hamer and others confirmed that finding.
In 1999, researchers led by George Rice at the University of Western Ontario in Canada studied the same brain region in 52 gay male sibling pairs and reported contradictory findings.
Clearly, more research is needed to determine whether homosexuality is inherited.