Sexual orientation: nature vs. nurture?

Medical Matters

December 08, 2003|By Judy Foreman | Judy Foreman,SPECIAL TO THE SUN

IS THERE A biological basis for homosexuality? With gay marriage now supported by Massachusetts' highest court and homosexuality likely to be a hot issue in the presidential campaign, the question of whether sexual orientation is an innate or acquired trait is increasingly urgent.

Since at least 1991, some scientific research has suggested that there is a biological basis to homosexuality - meaning sexual orientation is at least partly natural destiny, not a choice. But that point is open to debate, and our understanding is still fuzzy.

Some data on identical twins suggests that homosexuality - particularly in men - is inherited. Other scientists have tried to pin down anatomical differences in brain structure between gay and straight men.

Understanding homosexuality - or even heterosexuality - involves, among other things, figuring out how the brain, the seat of complex behavior, becomes male or female.

Until recently, researchers thought a surge in the male hormone testosterone set the brain on a male 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. That has been the dogma in the field for 30 years," said Dr. Eric Vilain, a geneticist and urologist at the David Geffen School of Medicine at the University of California at Los Angeles, who led the research.

The study's implications are many. About one in 4,000 babies is born with "ambiguous genitalia," making it difficult to tell whether it's 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 and sometimes perform surgery to make the anatomy conform. DNA analysis may soon help doctors figure out to which gender the child most appropriately belongs, Vilain says.

The 54 genes may also help explain transgenderism. Feeling that they were born the "wrong" sex, some transgendered people simply live and dress like the gender they feel, while others undergo sex-change surgery.

The UCLA study does not directly address homosexuality. But other data suggest that 75 percent of boys confused about their gender as children grow up to be gay.

Other studies on the genetic roots of homosexuality are mixed. Dr. Richard C. Pillard, a professor of psychiatry at the Boston University School of Medicine, says male sexual orientation is often inherited. In women, "sexuality is not as rigidly set."

In 1991, an autopsy study by Simon LeVay at the Salk Institute for Biological Studies in San Diego found that the brain's anterior hypothalamus was twice as large in heterosexual men as it was in homosexuals. But because all the gay men had AIDS, it is possible that the difference was linked to the disease, rather than their homosexuality.

Other studies that have tried to draw a biological link to homosexuality have also faced problems. In 1993, Dean Hamer, a molecular biologist at the National Cancer Institute, studied 40 pairs of gay brothers.

With a technique called linkage mapping, Hamer identified a region called Xq28 on the X chromosome (inherited from the mother) that was statistically correlated to homosexuality. In 1999, researchers at the University of Western Ontario studied the same brain region in pairs and reported contradictory findings.

Clearly, more research is needed. But Dr. Fred Berlin, a psychiatrist at the Johns Hopkins University School of Medicine, said the basic conclusion is already clear. Homosexuality, he said, "is not due to voluntary choice. None of us as kids sat down and said, `Do I want to be attracted to members of the same gender?'"

Judy Foreman is a lecturer at Harvard Medical School.

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