New Homosexuality Link Found in Brain : Medicine: UCLA researchers find a band of fibers larger than in women or heterosexual men. They suggest that sexual orientation is determined by changes throughout the organ.

UCLA researchers have discovered more evidence that the brains of homosexual males are fundamentally different from those of heterosexuals.

Studying brains obtained from autopsies, they have found that an important structure connecting the left and right sides of the brain, already known to be larger in women than in men, is larger still in homosexual men.

Combined with an earlier study at the Salk Institute for Biological Sciences, which found that another area of the brain was smaller in gay men, the findings reported today in the prestigious Proceedings of the National Academy of Sciences suggest that homosexuality is not linked to any single brain structure, but to changes throughout the brain.

“This is very significant because here we have a part of the brain that has nothing to do with sex,” said Salk neuroscientist Simon LeVay, who last year reported the first difference in the brains of homosexual men.

“Something unusual is clearly happening when the brain is organizing itself in fetal life,” LeVay said. “This strongly supports the idea that there is a biological basis for the determination of sexual orientation. It’s one more nail in the coffin” of critics who argue that homosexuality is a choice and thus immoral.

It also further discredits the myth that parental influence--such as the stereotypical domineering mother--turns children toward homosexuality.

“This study supports our belief that nature created us just the way we are and that there is no reason to fix anything because nothing is broken,” said Robert Bray, spokesman for the Gay and Lesbian Task Force in Washington. “It supports our assertion that we are born this way.”

The major criticism of the study is that--like LeVay’s highly publicized study last year--it involves only homosexual males. The researchers were unable to apply their research to lesbians, said UCLA neuroscientist Roger A. Gorski, because they were unable to obtain brains from women known to have been gay. Unlike the case with men who die of AIDS, the sexual orientation of women who die of the disease is not noted on death certificates, he said.

Gorski and his colleague, neuroscientist Laura S. Allen, obtained a total of 90 brains from homosexual men, heterosexual men and women whose sexual orientation was not known. They carefully studied the size of the anterior commissure, a band of fibers that connects the cortex of the left and right sides of the brain.

They found that the anterior commissure of homosexual men was 18% larger than that of the women studied and 34% larger than that of heterosexual men.

“It’s important to note that this region of the anterior commissure is larger, not smaller, in homosexuals,” Gorski said. “This dispels the notion that all physiological differences playing a role in sexual orientation will follow a pattern, or that brains of homosexuals would be smaller overall than brains of heterosexuals.”

Their discovery correlates well with previous observations of other researchers, especially neuro-psychologist Sandra Witelson of McMaster University in Hamilton, Canada. Witelson has shown that another area linking the cortex of the two sides of the brain, the corpus callosum, is also bigger in women then in men.

Witelson and others have argued that increased linkage between the two sides of the brain is the source of the greater verbal facility demonstrated by women on standardized tests. The lower communication between the two halves of the brain in men, in contrast, is thought to improve their scores on tests that involve spatial reasoning, such as the ability to mentally rotate a three-dimensional object.

Homosexual men typically score more like women on tests of both verbal facility and spatial reasoning.

The new study was prompted by Gorski’s previous discovery in rats that hormone levels in the womb determine both sexual behavior and the size of many brain structures. Subsequent studies have traced every difference in brain structure and function between males and females to early exposure to hormone levels.

“The brain is basically female and it has to be acted upon by testicular hormones to become male both structurally and functionally,” Gorski said. If for any reason there is a variation in the output of these hormones, it could lead to feminization of a male fetus.

That difference could very well be genetic in origin. As least four different laboratories around the country are looking for such a gene.

“If it (the gene) exists, the techniques are there to identify it,” Gorski said. “Progress in that area could be very rapid. . . . Neither our study nor LeVay’s proves conclusively that there is a biological basis for sexual orientation. The discovery of a gene would.”