I've got the reference and abstract from the first study at the Salk Institute, and the reference and abstract to an earlier study conducted by the UCLA group on the differences between the brains of men and women (same region). The second study is too new to have been entered into the online databases, so I couldn't find it. But it was published in the Proceedings of the Natl. Acad. of Sci. (I think) Jim --------------------------------------------------------------------------- 7 A DIFFERENCE IN HYPOTHALAMIC STRUCTURE BETWEEN HETEROSEXUAL AND HOMOSEXUA L MEN ARTICLE (REFS:28) by LEVAY-S(*R) SALK INST BIOL STUDIES/SAN DIEGO//CA/92186 (*R) SCIENCE VOL:253 (5023):1034-1037(1991) LOC:BIOL CHEM ENGR ASTR CENG GEOL KLAB The anterior hypothalamus of the brain participates in the regulation of male-typical sexual behavior. The volumes of four cell groups in this region *interstitial nuclei of the anterior hypothalamus (INAH) 1, 2, 3, and 4* were measured in postmortem tissue from three subject groups: women, men who were presumed to be heterosexual, and homosexual men. No differences were found between the groups in the volumes of INAH 1, 2, or 4. As has been reported previously, INAH 3 was more than twice as large in the heterosexual men as in the women. It was also, however, more than twice as large in the heterosexual men as in the homosexual men. This finding indicates that INAH is dimorphic with sexual orientation, at least in men, and suggests that sexual orientation has a biological substrate. -------------------------------------------------------------------------------- 16 SEXUAL DIMORPHISM OF THE ANTERIOR COMMISSURE AND MASSA INTERMEDIA OF THE HUMAN BRAIN ARTICLE (REFS:92) by ALLEN-LS(*R) GORSKI-RA UNIV CALIF LOS ANGELES,DEPT ANAT & CELL BIOL,405 HILGARD AVE/LOS ANGELES //CA/90024 (*R) JOURNAL OF COMPARATIVE NEUROLOGY VOL:312 (1):97-104(1991) LOC:BIOL Neuroanatomical sex differences were observed in the midsagittal area of both the anterior commissure and the massa intermedia on analysis of postmortem tissue from 100 age-matched male and female individuals. The anterior commissure, a fiber tract whose axons in primates primarily connect the two temporal lobes, was an average of 12%, or 1.17 mm2 larger in females than in males. The massa intermedia, a structure that crosses the third ventricle between the two thalami, was present in 78% of the females and 68% of the males. Among subjects with a massa intermedia, the structure was an average of 53.3% or 17.5 mm2 larger in females than in males. Inclusive of subjects with and without a massa intermedia, this structure was a mean of 76% or 16.93 mm2 greater in females than in males. These sex differences were present despite the fact that the brains of males were larger than those of females. Since a majority of subjects were adults, it is unknown when sexual differentiation occurred. Anatomical sex differences in structures that connect the two cerebral hemispheres may, in part, underlie functional sex differences in cognitive function and cerebral lateralization. ----------------------------------------------------------------------------- The following article was published on Aug. 1. I am reprinting it without permission. SCIENTISTS, FINDING SECOND IDIOSYNCRASY IN HOMOSEXUALS' BRAINS, SUGGEST ORIENTATION IS PHYSIOLOGICAL by Natalie Angier ____________________________ A cord of nerve fibers that allows the two halves of the brain to communicate with one another is larger in homosexual men than it is in either heterosexual men or in women, scientists have reported. The brain structure, called the anterior commissure, is not thought to influence sexual behavior directly, but the researchers believe the finding supports the idea that the brains of homosexuals differ in many subtle ways from those of heterosexuals, and that sexual orientation has a deep biological basis. The discovery remains to be confirmed by other laboratories, and some researchers remain skeptical that it has any relevance to an issue as complex as human sexuality. They warned that so little is known about how the brains of higher animals work that to assign function or meaning to slight discrepancies in anatomical features is a gross oversimplification. HOMOSEXUAL 'BY NATURE' Others embraced the finding as added proof that homosexuality is inborn rather than a chosen behavior, and that therefore all discrimination against homosexuals should be illegal, just as it is against minorities or women. "It points out that gay people are made this way by nature." said Robert Bray, head of public information at the National Gay and Lesbian Task Force, a gay-rights advocacy group in Washington. "It strikes at the heart of people who oppose gay rights and who think we don't deserve our rights because we're choosing to be the way we are." The report, from Dr. Laura S. Allen and Dr. Roger A. Gorski of the University of California at Los Angeles, appears today in the Proceedings of the National Academy of Sciences. It follows by less than a year another announcement of anatomical differences in the brains of homosexual and heterosexual men, a study that generated a debate nationwide. In the previous experiment, Dr. Simon LeVay of the Salk Institute in La Jolla, Calif., said that a region of the hypothalamus, a part of the brain that helps regulate sexual behavior, was smaller in homosexual men than in heterosexual men; it was equivalent to the dimensions seen in heterosexual women. Dr. LeVay, who is openly gay, praised the new research. "I think the work is very good, and I think it's correct," he said. "It's such a clear result, and in a sense it's more important than my own finding." In the new study, the researchers looked at the anterior commissure, a structure about the width of a baby's fingertip, which early research had suggested was larger in women than in men. The function of the commissure remains mysterious, but researchers believe that it allows the left and right hemispheres of the brain to integrate sensory information, just as another, more familiar structure called the corpus callosum does. The scientists examined brain sections from 34 men whose medical records indicated they were homosexual, 75 men presumed to be heterosexual, and 84 women also thought to be heterosexual. They were not able to obtain brain samples of known lesbians, because lesbians rarely die of sexually transmitted diseases like AIDS and thus their sexual orientation is almost never noted on medical charts. They found statistically significant differences among the three groups. The commissures of the heterosexual women were 13 percent bigger than those of heterosexual men. Those of the homosexual men were 18 percent larger than the samples from the women and 34 percent larger than those of heterosexual men. However, these differences were based on comparisons of the sample groups' average commissure dimensions. There were also vast differences within the groups. Among heterosexual women, for example, the largest commissure was three times the size of the smallest. REACTION IS MIXED The scientists are well known in the field of neuroanatomy, and peers said their methods were sound. "Gorski is a perfectly fine anatomist, and I think this is pretty interesting," said Dr. Allen Basbaum, a neuroanatomist at the University of California at San Francisco. "I have no problems believing, a priori, that there would be these differences in the brain, and that if someone looks carefully, they'll find them." But some scientists said the new work was likely to prove as questionable as many past studies of neuroanatomical differences between blacks and whites, men and women, or various ethnic groups. "There's really a trend in this area for each paper to be just another statistical fluke," said Dr. William Byne of the Columbia University College of Physicians and Surgeons in New York. "I just don't think sexual orientation is going to be represented in any particular brain structure. It's like looking in the brain for your political party affiliation." Others criticized the study for relying on brains from AIDS patients; all but six of the homosexual men studied had died of the disease. "We know that H.I.V. has an affinity for brain cells, and I don't think we can rule out that this is having an effect," said Dr. Robert P. Cabaj, a psychiatrist at the University of California at San Francisco. Dr. Allen said that six of her group's heterosexual subjects had also died of AIDS, and that the researchers did not think the commissure was affected by the disease. That's all folks!