The Blog


12:00 AM, Aug 5, 1996 • By JEFFREY MARSH
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Why are some men attracted sexually to other men, rather than to women? Since one of the most powerful forces in nature is the drive to perpetuate the species, the origin of this perverse sexual orientation is clearly of great interest to biologists. In A Separate Creation (Hyperion, 354 pages, $ 24.95), Chandler Burr provides a riveting depiction of the latest scientific investigations of the subject. Using the tools and techniques of molecular biology and guided by a theoretical framework encompassing endocrinology, genetics, and neuroanatomy, this research is rapidly advancing against a backdrop of heated debate among scientists and fierce controversy among laymen.

As presented by Burr, these are the main points that have been determined so far: Homosexual orientation is found in a small fraction of the male population (probably somewhere between 2 and 5 percent); it is probably fixed at an early stage of fetal development by some biological mechanism dependent on the flow of hormones; that mechanism is directed in some measure by a wayward gene on the single X chromosome men inherit from their mother. Burr also points out that while homosexual orientation is virtually impossible to alter, it neither compels homosexual behavior nor precludes heterosexual activity, because human beings possess free will.

Systematically reviewing the groundwork for these conclusions, Burr begins with an analogy between homosexual orientation and left-handedness. Both are found in a small minority of the population (and more frequently among males than females), both are already evident at about two years of age, both run in families, are transmitted through the mother, and are more often shared by identical twins than by other siblings. And while the external behavior connected with both traits can be suppressed, the intrinsic tendency remains. This confluence of observations suggests that homosexual orientation is fixed by nature, and quite likely by genetic inheritance. Burr tells us that this conclusion was powerful enough to persuade reputable scientists to search in a variety of directions for the underlying biological mechanisms.

Some researchers have looked for structures in the brain that might reflect differences in sexual orientation. Burr describes a controversial claim by Simon LeVay, a neuroanatomist at the Salk Institute in California, that two hypothalamic nuclei, pinhead-sized structures in part of the brain identified with the sex drive, are larger in homosexual than in heterosexual men. Other scientists question the validity of LeVay's findings. They cite, for example, doubts that the "nuclei" are anything more than artifacts of the stain used to observe them, as well as suggestions that the differences in size may have resulted from drugs taken by the deceased AIDS victims from whom the brains had been taken or from high levels of testosterone in the subjects' blood. Further, even if there is a difference in brain structure, the question remains whether it is a cause or an effect of sexual orientation.

Hormones clearly play a key role in sexual behavior, but their precise role in determining sexual orientation is unclear. Burr discusses research on a variety of bizarre sexual behavior in species including fruit flies, rats, and hyenas, both in the lab and in the field. Scientists observing these phenomena with a mixture of curiosity and prurience have connected them all to various chemical effects. However, like observations made on men and women suffering from severe hormone-related physiological and psychological disorders that involve contradictions between their genes, their anatomy, and their behavior, they shed little light on the origin of homosexual orientation in otherwise normal human males.

Taking another approach, Dean Hamer of the National Institutes of Health compared DNA samples from mothers and sons in families with both homosexual and heterosexual offspring. Using a technique called linkage analysis, he found with an extremely high level of confidence that the homosexual brothers, but not their heterosexual brothers, shared a particular variant of a gene located on the X chromosome. The gene discovered and named by Hamer is now listed in the geneticists' directory as GAY-I, locus Xq28.