Sexual orientation and biology

For several decades now scientists have been seeking evidence of biological factors conditioning sexual orientation. Many educated persons tend to be suspicious of this approach because of its apparent right-wing pedigree, yielding more than a whiff of the social Darwinism of a hundred years ago.

Moreover, these ideas of determinants operating outside our conscious control seem to pose a limit to human freedom, which we should be seeking to maximize instead of reducing. Yet most gay and lesbian people have known for a a long time that their orientation is not a mere choice, a pattern of behavior that we can assume or discard just as we choose. In some sense it chooses us.

In everyday life most would recognize that there is such a thing as gaydar. The pitch of the voice, the gait, the style of clothing--all these things may provide a clue. Clustering together, the traits suggest a near-certain conclusion. In a less sensitive era, we would have said that such a person was an “obvious” homosexual.

Well, yes, but this everyday approach is still more intuitive than rational. And what about the millions who are not “obvious”? Gaydar provides little help in detecting their orientation. Moreover. this common-garden approach tells us little about the underlying causal factors.
So why not explore the possibility of some element of biological or constitutional conditioning in the formation of sexual orientation? In fact, for some time now scientists have been doing just that. In this endeavor, they have had successes as well as failures.

Statistically, it has been claimed that gay men and lesbians have about a 50 percent greater chance of being left-handed or ambidextrous than straight men or women. The relative lengths of fingers may offer another hint. The index fingers of most straight men are shorter than their ring fingers, while for most women they are closer in length, or even reversed in ratio. But it has been noted that gay men are likely to have finger-length ratios more in line with those of straight women. Or so it seems.

Other markers may be less immediately discernible, but more significant. Researchers have traditionally used twin studies to try to distinguish genetic influences from environmental or other influences. One common type of twin study compares identical twins (known as monozygotic or "MZ twins") who both have a particular trait to non-identical or fraternal twins (known as dizygotic or "DZ twins") with that same trait. Since identical twins have the same genetic makeup (genotype) while non-identical twins share an average of 50% of their genes, a difference between these types of twins points to a genetic component. For example, if a high percentage of identical twins both have red hair (while a low percentage of non-identical twins both have red hair), that concordance implies that red hair has a genetic basis. On the other hand, if identical twins share a characteristic just as often as fraternal twins (such as love of music), that fact suggests that there is not a genetic basis for that trait.

A number of twin studies have attempted this kind of comparative study. Since it is important to use individuals of twin sets who have been raised apart (to guard against environmental contaminants), the numbers tend to be small. These are the ones who show a high degree of difference in orientation when identical twins (MZ) and fraternal twins (DZ) are compared. Critics say the studies are too small to guard against sample bias.

Later studies, performed on increasingly more representative samples, showed less concordance among MZ twins, although still significantly larger than among DZ twins. A recent second-order analysis by Hershberger (2001) compares the results of eight different twin studies: among those, all but two showed MZ twins having much higher concordance of sexual orientation than DZ twins, suggesting a non-negligible genetic component. Here are two additional examples: Bailey and Pillard (1991) in a study of gay twins found that 52% of MZ brothers and 22% of the DZ twins were concordant for homosexuality. In additiion, Bailey, Dunne and Martin (2000) used the Australian twin registry to obtain a sample of 4,901 twins. Self reported zygosity, sexual attraction, fantasy and behaviors were assessed by questionnaire and zygosity was serologically checked when in doubt. MZ twin concordance for homosexuality was found to be 30%. Averaging over all studies suggests that roughly 50 percent of the variance in sexual orientation can be attributed to inherited factors.

A recent study of all known adult twins in Sweden (more than 7,600 twins) found that same-sex behavior was explained by both heritable factors and individual-specific environmental sources (including prenatal environment, experience with illness and trauma, as well as peer groups, and sexual experiences), while influences of shared-environment variables such as familial environment and societal attitudes had a weaker, but significant effect.

In short, some of the causation seems to be environmental and some of it hereditary. The hereditary factor is still very significant, and this finding tells against the hyperenvironmental approach which denies any biological component in the formation of sexual orientation.

At this point perhaps a touch of levity is warranted. One wag puts it this way. “The etiology of homosexuality is complex. Fifty percent are born that way; the other fifty percent just get sucked into it.”

Still, the question of the actual mechanism remains. A famous answer has been provided by Simon LeVay, a British-born neuroscientist who works in California. In 1991 LeVay published "A difference in hypothalamic structure between heterosexual and homosexual men" in the prestigious weekly Science. For his study he obtained brains from 41 deceased hospital patients. The subjects were classified as follows: 19 gay men who had died of AIDS, 16 presumed heterosexual men (6 of whom had died of AIDS), and 6 presumed heterosexual women (1 of whom had died of AIDS). The AIDS patients in the heterosexual groups were all identified from medical records as intravenous drug abusers or recipients of blood transfusions, though only 2 of the men in this category had specifically denied homosexual activity. The records of the remaining heterosexual subjects contained no information about their sexual orientation; they were assumed to have been mostly or all heterosexual "on the basis of the numerical preponderance of heterosexual men in the population."

LeVay’s article reported a difference in average size between the third Interstitial Nucleus of the Anterior Hypothalamus (INAH3) in the brains of heterosexual men and homosexual men: INAH3 was more than twice as large in heterosexual men as in homosexual men. And yet the INAH3 size of homosexual men was the same as that of women. LeVay concluded that "This finding indicates that INAH is dimorphic with sexual orientation, at least in men, and suggests that sexual orientation has a biological substrate." LeVay added, "The existence of 'exceptions' in the present sample (that is, presumed heterosexual men with small INAH 3 nuclei, and homosexual men with large ones, hints at the possibility that sexual orientation, although an important variable, may not be the sole determinant of INAH 3 size. It is also possible, however, that these exceptions are due to technical shortcomings or to misassignment of subjects to their subject groups."

A few years later LeVay cautioned against misinterpreting his findings: "It’s important to stress what I didn’t find. I did not prove that homosexuality is genetic, or find a genetic cause for being gay. I didn’t show that gay men are born that way, the most common mistake people make in interpreting my work. Nor did I locate a gay center in the brain. The INAH3 is less likely to be the sole gay nucleus of the brain than a part of a chain of nuclei engaged in men and women's sexual behavior." Some critics of LeVay questioned the accuracy and appropriateness of his measurements, saying that the structures are difficult to see in tissue slices and that he measured in volume rather than cell count. LeVay has been criticized for "his small sample size and for compiling inadequate sexual histories." The use of brains of men who died of HIV/AIDS is also problematic. Some of the difference might be due to the pathology induced by the disease.

Chromosome linkage studies of sexual orientation suggest the presence of multiple contributing genetic factors throughout the genome. In 1993 Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families. Hamer et al. found that the gay men had more gay male uncles and cousins on the maternal side of the family than on the paternal side. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles. In another finding, thirty-three of the forty sibling pairs tested were found to have similar alleles in the distal region of Xq28, which was significantly higher than the expected rates of 50% for fraternal brothers. This feature was popularly (but inaccurately) dubbed as the “gay gene” in the media, causing significant controversy. Later studies have offered some, though limited support for this finding. Other genetic factors may be involved. For reasons that are not entirely clear, the approach does not seem to be a promising area of research.

For some time now, there has been widespread enthusiasm for using birth order as an explanation for human behavior. Blanchard and Klassen (1997) hold that each older brother increases the odds of a man being gay by 33%. This, they claim, is now "one of the most reliable epidemiological variables ever identified in the study of sexual orientation." To explain this finding, it has been proposed that male fetuses provoke a maternal immune reaction that becomes stronger with each successive male fetus. Male fetuses produce HY antigens which are "almost certainly involved in the sexual differentiation of vertebrates." It is this antigen which maternal H-Y antibodies are proposed to both react to and “remember.” Successive male fetuses are then attacked by H-Y antibodies which somehow decrease the ability of H-Y antigens to perform their usual function in brain masculinization.

The idea that male homosexuals are deficient in masculinity is inherently problematic. Moreover, Peter Bearman (2002) questions the sampling method of Blanchard and other scientists who report a link between fraternal birth order and sexual orientation. He says that the studies work with nonrepresentative samples and indirect reports on siblings’ sexual orientation. After repeating the experiment done by Blanchard, Bearman found "no association between same-sex attraction and number of older siblings, older brothers, or older sisters.”

As if all this were not enough, a surprising new finding has been added to this bewildering heap of evidence. Richard Lippa, a psychologist from California State University at Fullerton, has joined the ranks of the cataloguers of the many ways in which gay people are different. The University’s press release sums up the matter this way: “If the hair whorl on the top of your head circles counterclockwise and you are left-handed, then it’s possible that you are gay and that your sexual orientation can be biologically explained.”

Professor Lippa, author of “Gender, Nature and Nurture,” has examined about 500 male head whorls and is becoming convinced that that there is a connection. Women are excluded from the survey because they tend to have long hair, making it hard to ascertain their whorls.

As part of the study, Lippa also is collecting DNA samples by swabbing the inside cheeks of his subjects. The samples will be analyzed to determine whether there is a common gene responsible for causing hair whorl direction and whether people are left- or right-handed.

Preliminary results are “implicating a biological explanation because you are born with your hair whorl,” Lippa said. “You can’t change the direction in which it swirls. It sounds weird that hair whorl patterns could be associated with sexual orientation. I was very skeptical when I started this study, but I’m becoming convinced that there is a connection between hair whorl, left-handedness and sexual orientation.”

Perhaps sexual orientation is determined in the womb, he said. “If this is true, then that will be a remarkable finding.”

His broader horizons of research have led Professor Lippa to conclude that gay men and lesbians are more likely than heterosexuals to be left-handed; the more older brothers a man has, the more likely he is to be gay; the higher a woman’s sex drive, the more she desires both men and women; men prefer good looks in a mate while women prefer honesty, humor, kindness and dependability.

Specifically, more gay men (13 percent) than heterosexual men (11 percent) and more lesbians (11 percent) than heterosexual women (10 percent) reported being left-handed. In my view this is not a significant finding. However, Lippa goes further, saying that more bisexual men (12 percent) than gay or heterosexual men (8 percent) described themselves as ambidextrous, and more bisexual women (16 percent) than lesbians (12 percent) or heterosexual women (8 percent) described themselves as ambidextrous.

However, even Lippa hesitates to say that gay people are essentially different from straights. "Essentialism," he explains, "is the enemy of a lot of academics," because it shuts down inquiry into all the possible influences. Perhaps there are a dozen possible routes to homosexuality, any combination of which might produce a number of the traits being catalogued now. It might be that there is no single thing called homosexuality-that there are instead dozens of homosexualities, scores of potential outcomes in terms of personality, and endless potentials for describing them. "For example, do gay men who have older brothers show more or less feminine? Do gay men with counterclockwise hair have more masculine traits? One cause might create a more feminine homosexuality than another."

These caveats are useful. It is hard to miss the recurrent note of overconfidence that runs through so many of these studies. All the same. there is something important going on here, and the passage of time is sure to make the picture clearer.