Homosexuality and Biology

An introduction to a muddled and sometimes contentious world of scientific research—one whose findings, now as tentative as they are suggestive, may someday shed light on the sexual orientation of everyone
The Genetic Quest

In 1963 Kulbir Gill, a visiting scientist from India working at Yale University, was conducting research into genetic causes of female sterility. His experiments involved exposing the fruit fly Drosophila melanogaster, that workhorse of genetic research, to X-rays, and observing the behavior of the resulting offspring. Gill noticed that a certain group of mutant male flies were courting other males, following each other and vibrating their wings to make characteristic courtship "songs." Gill published his findings in a short note in the publication Drosophila Information Service and then returned to the question of female sterility.
A decade later Jeffrey Hall, a biologist at Brandeis University, followed up on Gill's odd discovery. Every discovered Drosophila gene mutation is given a name, and Gill had called his mutation "fruity." Hall, considering this name to be denigrating, redubbed it, still somewhat tongue-in-cheek, "fruitless." Hall explains that the fruitless mutation produces two distinct behaviors. First, fruitless-bearing male flies, unlike nonmutant male flies, actively court other males as well as females, although for reasons that remain poorly understood, they are unable actually to achieve intercourse with members of either sex. Second, fruitless-bearing males elicit and are receptive to courtship from other males, which nonmutant males reject.

Fruit flies can live for two or three months, and this "bisexual" fly strain has existed behaviorally unchanged through hundreds of generations. Some gene mutations are lethal to flies; fruitless is not one of these, nor does it cause illness. It is, Hall says, a nonpathological genetic mutation that causes a consistent, complex behavior. And fruitless displays an anatomical sexual dimorphism, bringing LeVay's study to mind. In the abdomen of male Drosophila flies there is a muscle, the so-called muscle of Lawrence, whose function is unknown; female fruit flies don't have it, and neither do fruitless males.

Although fruitless flies don't mate, the perpetuation of the fruitless trait is made possible by the fact that it is recessive--a full pair of the mutations is needed for fruitless behavior to be expressed. When males that carry a single fruitless gene mate with a fruitless-carrying female, a percentage of their offspring will carry the full pair and display typical fruitless behavior. If a genetic component of homosexuality in human beings exists, it could possibly operate by means of a comparable mechanism.

Angela Pattatucci, a geneticist at the National Institutes of Health, gave me a demonstration a few months ago in her lab. She took a small glass container of tiny Drosophila flies, popped off the top, and plugged an ether-soaked cotton ball into the mouth. Within a few seconds the flies were lying stunned on the glass floor. Using a plastic stick Pattatucci separated out a few of the flies into a larger glass jar. I looked at a group of males and females through a microscope, their bodies vibrating, red eyes bulging. Pattatucci showed me how to differentiate the genitalia at the end of the abdomen--smooth and light-colored for females, furry and dark for males.

Pattatucci said that researchers are relatively close to finding the actual fruitless gene. It is already known that fruitless is located physically on the right arm of the third chromosome. After establishing the precise location (or locations) of the mutation, researchers can determine the sequence of biochemical information in fruitless's genetic code—the order of thousands of units of the basic genetic components adenine, thymine, guanine, and cytosine. Once the combination is known, the search can begin for a similar combination—a fruitless analogue—in human beings.

In the jar the males, separated out, eventually came back to awareness. "Watch that one," Pattatucci said, pointing to a fly that had come up behind another fly, vibrating his wings in courtship. He then climbed on top of the male he was courting. I watched the two flies, one atop the other, the one on the bottom wandering around as if a bit bored. As noted, for a fruitless fly that is as far as things can go.

I once asked Jeffrey Hall if courtship alone could be satisfying for a fly. "Could be," he said. "Maybe it's delicious, maybe he's frustrated. But this becomes ludicrous. How do you know when a fruit fly is frustrated?" It is an important point: the danger of anthropomorphizing insect behavior is great, and I found myself doing it almost by reflex when watching Pattatucci's flies. How can we equate fly behavior with a vast something that in human beings generates aesthetic and intellectual perceptions—with something that encompasses emotional need and love and the pain of love? So Hall is careful to describe fruitless as "a mutation that leads to a mimic of bisexuality." He is skeptical that finding a fruitless analogue will lead to a full explanation of human homosexuality. DNA analogues for all sorts of fruit-fly genes do exist in human beings, and the process of looking for them is relatively straightforward. But, as Hall points out, "it is very unlikely that the genetics of homosexuality will ever devolve to a single factor in humans with such major effects as it has in Drosophila."

When biologists are interested in establishing whether genetics is involved in the appearance of certain characteristics or conditions, one obvious place to look is among people who are closely related to one another. In "A Genetic Study of Male Sexual Orientation," a study that has now achieved almost as much renown as LeVay's, the Northwestern University psychologist Michael Bailey and Boston University's Richard Pillard compared fifty-six "monozygotic" twins (identical twins, from the same zygote, or fertilized egg), fifty-four "dizygotic" (fraternal) twins, and fifty-seven genetically unrelated adopted brothers. Identical twins are important in sexual-orientation research because, of course, they have identical genomes, including the sex-chromosome pair. If homosexuality is largely genetic in origin, then the more closely related that people are, the greater should be the concordance of their sexual orientation.

That is, in fact, what the study found. Bailey and Pillard reported a gay-gay concordance rate of 11 percent for the adoptive brothers, 22 percent for the dizygotic twins, and 52 percent for the monozygotic twins. The findings suggest that homosexuality is highly attributable to genetics—by some measures up to 70 percent attributable, according to Pillard. This figure is based on something geneticists call "heritability," a painstakingly calculated indicator of how much genes have to do with a given variation among people. If heritability is less than 100 percent, then the characteristic being studied is by definition "multifactorial." Eye color is 100 percent dependent on genetics. Height, on the other hand, though about 90 percent genetic, is also affected by nutrition, and thus is multifactorial.

If a large contribution to homosexuality comes from genes; where does the rest of it come from? The range of environmental and biological inputs a developing child receives is both enormous and enormously complex. "Whatever the other variables are," Pillard says, "they must be present early in life. I think this because the genderatypical behavior that so strongly prefigures an adult homosexual orientation can be observed early in development." And he goes on: "There certainly could be different paths to the same outcome. With individual cases, there are doubtless some that are mostly or all genes, and others that might be all environment. Our analysis [of twins] doesn't say anything about the individual." Jeffrey Hall can be so underwhelmed by the prospect of finding a human analogue of the fruitless mutation because, as he points out, if we do find it, we still will not have fully accounted for the etiology of homosexuality even in identical twins. "You will effectively know nothing from this genetic knowledge," Hall says. A behavior as simple as jumping, he notes, is quite complex genetically, having to do with all kinds of genes and other, unknown factors. He says, "We are not about to create a genetic surgical procedure which makes you Michael Jordan." LeVay made the same point in the course of our conversation: "It's one thing to say that genes are involved, as they almost certainly are. It's a whole other thing to actually identify those genes, because homosexuality may be polygenic, with each gene having a small effect. "

Whatever the uncertainties ahead, though, the important point is that the genetic work is already fairly compelling. A new Bailey and Pillard genetic study of lesbian twins, to be published soon in the Archives of General Psychiatry, echoes the researchers' original male-twin findings with strikingly similar results. "We're getting a lot of consistency where we should be getting it," Bailey says.

The most interesting question is perhaps becoming not whether genetics plays a role in homosexuality but how. Why does nature preserve genes that influence sexual behavior and yet do not facilitate reproduction? Does less than 100 percent heritability mean that the Bailey and Pillard study is incompatible with a bipolar model of sexual orientation? In his study LeVay defined homosexuality in terms of the sex of a person's sexual-object choice: either men or women, either homosexual or heterosexual. Pillard and Bailey's multifactorial model suggests a shaded continuum of sexual orientations, and of origins and causes, more complex and subtle than a simple either-or model can accommodate, and closer to what may be the quirks and ambiguities of our real lives.

The Ramifications of Science

What does it all mean? As we have seen, scientists must sift for their conclusions through ambiguous results from a disparate group of studies that are excruciatingly difficult to interpret. Yet even at this relatively early date, out of the web of complexities it is becoming ever clearer that biological factors play a role in determining human sexual orientation. Richard Green said to me, "I suspect that at least in your lifetime we will find a gene that contributes substantially to sexual orientation." Michael Bailey says, "I would—and have—bet my career on homosexuality's being biologically determined." The pace of neurobiological and genetic research is only increasing.
The search is not without its opponents. Some, recalling earlier psychiatric "treatments" for homosexuality, discern in the biological quest the seeds of genocide. They conjure up the specter of the surgical or chemical "rewiring" of gay people, or of abortions of fetal homosexuals who have been hunted down in the womb. "I think all of us working in this field," Pattatucci says, "have delusions of grandeur in thinking we can control the way this knowledge will be used." Certainly the potential for abuse is there, but that is true of much biomedical knowledge. It is no reason to forswear knowledge of ourselves, particularly when the potential benefits are great.

Some of the benefits could be indirect. Laura Allen points out, for example, that there are many now-mysterious diseases—autism, dyslexia, schizophrenia—that affect men and women differently, hiding inside parts of the human mind and body that we cannot penetrate. Neurobiological research into sexual differentiation may help us to understand and cure these diseases, as well as to unlock other mysteries—the mysteries of sexuality.

And then there is the question with which we began—that of the acceptance of gay people in American society. The challenge posed by homosexuality is one of inclusion, and, as Evelyn Hooker would say, the facts must be allowed to speak. Five decades of psychiatric evidence demonstrates that homosexuality is immutable, and nonpathological, and a growing body of more recent evidence implicates biology in the development of sexual orientation.

Some would ask: How can one justify discriminating against people on the basis of such a characteristic? And many would answer: One cannot. Yet it would be wise to acknowledge that science can be a rickety platform on which to erect an edifice of rights. Science can enlighten, can instruct, can expose the mythologies we sometimes live by. It can make objective distinctions—as, for example, between sexual pathology on the one hand and sexual orientation on the other. But we cannot rely on science to supply full answers to fundamental questions involving human rights, human freedom, and human tolerance. The issue of gay people in American life did not arise in the laboratory. The principles needed to resolve it will not arise there either.

Presented by

Join the Discussion

After you comment, click Post. If you’re not already logged in you will be asked to log in or register with Disqus.

Please note that The Atlantic's account system is separate from our commenting system. To log in or register with The Atlantic, use the Sign In button at the top of every page.

blog comments powered by Disqus


The Horrors of Rat Hole Mining

"The river was our source of water. Now, the people won't touch it."


What's Your Favorite Slang Word?

From "swag" to "on fleek," tweens choose.


Cryotherapy's Dubious Appeal

James Hamblin tries a questionable medical treatment.


Confessions of Moms Around the World

In Europe, mothers get maternity leave, discounted daycare, and flexible working hours.


How Do Trees Know When It's Spring?

The science behind beautiful seasonal blooming

More in Technology

More back issues, Sept 1995 to present.

Just In