How can we reconcile this idea—of a single genetic switch that dominates one of the most profound dichotomies in human identity—with the fact that human gender identity in the real world appears in a continuous spectrum? Virtually every culture has recognized that gender does not exist in discrete half-moons of black and white, but in a thousand shades of gray. Even Otto Weininger, the Austrian philosopher now famous for his misogyny, conceded, “Is it really the case that all women and men are marked off sharply from each other ... ? There are transitional forms between the metals and nonmetals; between chemical combinations and simple mixtures, between animals and plants, between phanerogams and cryptogams, and between mammals and birds. ... The improbability may henceforth be taken for granted of finding in Nature a sharp cleavage between all that is masculine on the one side and all that is feminine on the other.”
In genetic terms, though, there is no contradiction: Master switches and hierarchical organizations of genes are perfectly compatible with continuous curves of behavior, identity, and physiology. The SRY gene indubitably controls sex determination in an on/off manner. Turn SRY on, and an animal becomes anatomically and physiologically male. Turn it off, and the animal becomes anatomically and physiologically female.
But to enable more profound aspects of gender determination and gender identity, SRY must act on dozens of targets—turning them on and off, activating some genes and repressing others, like a relay race that moves a baton from hand to hand. These genes, in turn, integrate inputs from the self and the environment—from hormones, behaviors, exposures, social performance, cultural role-playing, and memory—to engender gender. What we call gender, then, is an elaborate genetic and developmental cascade, with SRY at the tip of the hierarchy, and modifiers, integrators, instigators, and interpreters below. This geno-developmental cascade specifies gender identity. Genes are like single lines in a recipe that specifies gender. The SRY gene is the first line in the recipe: “Start with four cups of flour.” If you fail to start with the flour, you will certainly not bake anything close to a cake. But infinite variations fan out of that first line—from the crusty baguette of a French bakery to the eggy mooncakes of Chinatown.
The existence of a transgender identity provides powerful evidence for this geno-developmental cascade. In an anatomical and physiological sense, sex identity is quite binary: Just one gene governs sex identity, resulting in the striking anatomical and physiological dimorphism that we observe between males and females. But gender and gender identity are far from binary. Imagine a gene—call it TGY—that determines how the brain responds to SRY (or some other male hormone or signal). One child might inherit a TGY gene variant that is highly resistant to the action of SRY on the brain, resulting in a body that is anatomically male, but a brain that does not read or interpret that male signal. Such a brain might recognize itself as psychologically female; it might consider itself neither male or female, or imagine itself belonging to a third gender altogether.
These men (or women) have something akin to a Swyer syndrome of identity: Their chromosomal and anatomical gender is male (or female), but their chromosomal/anatomical state does not generate a synonymous signal in their brains. In rats, notably, such a syndrome can be caused by changing a single gene in the brains of female embryos or exposing embryos to a drug that blocks the signaling of “femaleness” to the brain. Female mice engineered with this altered gene or treated with this drug have all the anatomical and physiological features of femaleness, but perform the activities associated with male mice, including mounting females: these animals might be anatomically female, but they are behaviorally male.
The hierarchical organization of this genetic cascade illustrates a crucial principle about the link between genes and environments in general. The perennial debate rages on: nature or nurture, genes or environment? The battle has gone on for so long, and with such animosity, that both sides have capitulated. Identity, we are now told, is determined by nature and nurture, genes and environment, intrinsic and extrinsic inputs. But this too is nonsense—an armistice between fools. If genes that govern gender identity are hierarchically organized—starting with SRY on top and then fanning out into thousands of rivulets of information below—then whether nature predominates or nurture is not absolute, but depends quite acutely on the level of organization one chooses to examine.
At the top of the cascade, nature works forcefully and unilaterally. Up top, gender is quite simple—just one master gene flicking on and off. If we learned to toggle that switch—by genetic means or with a drug—we could control the production of men or women, and they would emerge with male versus female identity (and even large parts of anatomy) quite intact. At the bottom of the network, in contrast, a purely genetic view fails to perform; it does not provide a particularly sophisticated understanding of gender or its identity. Here, in the estuarine plains of crisscrossing information, history, society, and culture collide and intersect with genetics, like tides. Some waves cancel each other, while others reinforce each other. No force is particularly strong—but their combined effect produces the unique and rippled landscape that we call an individual’s identity.
