Little Drian Juarez didn’t play like other boys. Instead of Tonka trucks and wrestling, he preferred Barbie dolls and dressing up in his mother’s clothing. Whenever he asked for a dab of her lipstick, she told him it was “inappropriate for little boys.” Other kids made fun of everything from his mannerisms to his tone of voice. Some threatened to beat him up. “Something is wrong with me,” he thought. “Something is different.”

Juarez was born male, but he felt female.

Eric Vilain, a pediatric geneticist at the University of California, Los Angeles, probes the brain and genome for what determines whether Juarez and others feel male or female, and whether they’re attracted to the same or opposite sex. Last June, Vilain’s lab presented a model to predict homosexuality, and he hopes to test whether kids can outgrow gender dysphoria, challenging the recent push among parents to help them transition as early as possible. In fact, he remains skeptical of the very concept of gender identity, a stance that has sparked controversy in the communities he studies. “You talk about variations like the color of your skin,” he says in a thick French accent between sips of masala chai in a cozy tea lounge in San Francisco, where he was visiting to ring in the New Year. “But when it comes to sex, it’s very, very normative.”

Until Caitlyn Jenner made international news last summer, the concept of transgender was unfamiliar to most people outside the LGBTQ world. Granted, it’s a small population, although pinpointing the number of transgender people is difficult. Last year, a U.S. Census Bureau paper gave a rough estimate based on the numbers of people who changed their registered name and sex with the Social Security Administration. Among Americans who had responded to the 2010 Census, 89,667 had changed their registered names, and 21,833 had changed sex as well. Other surveys have calculated higher estimates, with 700,000 being the most commonly cited figure. In any case, the word “transgender” is entering the mainstream, and stirring controversy. North Carolina, for instance, just passed a law that prohibits trans people from using the bathroom that corresponds with their gender identity. Trans people are infamously vulnerable to violence, both suicide and murder. And there is no single transgender experience or definition, leaving many people, in and out of the queer world, confused. Vilain is using science, of all things, to help demystify gender.

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Vilain, 50, is dressed in fitted charcoal pants, a flannel button-up shirt and a black collared jacket, with a silver puzzle ring. He looks a bit like Quentin Tarantino with his square, stubbled jawline and heavy brow bone, jutting out over piercing, wide-set eyes. Don’t be so surprised. There’s a reason he defies the stereotype of the dowdy, disheveled scientist — gay men. Specifically, researchers at his lab, who gave their boss a makeover. “His hair was a disaster,” says Francisco Sánchez, a former postdoctoral researcher. They got him a hairdresser and dentist and suggested trading in his clunky specs for contact lenses.

As an adolescent in Paris — an “asexual time,” he says — Vilain was a geek. The son of a schoolteacher and a manager at a Colgate factory — both of whom encouraged an open-minded outlook — he excelled in school, even skipping a grade, and buried himself in the writings of Jean-Paul Sartre, Michel Foucault and other French philosophers, adopting their humanist credo that people should decide their own destiny. He also read the memoirs of Herculine Barbin, a 19th-century intersex person, discovered and published by Foucault. Designated female at birth, Barbin committed suicide after a medical exam revealed that she also had male genitalia. But Vilain didn’t give intersexuality any more thought until he was 19. At the time, he was fulfilling his childhood dream of becoming a pediatrician at L’hôpital Necker—Enfants Malades (Necker Hospital—Sick Children) in central Paris. His first rotation? The referral center for all intersex newborns born at the hospital.

If a trans woman believes she was once a girl trapped in a boy’s body, “Who am I to tell them, ‘That’s probably not true’?” Vilain says. “Can I do that as a scientist?”

Seeing his patients for the first time left Vilain shocked — and intensely fascinated. “I felt this urge to understand why on Earth it was possible to have genitals develop in such a broad variety,” he says. But the surgeons he worked with, according to Vilain, didn’t share his awe. When they presented case studies on Saturday mornings, they seemed less concerned with why an infant’s genitalia appeared a certain way than how to “fix” it. Reconstructive surgery to make the genitalia look “normal” would prevent psychological consequences — that was their reported stance, despite the lack of evidence to support their claims. In fact, surgery might even lead to loss of sensation and other complications, necessitating additional surgeries. But Vilain saw them make recommendations anyway. “It was very paternalistic,” he recalls.

Yves Aigrain, chief of the pediatric urology unit at Necker, wrote in an email that the attitude of French surgeons around the time of Vilain’s training was “for sure more paternalistic than it is today,” but “it is always very difficult to judge the medical decisions taken more than 20 years ago with today’s knowledge.” He “fully disagrees” that they were uninterested in understanding the development of their patients’ genitalia. Although they had based their decisions more on their own experience monitoring intersex children into adulthood than on controlled studies, they have since begun that research and now inform parents of the possible consequences of different courses of action. “The option of delaying any surgical decision until the child may ask himself or herself for a specific repair has to be considered,” Aigrain notes.

Disturbed by the hospital’s approach to intersex conditions, Vilain took a hiatus from his residency to pursue a Ph.D. in genetics at the Pasteur Institute. To improve care for intersex infants, he needed to understand the genetic mechanism of sex determination and how it can go awry. After earning his Ph.D. and completing his residency, he began a postdoctoral fellowship at UCLA on a sunny, 80-degree day in December 1995. Two years later, he accepted a faculty position and started his own research group, the Center for Gender-Based Biology.

At UCLA, Vilain earned a reputation for brashness. Sánchez recalls his mentor’s penchant for editorializing during colleagues’ talks. “He has no qualms with vocalizing how bad the presentation is … even though everyone in earshot hears him,” says Sánchez, now an assistant professor of at the University of Missouri. During the first talk he attended with Vilain, Sánchez sank sheepishly in his chair as some audience members snickered, while others glared. “If he thinks something is bad, he lets you know it’s bad. That’s just Eric.”

While unraveling the genetics of sex determination in the lab, Vilain also saw patients in the pediatric clinic, many of them older than the infants he had worked with in Paris. His mind turned to how sexuality develops later in life: What genes determine who we find attractive? Scientists had been pursuing the same question since at least the 1950s and ’60s, with the first studies of homosexuality in twins. They found that if one twin was gay, there was a greater likelihood that the other would be too — especially among identical twins, who share the same genetic code. Spurred by evidence supporting the heritability of homosexuality, researchers began their quest for the “gay gene.” In 1993, geneticist Dean Hamer published a paper in Science that reported a likely link between homosexuality and a region at the tip of the X chromosome, Xq28, bolstered by a larger study in 2014. But scientists have yet to pinpoint the precise genes involved in sexuality. “Maybe it’s a complex trait,” Vilain recalls thinking. “Maybe there’s a part of the environment that plays a role.” He began investigating how the environment might affect DNA by adding chemical markers that turn genes on and off, known as epigenetics.

Last June, there was a backlash after a talk that a former postdoctoral researcher in his lab, Tuck Ngun, gave on a project he had helped lead at the American Society of Human Genetics meeting. To tease apart the environment’s role, Ngun compared an epigenetic change called methylation — chemical tags that turn genes off — between gay and straight twins. Thirty-seven of the twin pairs in the study consisted of one gay and one straight twin, while in the remaining 10 pairs, both were gay. Ngun scanned DNA extracted from the twins’ saliva for 400,000 methylation tags. Among those, five differed significantly between gay and straight twins. He then used those tags to develop a model that predicts a man’s sexual orientation with 67 percent accuracy. Media outlets ran with the findings, while members of the scientific community criticized the small sample size and statistical methods. Emory University geneticist Peng Jin adds that the methylation differences between gay and straight twins might be too small to translate into any detectable differences in biological function. (Vilain says his lab is recruiting more twins, but he’s confident about the statistical methods used for the preliminary sample. Ngun could not be reached for comment.)

Vilain has built on his work in the science of sexuality and started investigating the science of gender identity, or whether we perceive ourselves as male or female. Among male-to-female trans people, gender identity often follows a so-called feminine essence narrative — someone born biologically male feeling like a woman trapped inside the wrong body. Some researchers have proposed that certain brain regions differ in men and women, and that transgenderism results when one or more of these regions aren’t consistent with the individual’s biological sex. One of Vilain’s projects involves scanning the brains of male-to-female transsexuals for evidence of these “male” and “female” brain regions. He hasn’t found anything compelling so far — and neither have other researchers. Although he can’t rule out the possibility yet, Vilain remains skeptical. His lips tremble nervously, and he stares into his Greek yogurt, grasping for the precise words, aware of their weight: “The idea of gender identity as something innate — I’m not sure it exists.” It may simply be a reflection of how others perceive our gender, which suggests that “transgenderism might not exist.”

In a Los Angeles Times op-ed he co-wrote last year, Vilain questioned the increasingly common practice of encouraging gender-nonconforming children to socially transition (calling them by an opposite gender name, for instance). He noted that while some gender-nonconforming boys later identify as trans women, the vast majority — more than 80 percent — outgrow their gender dysphoria by puberty, identifying as gay men. Vilain wants to investigate whether the same genetic or epigenetic changes in gay men are found in gender-nonconforming boys. If so, that would suggest gender dysphoria is more often a precursor to homosexuality than to transgenderism. Studies have also found men pushed to transition as children are more likely to identify as transgender — but, Vilain wonders, is that what’s best for them? Later, they might decide to fully transition with surgery and hormones, which carry their own risks and side effects. They might even outgrow their gender dysphoria and regret the decision.

Dana Beyer, executive director of Gender Rights Maryland, worries that relying too heavily on findings that most gender-nonconforming boys don’t identify as trans could lead parents and others to deny them the right to transition. “You can’t harm the trans girls because you want to protect those gay boys,” she says. But Vilain says he’s a “big proponent” of parents allowing gender-nonconforming children to express themselves in ways typical of the opposite sex — letting their sons play with dolls or grow out their hair. “That’s OK,” he says, “but that does not mean you’re a girl.” For him, it amounts to another attempt to box people into normative gender categories. Perhaps parents should allow children to dwell in the middle of the spectrum, rather than in the black-and-white extremes. Why can’t a boy who likes wearing dresses identify as male?

These gender categories don’t exist in nature, points out Juarez, now a transgender woman and program manager of the Transgender Economic Empowerment Project at the Los Angeles LGBT Center. “An elephant doesn’t collect pink flowers for a girl baby elephant,” she says. “Humans created narratives about what it means to be masculine or feminine.” Restricting ourselves to two neat identities is our way to bring order to the maelstrom of gender and sexuality; we fear uncertainty. Even Juarez wonders, what if she had grown up in an environment where she could explore the in-between region of the gender spectrum? “I might not have landed on a specific male or female identity,” she says. The new generation is more open to fluidity, she says, with the emergence of “heteroflexible,” “pansexual” and a multitude of other labels. Ultimately, however, “gender might be something bigger than we even have the language to describe,” she says, “and that’s OK.”

***

But it also makes clashes inevitable. Vilain’s fervid devotion to science above everything else — public opinion especially — has triggered intense reactions. Over the years, he’s weathered criticism, invectives, even threats. He recalls a Traditional Values Coalition blog post that slammed the use of taxpayer dollars to fund his research on “deviant sexual behaviors.” (Traditional Values Coalition President Andrea Lafferty wrote in an email that the organization “continues to monitor taxpayer-funded research, the grant process and the abuse of these funds.”) One trans activist even labeled him a monster. He has received letters containing what he describes as “veiled suggestions of violence.” Some of the concerns are ethical. People have accused him of “promoting a type of science,” he says, one that could be used to screen and abort fetuses. Albert Einstein College of Medicine bioethicist Tia Powell agrees that whether someone decides to distort scientists’ research findings lies beyond their purview. Still, they should mention how others might abuse their work, which Vilain has done in public lectures about his research, dismissing myths about the cause of homosexuality — clarifying that they don’t include a distant father or controlling mother, for instance. “It’s impossible to end this kind of research and inappropriate to do so,” Powell says. “Questions around sexual attraction and what people think about romantic love are among the questions people care most about in the world.”

And the discovery that the environment can influence sexuality implies that it can be changed through conversion therapy or other means. Others point out that it undermines the argument often invoked to secure equal rights for LGBT individuals — that they can’t change their orientation. Vilain finds it “a very strange use of the science.” Although his findings don’t entirely rule out genetics, he says LGBT individuals deserve basic human rights regardless. Vilain also knows his research lies at odds with the narratives of the very communities he studies, and he grapples with the existential questions it raises. If a trans woman genuinely believes she was once a girl trapped in a boy’s body, “Who am I to tell them, ‘That’s probably not true’?” Vilain says. “Can I do that as a scientist?”

Trans critics often point out that, as a white cis man — a male who was born biologically male — Vilain can easily discount the feeling of being trapped in the wrong body, having never experienced it himself. Juarez “can’t help but think that would influence his work. Whether the individual would acknowledge it or not, he’s been given privilege.” Trans people are tired of cis academics treating them like curiosities and telling their stories, she says. Vilain, though, claims not to have an identity — in fact, he staunchly spurns the very concept. Questions about his own experiences with gender and sexual orientation make him cagey. Sánchez described attending a birthday party hosted by Vilain and his wife at their Brentwood home, but Vilain neither confirms nor denies his marital status. “I don’t identify as LGBT” — he pauses — “or straight either. It’s not something I wake up in the morning and think about.” Even if he did identify as cis, he notes that just as atheists can still study the impact of religion on behavior, gender and sexuality researchers needn’t be sexual minorities themselves to understand them. “That’s not how science works.”

Vilain insists that his only agenda is to understand what makes us human. Our sexuality forms “a huge part of who we are,” he says. “Society can decide what to do with the science.” Still, amid the constant opposition and scant funding, Vilain sometimes feels tempted to walk away. A few months ago, a UCLA colleague invited him to collaborate on a project to investigate Ebola susceptibility in the Congo. Securing grant money would be a breeze, and who would protest Ebola research? “I could have it easier,” Vilain admits, “but I feel like my work is not done.”

This article has been updated to reflect Francisco Sánchez’s latest position.