A great sports story tells us about who we are, and who we are in 2015 is Fallon Fox. Fox, a professional women’s mixed martial arts fighter since 2012, is five-foot-seven. In the ring, she’s brutal, tactical and efficient if not flashy; in a 2013 match against Allanna Jones, Fox delivered a barrage of thigh kicks and straight punches, ultimately winning with a knee-ride choke-out. Neither woman has half Muhammad Ali’s grace, and that’s meaningful. Women’s MMA allows for bruisers who transgress our expectations of women in the ring and in life. But as MMA’s only transgender fighter, Fox prompts an even bigger question: How do we define gender in the first place?

Trans athletes, women specifically, face discrimination that reveals a cultural preoccupation with male superiority rather than “competitive advantage”—how critics phrase their claim that trans women retain the physical advantages they had before transitioning. UFC women’s bantamweight champion Ronda Rousey, widely considered the best women’s fighter in the world, is among Fox’s loudest detractors. “If you go through puberty as a man, it’s not something you can reverse,” she told TMZ last year. “There’s no undo button on that.”

But there is. In 2004, the International Olympic Committee became the first major athletic organization to adopt a policy governing the participation of transgender athletes. The committee concluded that trans women who transition after puberty lose any advantage after two years on estrogen. USA Track and Field, the Association of Boxing Commissions, the U.K.’s Football Association and the Ladies Professional Golf Association have announced similar policies.

Discomfort with trans women in sports goes back decades. In 1976 trans tennis player Renée Richards’s bid to compete in the U.S. Open was refused based on the United States Tennis Association’s female-at-birth policy. After a controversial New York Supreme Court ruling reversed the decision, Richards demonstrated her distinct lack of advantage by not making it past the first round of the 1977 U.S. Open singles competition—a fact conveniently forgotten by critics.

Critics claimed then that men would transition to compete in women’s sports, the likely root of the “advantage” argument. “I don’t think anybody becomes a woman for competitive advantage,” asserts leading gender-reassignment surgeon Dr. Marci Bowers. “If Mike Tyson became a woman and went into women’s boxing, Mike Tyson would look like a round, dumpy woman. That muscle would turn to fat.”

The advantage argument is a straw man for a larger cultural anxiety, which is why basic biological facts don’t pacify those who refuse to understand the reality of trans bodies in the first place.

“It’s like global warming,” Fox says. “Scientists know it’s happening, but it’s not something most people can measure for themselves. They think it’s a conspiracy because it’s over their heads. It becomes their own personal religion.”

You can’t watch the Olympics without noticing that physical variation, even within the sexes, is enormous. But in the world of sex-segregated sports, abnormal ability, even in highly conditioned athletes, raises eyebrows.

Take Caster Semenya, the South African who won the 800-meter women’s world championship in 2009 and was forced to submit to gender-verification testing after her performance raised suspicion. The International Association of Athletics Federations cleared her to race—but only after an 11-month waiting period and negative media attention that seem to have derailed the career of the rising star.

How do we make athletics fair, given that the genetic lottery is part and parcel of athleticism? “It’s tough to pick one thing and say it’s a competitive advantage,” Bowers says. Some born female exhibit the same advantages—greater bone density and height, narrower pelvic structure—attributed to trans women.

If we want to talk biology, we should discuss Fox’s competitive disadvantage. She barely produces testosterone, clocking an average of 12 to 19 nanograms per deciliter of blood. According to the Mayo Clinic, women over 19 produce eight to 60 nanograms on average; men over 19 produce 240 to 950 nanograms. Given testosterone’s role in muscle production, Fox’s low levels require her to compensate via rigorous training.

“If you have a predominance of estrogens, you should be considered female, and vice versa,” Bowers says. “It gets messy if you try to pin it down too much.” Seems simple enough. And yet….

What are Rousey and her ilk really talking about when they cite “competitive advantage”? Fox most often hears accusations involving “bone density,” another straw man. “Black women’s bones are denser than your average white man’s bones,” Fox says. “Asians are the least dense of all ethnicities. What does that mean? That black women can’t compete against Asian or Caucasian women because their bones are too dense?”

Black athletes have long been subject to the offensive “slave genes” myth, which racists use to suggest that descendants of slaves have a competitive advantage that is unfair to white athletes. “Every time a minority tries to enter a sport, they try to use biology to bar them,” Fox says.

The truth is that many people are afraid. History has taught us that when people in power operate in fear, they try to eradicate and hide that which confuses or troubles them.

The world is changing once again. Arguments lobbed against athletes like Fox are a product of a world organized around genders we can identify with our eyes. That categorization turns out to be imprecise. Transgender athletes and the controversies they incite have demonstrated that gender is not verifiable with a pants check or a birth certificate. Some non-transgender women have a Y chromosome. Some men, trans and not, have low testosterone or atypically formed testes. Trans teens are treated with hormone blockers that repress puberty. And there are trans men like me who inject testosterone once a week so our bodies reflect our reality not as we were born but as we are.

The future is now: Laverne Cox is on the cover of Time and Caitlyn Jenner on Vanity Fair. Transparent and Orange Is the New Black rule our living rooms. Women such as Fox force a frightened country to reconcile its ideal of liberty with reality, as it has done many times before. Our sports are our stories, our greatest cultural and social narratives in all their messy humanity, from Muhammad Ali to Billie Jean King. They begin, as Fox knows, with visibility.

“Competing puts us on the map,” she says. “It breaks the silence, and silence is what has kept us down for so long.”